matrix_sdk_ui/timeline/controller/

mod.rs

// Copyright 2023 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use std::{
    collections::BTreeSet,
    fmt,
    ops::Deref,
    sync::{Arc, OnceLock},
};

use as_variant::as_variant;
use eyeball_im::{VectorDiff, VectorSubscriberStream};
use eyeball_im_util::vector::{FilterMap, VectorObserverExt};
use futures_core::Stream;
use futures_util::future::try_join_all;
use imbl::{HashSet, Vector};
use matrix_sdk::{
    deserialized_responses::{ThreadSummary, TimelineEvent},
    event_cache::{
        DecryptionRetryRequest, EventCache, EventFocusedCache, PaginationStatus, PinnedEventsCache,
        RoomEventCache, Subscriber as EventCacheSubscriber, ThreadEventCache,
        ThreadEventCacheUpdate,
    },
    send_queue::{LocalEcho, LocalEchoContent, RoomSendQueueUpdate, SendHandle},
    task_monitor::BackgroundTaskHandle,
};
use ruma::{
    EventId, MilliSecondsSinceUnixEpoch, OwnedEventId, OwnedTransactionId, OwnedUserId, RoomId,
    TransactionId, UserId,
    api::client::receipt::create_receipt::v3::ReceiptType as SendReceiptType,
    events::{
        AnyMessageLikeEventContent, AnySyncMessageLikeEvent, AnySyncTimelineEvent,
        MessageLikeEventType,
        poll::unstable_start::UnstablePollStartEventContent,
        reaction::ReactionEventContent,
        receipt::{Receipt, ReceiptEventContent, ReceiptThread, ReceiptType},
        relation::{Annotation, RelationType},
        room::message::{MessageType, Relation},
    },
    room_version_rules::RoomVersionRules,
};
use tokio::sync::{RwLock, RwLockWriteGuard};
use tracing::{
    Instrument as _, Span, debug, error, field::debug, info, info_span, instrument, trace, warn,
};

pub(super) use self::{
    metadata::{RelativePosition, TimelineMetadata},
    observable_items::{
        AllRemoteEvents, ObservableItems, ObservableItemsEntry, ObservableItemsTransaction,
        ObservableItemsTransactionEntry,
    },
    state::TimelineState,
    state_transaction::TimelineStateTransaction,
};
use super::{
    DateDividerMode, EmbeddedEvent, Error, EventSendState, EventTimelineItem, InReplyToDetails,
    MediaUploadProgress, Profile, TimelineDetails, TimelineEventItemId, TimelineFocus,
    TimelineItem, TimelineItemContent, TimelineItemKind, TimelineReadReceiptTracking,
    VirtualTimelineItem,
    algorithms::{rfind_event_by_id, rfind_event_item},
    event_item::RemoteEventOrigin,
    item::TimelineUniqueId,
    subscriber::TimelineSubscriber,
    traits::RoomDataProvider,
};
use crate::{
    timeline::{
        MsgLikeContent, MsgLikeKind, Room, SendTarget, TimelineEventFilterFn,
        TimelineEventFocusThreadMode,
        algorithms::rfind_event_by_item_id,
        controller::decryption_retry_task::compute_redecryption_candidates,
        date_dividers::DateDividerAdjuster,
        event_item::TimelineItemHandle,
        tasks::{event_focused_task, pinned_events_task, thread_updates_task},
    },
    unable_to_decrypt_hook::UtdHookManager,
};

pub(in crate::timeline) mod aggregations;
mod decryption_retry_task;
mod metadata;
mod observable_items;
mod read_receipts;
mod state;
mod state_transaction;

pub(super) use aggregations::*;
pub(super) use decryption_retry_task::{CryptoDropHandles, spawn_crypto_tasks};
use matrix_sdk_base::{CallIntentConsensus, RoomInfo};

/// The outcome of [`TimelineController::should_send_receipt`].
pub(super) enum SendReceiptDecision {
    /// No read receipt should be sent.
    DoNotSend,

    /// A read receipt should be sent, targeting this event.
    ///
    /// This may differ from the event the caller asked about, since a read
    /// receipt should not point at one of the user's own events.
    SendTo(OwnedEventId),
}

/// Data associated to the current timeline focus.
///
/// This is the private counterpart of [`TimelineFocus`], and it is an augmented
/// version of it, including extra state that makes it useful over the lifetime
/// of a timeline.
#[derive(Debug)]
pub(in crate::timeline) enum TimelineFocusKind {
    /// The timeline receives live events from the sync.
    Live {
        /// Whether to hide in-thread events from the timeline.
        hide_threaded_events: bool,

        /// The cache holding all the events for this focus.
        event_cache: RoomEventCache,
    },

    /// The timeline is focused on a single event, and it can expand in one
    /// direction or another.
    Event {
        /// The focused event ID.
        focused_event_id: OwnedEventId,

        /// If the focused event is part or the root of a thread, what's the
        /// thread root?
        ///
        /// This is determined once when initializing the event-focused cache,
        /// and then it won't change for the duration of this timeline.
        thread_root: OnceLock<OwnedEventId>,

        /// The thread mode to use for this event-focused timeline, which is
        /// part of the key for the memoized event-focused cache.
        thread_mode: TimelineEventFocusThreadMode,

        /// The cache holding all the events for this focus.
        event_cache: EventFocusedCache,
    },

    /// A live timeline for a thread.
    Thread {
        /// The root event for the current thread.
        thread_id: OwnedEventId,

        /// The cache holding all the events for this focus.
        event_cache: ThreadEventCache,
    },

    PinnedEvents {
        /// The cache holding all the events for this focus.
        event_cache: PinnedEventsCache,
    },
}

impl TimelineFocusKind {
    /// Get the room ID of this timeline.
    pub(super) fn room_id(&self) -> &RoomId {
        match self {
            TimelineFocusKind::Live { event_cache, .. } => event_cache.room_id(),
            TimelineFocusKind::Thread { event_cache, .. } => event_cache.room_id(),
            TimelineFocusKind::Event { event_cache, .. } => event_cache.room_id(),
            TimelineFocusKind::PinnedEvents { event_cache } => event_cache.room_id(),
        }
    }
    /// Returns the [`ReceiptThread`] that should be used for the current
    /// timeline focus.
    ///
    /// Live and event timelines will use the unthreaded read receipt type in
    /// general, unless they hide in-thread events, in which case they will use
    /// the main thread.
    pub(super) fn receipt_thread(&self) -> ReceiptThread {
        if let Some(thread_root) = self.thread_root() {
            ReceiptThread::Thread(thread_root.to_owned())
        } else if self.hide_threaded_events() {
            ReceiptThread::Main
        } else {
            ReceiptThread::Unthreaded
        }
    }

    /// Whether to hide in-thread events from the timeline.
    fn hide_threaded_events(&self) -> bool {
        match self {
            TimelineFocusKind::Live { hide_threaded_events, .. } => *hide_threaded_events,
            TimelineFocusKind::Event { thread_mode, .. } => {
                matches!(
                    thread_mode,
                    TimelineEventFocusThreadMode::Automatic { hide_threaded_events: true }
                )
            }
            TimelineFocusKind::Thread { .. } | TimelineFocusKind::PinnedEvents { .. } => false,
        }
    }

    /// Whether the focus is on a thread (from a live thread or a thread
    /// permalink).
    fn is_thread(&self) -> bool {
        self.thread_root().is_some()
    }

    /// If the focus is a thread or event-focused, returns its thread root event
    /// ID if any.
    fn thread_root(&self) -> Option<&EventId> {
        match self {
            TimelineFocusKind::Event { thread_root, .. } => thread_root.get().map(|v| &**v),
            TimelineFocusKind::Live { .. } | TimelineFocusKind::PinnedEvents { .. } => None,
            TimelineFocusKind::Thread { thread_id, .. } => Some(thread_id),
        }
    }
}

#[derive(Clone, Debug)]
pub(super) struct TimelineController<P: RoomDataProvider = Room> {
    /// Inner mutable state.
    state: Arc<RwLock<TimelineState<P>>>,

    /// Focus data.
    focus: Arc<TimelineFocusKind>,

    /// A [`RoomDataProvider`] implementation, providing data.
    ///
    /// The type is a `RoomDataProvider` to allow testing. In the real world,
    /// this would normally be a [`Room`].
    pub(crate) room_data_provider: P,

    /// Settings applied to this timeline.
    pub(super) settings: TimelineSettings,
}

#[derive(Clone)]
pub(super) struct TimelineSettings {
    /// Should the read receipts and read markers be handled and on which event
    /// types?
    pub(super) track_read_receipts: TimelineReadReceiptTracking,

    /// Event filter that controls what's rendered as a timeline item (and thus
    /// what can carry read receipts).
    pub(super) event_filter: Arc<TimelineEventFilterFn>,

    /// Are unparsable events added as timeline items of their own kind?
    pub(super) add_failed_to_parse: bool,

    /// Should the timeline items be grouped by day or month?
    pub(super) date_divider_mode: DateDividerMode,
}

#[cfg(not(tarpaulin_include))]
impl fmt::Debug for TimelineSettings {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TimelineSettings")
            .field("track_read_receipts", &self.track_read_receipts)
            .field("add_failed_to_parse", &self.add_failed_to_parse)
            .finish_non_exhaustive()
    }
}

impl Default for TimelineSettings {
    fn default() -> Self {
        Self {
            track_read_receipts: TimelineReadReceiptTracking::Disabled,
            event_filter: Arc::new(default_event_filter),
            add_failed_to_parse: true,
            date_divider_mode: DateDividerMode::Daily,
        }
    }
}

/// The default event filter for
/// [`crate::timeline::TimelineBuilder::event_filter`].
///
/// It filters out events that are not rendered by the timeline, including but
/// not limited to: reactions, edits, redactions on existing messages.
///
/// If you have a custom filter, it may be best to chain yours with this one if
/// you do not want to run into situations where a read receipt is not visible
/// because it's living on an event that doesn't have a matching timeline item.
pub fn default_event_filter(event: &AnySyncTimelineEvent, rules: &RoomVersionRules) -> bool {
    match event {
        AnySyncTimelineEvent::MessageLike(AnySyncMessageLikeEvent::RoomRedaction(ev)) => {
            if ev.redacts(&rules.redaction).is_some() {
                // This is a redaction of an existing message, we'll only update
                // the previous message and not render a new entry.
                false
            } else {
                // This is a redacted entry, that we'll show only if the
                // redacted entity wasn't a reaction.
                ev.event_type() != MessageLikeEventType::Reaction
            }
        }

        AnySyncTimelineEvent::MessageLike(msg) => {
            match msg.original_content() {
                None => {
                    // This is a redacted entry, that we'll show only if the
                    // redacted entity wasn't a reaction.
                    msg.event_type() != MessageLikeEventType::Reaction
                }

                Some(original_content) => {
                    match original_content {
                        AnyMessageLikeEventContent::RoomMessage(content) => {
                            if content
                                .relates_to
                                .as_ref()
                                .is_some_and(|rel| matches!(rel, Relation::Replacement(_)))
                            {
                                // Edits aren't visible by default.
                                return false;
                            }

                            match content.msgtype {
                                MessageType::Audio(_)
                                | MessageType::Emote(_)
                                | MessageType::File(_)
                                | MessageType::Image(_)
                                | MessageType::Location(_)
                                | MessageType::Notice(_)
                                | MessageType::ServerNotice(_)
                                | MessageType::Text(_)
                                | MessageType::Video(_)
                                | MessageType::VerificationRequest(_) => true,
                                #[cfg(feature = "unstable-msc4274")]
                                MessageType::Gallery(_) => true,
                                _ => false,
                            }
                        }

                        AnyMessageLikeEventContent::Sticker(_)
                        | AnyMessageLikeEventContent::UnstablePollStart(
                            UnstablePollStartEventContent::New(_),
                        )
                        | AnyMessageLikeEventContent::CallInvite(_)
                        | AnyMessageLikeEventContent::RtcNotification(_)
                        | AnyMessageLikeEventContent::RoomEncrypted(_) => true,

                        // Beacon location-update events are aggregated onto
                        // their parent `beacon_info` state event's timeline
                        // item. They are never rendered as standalone items.
                        AnyMessageLikeEventContent::Beacon(_) => false,
                        // Ignore decline events, the matching RtcNotification
                        // event will be updated to reflect the decline.
                        AnyMessageLikeEventContent::RtcDecline(_) => false,

                        _ => false,
                    }
                }
            }
        }

        AnySyncTimelineEvent::State(_) => {
            // All the state events may get displayed by default.
            true
        }
    }
}

/// Result of calling [`TimelineController::init_focus`].
pub(super) struct InitFocusResult {
    /// Did the initialization result in having some events in the timeline?
    pub has_events: bool,
    /// If the timeline is a non-live timeline, an extra task that subscribes to
    /// changes to the focus source.
    pub focus_task: Option<BackgroundTaskHandle>,
}

/// Holds the various info about the current call
#[derive(Clone, Debug, PartialEq)]
pub struct ActiveCallInfo {
    /// The list of users in the call
    pub active_members: HashSet<OwnedUserId>,
    /// The consensus intent of the call, audio/video
    pub call_intent: CallIntentConsensus,
    /// True if the user (with any device) is currently in the call, meaning
    /// they have joined and haven't left yet.
    pub is_joined: bool,
    /// The timestamp of when the call started, in milliseconds since the unix
    /// epoch. Currently, this is the origin_server_ts of the rtc.notification
    /// event.
    pub call_started_ts_millis: Option<MilliSecondsSinceUnixEpoch>,
}

impl ActiveCallInfo {
    pub(crate) fn from_info(room_info: RoomInfo, owned_user_id: OwnedUserId) -> Option<Self> {
        if room_info.has_active_room_call() {
            Some(ActiveCallInfo {
                active_members: HashSet::from(room_info.active_room_call_participants()),
                call_intent: room_info.active_room_call_consensus_intent(),
                is_joined: room_info.active_room_call_participants().contains(&owned_user_id),
                call_started_ts_millis: None,
            })
        } else {
            None
        }
    }

    pub(crate) fn with_start_time(self, timestamp: Option<MilliSecondsSinceUnixEpoch>) -> Self {
        Self { call_started_ts_millis: timestamp, ..self }
    }
}

impl<P: RoomDataProvider> TimelineController<P> {
    pub(super) async fn new(
        room_data_provider: P,
        focus: &TimelineFocus,
        event_cache: &EventCache,
        internal_id_prefix: Option<String>,
        unable_to_decrypt_hook: Option<Arc<UtdHookManager>>,
        is_room_encrypted: bool,
        settings: TimelineSettings,
    ) -> Result<Self, Error> {
        let room_id = room_data_provider.room_id();

        let focus = match focus {
            TimelineFocus::Live { hide_threaded_events } => TimelineFocusKind::Live {
                hide_threaded_events: *hide_threaded_events,
                event_cache: event_cache.room(room_id).await?.0,
            },

            TimelineFocus::Event { target, thread_mode, num_context_events, .. } => {
                TimelineFocusKind::Event {
                    event_cache: event_cache
                        .event_focused(room_id, target, (*thread_mode).into(), *num_context_events)
                        .await?
                        .0,
                    focused_event_id: target.clone(),
                    // This will be initialised in `Self::init_focus`.
                    thread_root: OnceLock::new(),
                    thread_mode: *thread_mode,
                }
            }

            TimelineFocus::Thread { thread_id, .. } => TimelineFocusKind::Thread {
                event_cache: event_cache.thread(room_id, thread_id).await?.0,
                thread_id: thread_id.clone(),
            },

            TimelineFocus::PinnedEvents => TimelineFocusKind::PinnedEvents {
                event_cache: event_cache.pinned_events(room_id).await?.0,
            },
        };

        let focus = Arc::new(focus);
        let state = Arc::new(RwLock::new(TimelineState::new(
            event_cache.clone(),
            focus.clone(),
            room_data_provider.own_user_id().to_owned(),
            room_data_provider.room_version_rules(),
            internal_id_prefix,
            unable_to_decrypt_hook,
            is_room_encrypted,
            None,
        )));

        Ok(Self { state, focus, room_data_provider, settings })
    }

    /// Listens to encryption state changes for the room in
    /// [`matrix_sdk_base::RoomInfo`] and applies the new value to the existing
    /// timeline items. This will then cause a refresh of those timeline items.
    pub async fn handle_encryption_state_changes(&self) {
        let mut room_info = self.room_data_provider.room_info();

        // Small function helper to help mark as encrypted.
        let mark_encrypted = || async {
            let mut state = self.state.write().await;
            state.meta.is_room_encrypted = true;
            state.mark_all_events_as_encrypted();
        };

        if room_info.get().encryption_state().is_encrypted() {
            // If the room was already encrypted, it won't toggle to
            // unencrypted, so we can shut down this task early.
            mark_encrypted().await;
            return;
        }

        while let Some(info) = room_info.next().await {
            if info.encryption_state().is_encrypted() {
                mark_encrypted().await;
                // Once the room is encrypted, it cannot switch back to
                // unencrypted, so our work here is done.
                break;
            }
        }
    }

    /// Run a lazy backwards pagination (in live mode).
    ///
    /// It adjusts the `count` value of the `Skip` higher-order stream so that
    /// more items are pushed front in the timeline.
    ///
    /// If no more items are available (i.e. if the `count` is zero), this
    /// method returns `Some(needs)` where `needs` is the number of events that
    /// must be unlazily backwards paginated.
    pub(super) async fn live_lazy_paginate_backwards(&self, num_events: u16) -> Option<usize> {
        let state = self.state.read().await;

        let (count, needs) = state
            .meta
            .subscriber_skip_count
            .compute_next_when_paginating_backwards(num_events.into());

        // This always happens on a live timeline.
        let is_live_timeline = true;
        state.meta.subscriber_skip_count.update(count, is_live_timeline);

        needs
    }

    /// Is this timeline receiving events from sync (aka has a live focus)?
    pub(super) fn is_live(&self) -> bool {
        matches!(&*self.focus, TimelineFocusKind::Live { .. })
    }

    /// Is this timeline focused on a thread?
    pub(super) fn is_threaded(&self) -> bool {
        self.focus.is_thread()
    }

    /// The root of the current thread, for a live thread timeline or a
    /// permalink to a thread message.
    pub(super) fn thread_root(&self) -> Option<OwnedEventId> {
        self.focus.thread_root().map(ToOwned::to_owned)
    }

    /// Get a copy of the current items in the list.
    ///
    /// Cheap because `im::Vector` is cheap to clone.
    pub(super) async fn items(&self) -> Vector<Arc<TimelineItem>> {
        self.state.read().await.items.clone_items()
    }

    #[cfg(test)]
    pub(super) async fn subscribe_raw(
        &self,
    ) -> (Vector<Arc<TimelineItem>>, VectorSubscriberStream<Arc<TimelineItem>>) {
        self.state.read().await.items.subscribe().into_values_and_stream()
    }

    pub(super) async fn subscribe(&self) -> (Vector<Arc<TimelineItem>>, TimelineSubscriber) {
        let state = self.state.read().await;

        TimelineSubscriber::new(&state.items, &state.meta.subscriber_skip_count)
    }

    pub(super) async fn subscribe_filter_map<U, F>(
        &self,
        f: F,
    ) -> (Vector<U>, FilterMap<VectorSubscriberStream<Arc<TimelineItem>>, F>)
    where
        U: Clone,
        F: Fn(Arc<TimelineItem>) -> Option<U>,
    {
        self.state.read().await.items.subscribe().filter_map(f)
    }

    /// Toggle a reaction locally.
    ///
    /// Returns true if the reaction was added, false if it was removed.
    #[instrument(skip_all)]
    pub(super) async fn toggle_reaction_local(
        &self,
        item_id: &TimelineEventItemId,
        key: &str,
        extra_content: Option<serde_json::Map<String, serde_json::Value>>,
    ) -> Result<bool, Error> {
        let mut state = self.state.write().await;

        let Some((item_pos, item)) = rfind_event_by_item_id(&state.items, item_id) else {
            warn!("Timeline item not found, can't add reaction");
            return Err(Error::FailedToToggleReaction);
        };

        let user_id = self.room_data_provider.own_user_id();
        let target = item.identifier();

        // The item says whether we reacted; the registry has the handle and
        // event id.
        let has_reaction =
            item.reactions().get(key).is_some_and(|by_user| by_user.contains_key(user_id));
        let previous = has_reaction
            .then(|| state.meta.aggregations.find_reaction(&target, key, user_id).cloned())
            .flatten();

        if has_reaction && previous.is_none() {
            warn!("reaction is on the item but unknown to the aggregations");
            return Ok(false);
        }

        let Some(previous) = previous else {
            // Adding the new reaction.
            match item.handle() {
                TimelineItemHandle::Local(send_handle) => {
                    if send_handle
                        .react(key.to_owned())
                        .await
                        .map_err(|err| Error::SendQueueError(err.into()))?
                        .is_some()
                    {
                        trace!("adding a reaction to a local echo");
                        return Ok(true);
                    }

                    warn!("couldn't toggle reaction for local echo");
                    return Ok(false);
                }

                TimelineItemHandle::Remote(event_id) => {
                    // Add a reaction through the room data provider. No need to
                    // reflect the effect locally, since the local echo handling
                    // will take care of it.
                    trace!("adding a reaction to a remote echo");
                    let annotation = Annotation::new(event_id.to_owned(), key.to_owned());
                    self.room_data_provider
                        .send(ReactionEventContent::from(annotation).into(), extra_content)
                        .await?;
                    return Ok(true);
                }
            }
        };

        trace!("removing a previous reaction");

        if previous.is_local() {
            // Aborting the local echo is enough: its discard removes the
            // reaction.
            if let Some(handle) = &previous.send_handle {
                if !handle.abort().await.map_err(|err| Error::SendQueueError(err.into()))? {
                    // Impossible state: the reaction has moved from local to
                    // echo under our feet, but the timeline was supposed to be
                    // locked!
                    warn!("unexpectedly unable to abort sending of local reaction");
                }
            } else {
                warn!("no send handle (this should only happen in testing contexts)");
            }
            return Ok(false);
        }

        let TimelineEventItemId::EventId(event_id) = previous.own_id else {
            warn!("sent reaction without an event id");
            return Ok(false);
        };

        // Assume the redaction will work; we'll re-add the reaction if it
        // didn't.
        let Some(annotated_event_id) =
            item.as_remote().map(|event_item| event_item.event_id.clone())
        else {
            warn!("remote reaction to remote event, but the associated item isn't remote");
            return Ok(false);
        };

        let mut reactions = item.reactions().clone();
        let reaction_info = reactions.remove_reaction(user_id, key);

        if reaction_info.is_some() {
            let new_item = item.with_reactions(reactions);
            state.items.replace(item_pos, new_item);
        } else {
            warn!(
                "reaction is missing on the item, not removing it locally, \
                 but sending redaction."
            );
        }

        // Release the lock before running the request.
        drop(state);

        trace!("sending redact for a previous reaction");
        if let Err(err) = self.room_data_provider.redact(&event_id, None, None).await {
            if let Some(reaction_info) = reaction_info {
                debug!("sending redact failed, adding the reaction back to the list");

                let mut state = self.state.write().await;
                if let Some((item_pos, item)) = rfind_event_by_id(&state.items, &annotated_event_id)
                {
                    // Re-add the reaction to the mapping.
                    let mut reactions = item.reactions().clone();
                    reactions
                        .entry(key.to_owned())
                        .or_default()
                        .insert(user_id.to_owned(), reaction_info);
                    let new_item = item.with_reactions(reactions);
                    state.items.replace(item_pos, new_item);
                } else {
                    warn!(
                        "couldn't find item to re-add reaction anymore; \
                         maybe it's been redacted?"
                    );
                }
            }

            return Err(err);
        }

        Ok(false)
    }

    /// The handle for a pending send on an item, see [`SendTarget`].
    ///
    /// `None` if there's nothing of that kind left to act on, which a caller
    /// racing the remote echo can legitimately run into.
    pub(super) async fn pending_send_handle(
        &self,
        item_id: &TimelineEventItemId,
        target: SendTarget,
    ) -> Result<Option<SendHandle>, Error> {
        let state = self.state.read().await;

        let Some((_, item)) = rfind_event_by_item_id(&state.items, item_id) else {
            return Err(Error::EventNotInTimeline(item_id.clone()));
        };

        let own_user_id = self.room_data_provider.own_user_id();
        let target_id = item.identifier();
        let aggregations = &state.meta.aggregations;

        let handle = match &target {
            SendTarget::Event => item.local_echo_send_handle(),
            SendTarget::Edit => aggregations
                .pending_send_handle(&target_id, |kind| matches!(kind, AggregationKind::Edit(_))),
            SendTarget::Redaction => aggregations
                .pending_send_handle(&target_id, |kind| matches!(kind, AggregationKind::Redaction)),
            SendTarget::Reaction { key } => {
                aggregations.pending_send_handle(&target_id, |kind| {
                    matches!(kind, AggregationKind::Reaction { key: k, sender, .. } if k == key && sender == own_user_id)
                })
            }
        };

        Ok(handle)
    }

    /// Handle updates on events as [`VectorDiff`]s.
    pub(super) async fn handle_remote_events_with_diffs(
        &self,
        diffs: Vec<VectorDiff<TimelineEvent>>,
        origin: RemoteEventOrigin,
    ) {
        if diffs.is_empty() {
            return;
        }

        let mut state = self.state.write().await;
        state
            .handle_remote_events_with_diffs(
                diffs,
                origin,
                &self.room_data_provider,
                &self.settings,
            )
            .await
    }

    /// Only handle aggregations received as [`VectorDiff`]s.
    pub(super) async fn handle_remote_aggregations(
        &self,
        diffs: Vec<VectorDiff<TimelineEvent>>,
        origin: RemoteEventOrigin,
    ) {
        if diffs.is_empty() {
            return;
        }

        let mut state = self.state.write().await;
        state
            .handle_remote_aggregations(diffs, origin, &self.room_data_provider, &self.settings)
            .await
    }

    /// Handle an update of the thread summary of a single event that is a
    /// thread root.
    pub(super) async fn handle_thread_summary(
        &self,
        thread_root: OwnedEventId,
        thread_summary: ThreadSummary,
    ) {
        let mut state = self.state.write().await;
        state.handle_thread_summary(thread_root, thread_summary, &self.room_data_provider).await
    }

    pub(super) async fn clear(&self) {
        self.state.write().await.clear();
    }

    /// Replaces the content of the current timeline with initial events.
    ///
    /// Also sets up read receipts and the read marker for a live timeline of a
    /// room.
    ///
    /// This is all done with a single lock guard, since we don't want the state
    /// to be modified between the clear and re-insertion of new events.
    pub(super) async fn replace_with_initial_remote_events<Events>(
        &self,
        events: Events,
        origin: RemoteEventOrigin,
    ) where
        Events: IntoIterator,
        <Events as IntoIterator>::Item: Into<TimelineEvent>,
    {
        let mut state = self.state.write().await;

        let track_read_markers = &self.settings.track_read_receipts;
        if track_read_markers.is_enabled() {
            state.populate_initial_user_receipt(&self.room_data_provider, ReceiptType::Read).await;
            state
                .populate_initial_user_receipt(&self.room_data_provider, ReceiptType::ReadPrivate)
                .await;
        }

        // Replace the events if either the current event list or the new one
        // aren't empty. Previously we just had to check the new one wasn't
        // empty because we did a clear operation before so the current one
        // would always be empty, but now we may want to replace a populated
        // timeline with an empty one.
        let mut events = events.into_iter().peekable();
        if !state.items.is_empty() || events.peek().is_some() {
            state
                .replace_with_remote_events(
                    events,
                    origin,
                    &self.room_data_provider,
                    &self.settings,
                )
                .await;
        }

        if track_read_markers.is_enabled() {
            if let Some(fully_read_event_id) =
                self.room_data_provider.load_fully_read_marker().await
            {
                state.handle_fully_read_marker(fully_read_event_id);
            } else if let Some(latest_receipt_event_id) = state
                .latest_user_read_receipt_timeline_event_id(self.room_data_provider.own_user_id())
            {
                // Fall back to read receipt if no fully read marker exists.
                debug!("no `m.fully_read` marker found, falling back to read receipt");
                state.handle_fully_read_marker(latest_receipt_event_id);
            }
        }
    }

    pub(super) async fn handle_fully_read_marker(&self, fully_read_event_id: OwnedEventId) {
        self.state.write().await.handle_fully_read_marker(fully_read_event_id);
    }

    pub(super) async fn handle_active_call_update(
        &self,
        maybe_active_call: Option<ActiveCallInfo>,
    ) {
        let mut state = self.state.write().await;
        let mut txn = state.transaction();

        // Store the current active call info in metadata for new
        // RtcNotification items
        txn.meta.active_call = maybe_active_call.clone();

        if let Some(existing_event_id) = &txn.meta.active_rtc_notification_event_id {
            // Clean up the notification event
            let last_notification = rfind_event_by_id(&txn.items, existing_event_id);
            if let Some((last_idx, last_notification)) = last_notification {
                let updated_content = match last_notification.content() {
                    TimelineItemContent::RtcNotification {
                        call_intent,
                        declined_by,
                        active_call_info: _active_call_info,
                    } => Some(TimelineItemContent::RtcNotification {
                        call_intent: call_intent.to_owned(),
                        declined_by: declined_by.clone(),
                        active_call_info: maybe_active_call
                            .clone()
                            .map(|info| info.with_start_time(last_notification.timestamp.into())),
                    }),
                    _ => None,
                };
                if let Some(new_content) = updated_content {
                    let new_event_item = last_notification.inner.with_content(new_content);
                    let new_timeline_item =
                        TimelineItem::new(new_event_item, last_notification.internal_id.clone());
                    txn.items.replace(last_idx, new_timeline_item);
                }

                if maybe_active_call.is_none() {
                    // There is no active rtc_notification anymore
                    txn.meta.active_rtc_notification_event_id = None;
                }
            }
        }

        txn.commit();
    }

    pub(super) async fn handle_read_receipt_event(&self, event: ReceiptEventContent) {
        // Don't even take the lock if there are no events to process.
        if event.is_empty() {
            return;
        }

        let mut state = self.state.write().await;
        state.handle_read_receipt(event, &self.room_data_provider).await;
    }

    /// Creates the local echo for an event we're sending.
    #[instrument(skip_all)]
    pub(super) async fn handle_local_event(
        &self,
        txn_id: OwnedTransactionId,
        content: AnyMessageLikeEventContent,
        send_handle: Option<SendHandle>,
    ) {
        let sender = self.room_data_provider.own_user_id().to_owned();
        let profile = self.room_data_provider.profile_from_user_id(&sender).await;

        let date_divider_mode = self.settings.date_divider_mode.clone();

        let mut state = self.state.write().await;
        state
            .handle_local_event(sender, profile, date_divider_mode, txn_id, send_handle, content)
            .await;
    }

    /// Update the send state of a local event represented by a transaction ID.
    ///
    /// If the corresponding local timeline item is missing, a warning is
    /// raised.
    #[instrument(skip(self))]
    pub(super) async fn update_event_send_state(
        &self,
        txn_id: &TransactionId,
        send_state: EventSendState,
    ) {
        let mut state = self.state.write().await;
        let mut txn = state.transaction();

        let new_event_id: Option<&EventId> =
            as_variant!(&send_state, EventSendState::Sent { event_id } => event_id);

        // The local echoes are always at the end of the timeline, we must first
        // make sure the remote echo hasn't showed up yet.
        if rfind_event_item(&txn.items, |it| {
            new_event_id.is_some() && it.event_id() == new_event_id && it.as_remote().is_some()
        })
        .is_some()
        {
            // Remote echo already received. This is very unlikely.
            trace!("Remote echo received before send-event response");

            let local_echo = rfind_event_item(&txn.items, |it| it.transaction_id() == Some(txn_id));

            // If there's both the remote echo and a local echo, that means the
            // remote echo was received before the response _and_ contained no
            // transaction ID (and thus duplicated the local echo).
            if let Some((idx, _)) = local_echo {
                warn!("Message echo got duplicated, removing the local one");
                txn.items.remove(idx);

                // Adjust the date dividers, if needs be.
                let mut adjuster =
                    DateDividerAdjuster::new(self.settings.date_divider_mode.clone());
                adjuster.run(&mut txn.items, &mut txn.meta);
            }

            txn.commit();
            return;
        }

        // Look for the local event by the transaction ID or event ID.
        let result = rfind_event_item(&txn.items, |it| {
            it.transaction_id() == Some(txn_id)
                || new_event_id.is_some()
                    && it.event_id() == new_event_id
                    && it.as_local().is_some()
        });

        let Some((idx, item)) = result else {
            // Not a standalone item: maybe one of our aggregations.
            if txn.meta.aggregations.update_send_state(
                txn_id.to_owned(),
                send_state,
                &mut txn.items,
                &txn.meta.room_version_rules,
            ) {
                trace!("Updated the send state of an aggregation");
                txn.commit();
                return;
            }

            warn!("Timeline item not found, can't update send state");
            return;
        };

        let Some(local_item) = item.as_local() else {
            warn!("We looked for a local item, but it transitioned to remote.");
            return;
        };

        // The event was already marked as sent, that's a broken state, let's
        // emit an error but also override to the given sent state.
        if let EventSendState::Sent { event_id: existing_event_id } = &local_item.send_state {
            error!(?existing_event_id, ?new_event_id, "Local echo already marked as sent");
        }

        // If the event has just been marked as sent, update the aggregations
        // mapping to take that into account.
        if let Some(new_event_id) = new_event_id {
            txn.meta.aggregations.mark_target_as_sent(txn_id.to_owned(), new_event_id.to_owned());
        }

        let new_item = item.with_inner_kind(local_item.with_send_state(send_state));
        txn.items.replace(idx, new_item);

        txn.commit();
    }

    pub(super) async fn discard_local_echo(&self, txn_id: &TransactionId) -> bool {
        let mut state = self.state.write().await;

        if let Some((idx, _)) =
            rfind_event_item(&state.items, |it| it.transaction_id() == Some(txn_id))
        {
            let mut txn = state.transaction();

            txn.items.remove(idx);

            // A read marker or a date divider may have been inserted before the
            // local echo. Ensure both are up to date.
            let mut adjuster = DateDividerAdjuster::new(self.settings.date_divider_mode.clone());
            adjuster.run(&mut txn.items, &mut txn.meta);

            txn.meta.update_read_marker(&mut txn.items);

            txn.commit();

            debug!("discarded local echo");
            return true;
        }

        // Avoid multiple mutable and immutable borrows of the lock guard by
        // explicitly dereferencing it once.
        let mut txn = state.transaction();

        // Look if this was a local aggregation.
        let found_aggregation = match txn.meta.aggregations.try_remove_aggregation(
            &TimelineEventItemId::TransactionId(txn_id.to_owned()),
            &mut txn.items,
        ) {
            Ok(val) => val,
            Err(err) => {
                warn!("error when discarding local echo for an aggregation: {err}");
                // The aggregation has been found, it's just that we couldn't
                // discard it.
                true
            }
        };

        if found_aggregation {
            txn.commit();
        }

        found_aggregation
    }

    pub(super) async fn replace_local_echo(
        &self,
        txn_id: &TransactionId,
        content: AnyMessageLikeEventContent,
    ) -> bool {
        let AnyMessageLikeEventContent::RoomMessage(content) = content else {
            // Ideally, we'd support replacing local echoes for a reaction,
            // etc., but handling RoomMessage should be sufficient in most
            // cases. Worst case, the local echo will be sent Soon™ and we'll
            // get another chance at editing the event then.
            warn!("Replacing a local echo for a non-RoomMessage-like event NYI");
            return false;
        };

        let mut state = self.state.write().await;
        let mut txn = state.transaction();

        let Some((idx, prev_item)) =
            rfind_event_item(&txn.items, |it| it.transaction_id() == Some(txn_id))
        else {
            // Not a standalone item: maybe one of our pending edits.
            if let Some(Relation::Replacement(replacement)) = content.relates_to
                && txn.meta.aggregations.replace_local_edit(
                    txn_id,
                    replacement,
                    &mut txn.items,
                    &txn.meta.room_version_rules,
                )
            {
                debug!("Replaced local echo of an edit");
                txn.commit();
                return true;
            }

            debug!("Can't find local echo to replace");
            return false;
        };

        // Reuse the previous local echo's state, but reset the send state to
        // not sent (per API contract).
        let ti_kind = {
            let Some(prev_local_item) = prev_item.as_local() else {
                warn!("We looked for a local item, but it transitioned as remote??");
                return false;
            };
            // If the local echo had an upload progress, retain it.
            let progress = as_variant!(&prev_local_item.send_state,
                EventSendState::NotSentYet { progress } => progress.clone())
            .flatten();
            prev_local_item.with_send_state(EventSendState::NotSentYet { progress })
        };

        // Replace the local-related state (kind) and the content state.
        let new_item = TimelineItem::new(
            prev_item.with_kind(ti_kind).with_content(TimelineItemContent::message(
                content.msgtype,
                content.mentions,
                prev_item.content().thread_root(),
                prev_item.content().in_reply_to(),
                prev_item.content().thread_summary(),
            )),
            prev_item.internal_id.to_owned(),
        );

        txn.items.replace(idx, new_item);

        // This doesn't change the original sending time, so there's no need to
        // adjust date dividers.

        txn.commit();

        debug!("Replaced local echo");
        true
    }

    pub(super) async fn compute_redecryption_candidates(
        &self,
    ) -> (BTreeSet<String>, BTreeSet<String>) {
        let state = self.state.read().await;
        compute_redecryption_candidates(&state.items)
    }

    pub(super) async fn set_sender_profiles_pending(&self) {
        self.set_non_ready_sender_profiles(TimelineDetails::Pending).await;
    }

    pub(super) async fn set_sender_profiles_error(&self, error: Arc<matrix_sdk::Error>) {
        self.set_non_ready_sender_profiles(TimelineDetails::Error(error)).await;
    }

    async fn set_non_ready_sender_profiles(&self, profile_state: TimelineDetails<Profile>) {
        self.state.write().await.items.for_each(|mut entry| {
            let Some(event_item) = entry.as_event() else { return };
            if !matches!(event_item.sender_profile(), TimelineDetails::Ready(_)) {
                let new_item = entry.with_kind(TimelineItemKind::Event(
                    event_item.with_sender_profile(profile_state.clone()),
                ));
                ObservableItemsEntry::replace(&mut entry, new_item);
            }
        });
    }

    pub(super) async fn update_missing_sender_profiles(&self) {
        trace!("Updating missing sender profiles");

        let mut state = self.state.write().await;
        let mut entries = state.items.entries();
        while let Some(mut entry) = entries.next() {
            let Some(event_item) = entry.as_event() else { continue };
            let event_id = event_item.event_id().map(debug);
            let transaction_id = event_item.transaction_id().map(debug);

            if event_item.sender_profile().is_ready() {
                trace!(event_id, transaction_id, "Profile already set");
                continue;
            }

            match self.room_data_provider.profile_from_user_id(event_item.sender()).await {
                Some(profile) => {
                    trace!(event_id, transaction_id, "Adding profile");
                    let updated_item =
                        event_item.with_sender_profile(TimelineDetails::Ready(profile));
                    let new_item = entry.with_kind(updated_item);
                    ObservableItemsEntry::replace(&mut entry, new_item);
                }
                None => {
                    if !event_item.sender_profile().is_unavailable() {
                        trace!(event_id, transaction_id, "Marking profile unavailable");
                        let updated_item =
                            event_item.with_sender_profile(TimelineDetails::Unavailable);
                        let new_item = entry.with_kind(updated_item);
                        ObservableItemsEntry::replace(&mut entry, new_item);
                    } else {
                        debug!(event_id, transaction_id, "Profile already marked unavailable");
                    }
                }
            }
        }

        trace!("Done updating missing sender profiles");
    }

    /// Update the profiles of the given senders, even if they are ready.
    pub(super) async fn force_update_sender_profiles(&self, sender_ids: &BTreeSet<&UserId>) {
        trace!("Forcing update of sender profiles: {sender_ids:?}");

        let mut state = self.state.write().await;
        let mut entries = state.items.entries();
        while let Some(mut entry) = entries.next() {
            let Some(event_item) = entry.as_event() else { continue };
            if !sender_ids.contains(event_item.sender()) {
                continue;
            }

            let event_id = event_item.event_id().map(debug);
            let transaction_id = event_item.transaction_id().map(debug);

            match self.room_data_provider.profile_from_user_id(event_item.sender()).await {
                Some(profile) => {
                    if matches!(event_item.sender_profile(), TimelineDetails::Ready(old_profile) if *old_profile == profile)
                    {
                        debug!(event_id, transaction_id, "Profile already up-to-date");
                    } else {
                        trace!(event_id, transaction_id, "Updating profile");
                        let updated_item =
                            event_item.with_sender_profile(TimelineDetails::Ready(profile));
                        let new_item = entry.with_kind(updated_item);
                        ObservableItemsEntry::replace(&mut entry, new_item);
                    }
                }
                None => {
                    if !event_item.sender_profile().is_unavailable() {
                        trace!(event_id, transaction_id, "Marking profile unavailable");
                        let updated_item =
                            event_item.with_sender_profile(TimelineDetails::Unavailable);
                        let new_item = entry.with_kind(updated_item);
                        ObservableItemsEntry::replace(&mut entry, new_item);
                    } else {
                        debug!(event_id, transaction_id, "Profile already marked unavailable");
                    }
                }
            }
        }

        trace!("Done forcing update of sender profiles");
    }

    #[cfg(test)]
    pub(super) async fn handle_read_receipts(&self, receipt_event_content: ReceiptEventContent) {
        let own_user_id = self.room_data_provider.own_user_id();
        self.state.write().await.handle_read_receipts(receipt_event_content, own_user_id);
    }

    /// Get the latest read receipt for the given user.
    ///
    /// Useful to get the latest read receipt, whether it's private or public.
    pub(super) async fn latest_user_read_receipt(
        &self,
        user_id: &UserId,
    ) -> Option<(OwnedEventId, Receipt)> {
        let receipt_thread = self.focus.receipt_thread();

        self.state
            .read()
            .await
            .latest_user_read_receipt(
                user_id,
                receipt_thread,
                &self.room_data_provider,
                read_receipts::ImplicitReadReceipts::Include,
            )
            .await
    }

    /// Get the ID of the timeline event with the latest read receipt for the
    /// given user.
    pub(super) async fn latest_user_read_receipt_timeline_event_id(
        &self,
        user_id: &UserId,
    ) -> Option<OwnedEventId> {
        self.state.read().await.latest_user_read_receipt_timeline_event_id(user_id)
    }

    /// Subscribe to changes in the read receipts of our own user.
    pub async fn subscribe_own_user_read_receipts_changed(
        &self,
    ) -> impl Stream<Item = ()> + use<P> {
        self.state.read().await.meta.read_receipts.subscribe_own_user_read_receipts_changed()
    }

    /// Handle a room send update that's a new local echo.
    pub(crate) async fn handle_local_echo(&self, echo: LocalEcho) {
        match echo.content {
            LocalEchoContent::Event { serialized_event, send_handle, send_error } => {
                let content = match serialized_event.deserialize() {
                    Ok(d) => d,
                    Err(err) => {
                        warn!("error deserializing local echo: {err}");
                        return;
                    }
                };

                self.handle_local_event(echo.transaction_id.clone(), content, Some(send_handle))
                    .await;

                if let Some(send_error) = send_error {
                    self.update_event_send_state(
                        &echo.transaction_id,
                        EventSendState::SendingFailed {
                            error: Arc::new(matrix_sdk::Error::SendQueueWedgeError(Box::new(
                                send_error,
                            ))),
                            is_recoverable: false,
                        },
                    )
                    .await;
                }
            }

            LocalEchoContent::React { key, send_handle, applies_to } => {
                self.handle_local_reaction(key, send_handle, applies_to).await;
            }

            LocalEchoContent::Redaction { redacts, send_handle, send_error, .. } => {
                self.handle_local_redaction(
                    echo.transaction_id.clone(),
                    redacts,
                    Some(send_handle),
                )
                .await;

                if let Some(send_error) = send_error {
                    self.update_event_send_state(
                        &echo.transaction_id,
                        EventSendState::SendingFailed {
                            error: Arc::new(matrix_sdk::Error::SendQueueWedgeError(Box::new(
                                send_error,
                            ))),
                            is_recoverable: false,
                        },
                    )
                    .await;
                }
            }
        }
    }

    /// Adds a reaction (local echo) to a local echo.
    #[instrument(skip(self, send_handle))]
    async fn handle_local_reaction(
        &self,
        reaction_key: String,
        send_handle: SendHandle,
        applies_to: OwnedTransactionId,
    ) {
        let mut state = self.state.write().await;
        let mut tr = state.transaction();

        let target = TimelineEventItemId::TransactionId(applies_to);

        let reaction_txn_id = send_handle.transaction_id().to_owned();
        let aggregation = Aggregation::new_local(
            TimelineEventItemId::TransactionId(reaction_txn_id),
            AggregationKind::Reaction {
                key: reaction_key.clone(),
                sender: self.room_data_provider.own_user_id().to_owned(),
                timestamp: MilliSecondsSinceUnixEpoch::now(),
            },
            Some(send_handle),
        );

        tr.meta.aggregations.add(target.clone(), aggregation.clone());
        find_item_and_apply_aggregation(
            &tr.meta.aggregations,
            &mut tr.items,
            &target,
            aggregation,
            &tr.meta.room_version_rules,
        );

        tr.commit();
    }

    /// Applies a local echo of a redaction.
    pub(super) async fn handle_local_redaction(
        &self,
        txn_id: OwnedTransactionId,
        redacts: OwnedEventId,
        send_handle: Option<SendHandle>,
    ) {
        let mut state = self.state.write().await;
        let mut tr = state.transaction();

        let target = TimelineEventItemId::EventId(redacts);

        let aggregation = Aggregation::new_local(
            TimelineEventItemId::TransactionId(txn_id),
            AggregationKind::Redaction,
            send_handle,
        );

        tr.meta.aggregations.add(target.clone(), aggregation.clone());
        find_item_and_apply_aggregation(
            &tr.meta.aggregations,
            &mut tr.items,
            &target,
            aggregation,
            &tr.meta.room_version_rules,
        );

        tr.commit();
    }

    /// Handle a single room send queue update.
    pub(crate) async fn handle_room_send_queue_update(&self, update: RoomSendQueueUpdate) {
        match update {
            RoomSendQueueUpdate::NewLocalEvent(echo) => {
                self.handle_local_echo(echo).await;
            }

            RoomSendQueueUpdate::CancelledLocalEvent { transaction_id } => {
                if !self.discard_local_echo(&transaction_id).await {
                    warn!("couldn't find the local echo to discard");
                }
            }

            RoomSendQueueUpdate::ReplacedLocalEvent { transaction_id, new_content } => {
                let content = match new_content.deserialize() {
                    Ok(d) => d,
                    Err(err) => {
                        warn!("error deserializing local echo (upon edit): {err}");
                        return;
                    }
                };

                if !self.replace_local_echo(&transaction_id, content).await {
                    warn!("couldn't find the local echo to replace");
                }
            }

            RoomSendQueueUpdate::SendError { transaction_id, error, is_recoverable } => {
                self.update_event_send_state(
                    &transaction_id,
                    EventSendState::SendingFailed { error, is_recoverable },
                )
                .await;
            }

            RoomSendQueueUpdate::RetryEvent { transaction_id } => {
                self.update_event_send_state(
                    &transaction_id,
                    EventSendState::NotSentYet { progress: None },
                )
                .await;
            }

            RoomSendQueueUpdate::SentEvent { transaction_id, event_id } => {
                self.update_event_send_state(&transaction_id, EventSendState::Sent { event_id })
                    .await;
            }

            RoomSendQueueUpdate::MediaUpload { related_to, index, progress, .. } => {
                self.update_event_send_state(
                    &related_to,
                    EventSendState::NotSentYet {
                        progress: Some(MediaUploadProgress { index, progress }),
                    },
                )
                .await;
            }
        }
    }

    /// Insert a timeline start item at the beginning of the room, if it's
    /// missing.
    pub async fn insert_timeline_start_if_missing(&self) {
        let mut state = self.state.write().await;
        let mut txn = state.transaction();
        txn.items.push_timeline_start_if_missing(
            txn.meta.new_timeline_item(VirtualTimelineItem::TimelineStart),
        );
        txn.commit();
    }

    /// Create a [`EmbeddedEvent`] from an arbitrary event, be it in the
    /// timeline or not.
    ///
    /// Can be `None` if the event cannot be represented as a standalone item,
    /// because it's an aggregation.
    pub(super) async fn make_replied_to(
        &self,
        event: TimelineEvent,
    ) -> Result<Option<EmbeddedEvent>, Error> {
        let state = self.state.read().await;
        EmbeddedEvent::try_from_timeline_event(event, &self.room_data_provider, &state.meta).await
    }
}

impl TimelineController {
    pub(super) fn room(&self) -> &Room {
        &self.room_data_provider
    }

    /// Initializes the configured timeline focus with appropriate data.
    ///
    /// Should be called only once after creation of the [`TimelineController`],
    /// with all its fields set.
    pub(super) async fn init_focus(&self) -> Result<InitFocusResult, Error> {
        match self.focus.deref() {
            TimelineFocusKind::Live { event_cache, .. } => {
                // Retrieve the cached events, and add them to the timeline.
                let events = event_cache.events().await?;

                let has_events = !events.is_empty();

                self.replace_with_initial_remote_events(events, RemoteEventOrigin::Cache).await;

                match event_cache.pagination().status().get() {
                    PaginationStatus::Idle { hit_timeline_start } => {
                        if hit_timeline_start {
                            // Eagerly insert the timeline start item, since
                            // pagination claims we've already hit the timeline
                            // start.
                            self.insert_timeline_start_if_missing().await;
                        }
                    }
                    PaginationStatus::Paginating => {}
                }

                Ok(InitFocusResult { has_events, focus_task: None })
            }

            TimelineFocusKind::Event {
                focused_event_id: event_id,
                thread_mode,
                thread_root: focus_thread_root,
                event_cache,
                ..
            } => {
                let (events, receiver) = event_cache.subscribe().await?;

                let has_events = !events.is_empty();

                // Ask the cache for the thread root, if it managed to extract
                // one or decided that the target event was the thread root.
                if let Some(thread_root) = event_cache.thread_root().await? {
                    focus_thread_root.get_or_init(|| thread_root);
                }

                self.replace_with_initial_remote_events(events, RemoteEventOrigin::Pagination)
                    .await;

                let task = self
                    .room_data_provider
                    .client()
                    .task_monitor()
                    .spawn_infinite_task(
                        "timeline::event_focused_cache_updates",
                        event_focused_task(
                            event_id.clone(),
                            (*thread_mode).into(),
                            event_cache.clone(),
                            self.clone(),
                            receiver,
                        ),
                    )
                    .abort_on_drop();

                Ok(InitFocusResult { has_events, focus_task: Some(task) })
            }

            TimelineFocusKind::Thread { event_cache, .. } => {
                let (has_events, subscriber) = self.init_with_thread_root(event_cache).await?;

                let room = &self.room_data_provider;
                let span = info_span!(
                    parent: Span::none(),
                    "thread_live_update_handler",
                    room_id = ?room.room_id(),
                );
                span.follows_from(Span::current());

                let task = room
                    .client()
                    .task_monitor()
                    .spawn_infinite_task(
                        "timeline::thread_event_cache_updates",
                        thread_updates_task(subscriber, event_cache.clone(), self.clone())
                            .instrument(span),
                    )
                    .abort_on_drop();

                Ok(InitFocusResult { has_events, focus_task: Some(task) })
            }

            TimelineFocusKind::PinnedEvents { event_cache } => {
                let (initial_events, pinned_events_recv) = event_cache.subscribe().await?;

                let has_events = !initial_events.is_empty();

                self.replace_with_initial_remote_events(
                    initial_events,
                    RemoteEventOrigin::Pagination,
                )
                .await;

                let task = self
                    .room_data_provider
                    .client()
                    .task_monitor()
                    .spawn_infinite_task(
                        "timeline::pinned_events_cache_updates",
                        pinned_events_task(event_cache.clone(), self.clone(), pinned_events_recv),
                    )
                    .abort_on_drop();

                Ok(InitFocusResult { has_events, focus_task: Some(task) })
            }
        }
    }

    /// (Re-)initialise a timeline using [`TimelineFocus::Thread`] with cached
    /// threaded events and secondary relations.
    ///
    /// Returns whether there were any events added to the timeline, and a
    /// receiver to return updates after the initial events have been inserted
    /// in the timeline.
    pub(super) async fn init_with_thread_root(
        &self,
        event_cache: &ThreadEventCache,
    ) -> Result<(bool, EventCacheSubscriber<ThreadEventCacheUpdate>), Error> {
        let (events, subscriber) = event_cache.subscribe().await?;
        let has_events = !events.is_empty();

        // For each event, we also need to find the related events, as they
        // don't include the thread relationship, they won't be included in the
        // initial list of events.
        //
        // The lookups are independent store queries, so run them together
        // rather than awaiting them one after the other. `try_join_all` keeps
        // the input order, so the related events are collected in the same
        // order as before.
        let lookups = events
            .iter()
            .filter_map(|event| event.event_id())
            .map(|event_id| event_cache.find_event_with_relations(event_id, None));

        let mut related_events = Vector::new();
        for (_original, related) in try_join_all(lookups).await?.into_iter().flatten() {
            related_events.extend(related);
        }

        self.replace_with_initial_remote_events(events, RemoteEventOrigin::Cache).await;

        // Now that we've inserted the thread events, add the aggregations too.
        if !related_events.is_empty() {
            self.handle_remote_aggregations(
                vec![VectorDiff::Append { values: related_events }],
                RemoteEventOrigin::Cache,
            )
            .await;
        }

        Ok((has_events, subscriber))
    }

    /// Given an event identifier, will fetch the details for the event it's
    /// replying to, if applicable.
    #[instrument(skip(self))]
    pub(super) async fn fetch_in_reply_to_details(&self, event_id: &EventId) -> Result<(), Error> {
        let state_guard = self.state.write().await;
        let (index, item) = rfind_event_by_id(&state_guard.items, event_id)
            .ok_or(Error::EventNotInTimeline(TimelineEventItemId::EventId(event_id.to_owned())))?;
        let remote_item = item
            .as_remote()
            .ok_or(Error::EventNotInTimeline(TimelineEventItemId::EventId(event_id.to_owned())))?
            .clone();

        let TimelineItemContent::MsgLike(msglike) = item.content().clone() else {
            debug!("Event is not a message");
            return Ok(());
        };
        let Some(in_reply_to) = msglike.in_reply_to.clone() else {
            debug!("Event is not a reply");
            return Ok(());
        };
        if let TimelineDetails::Pending = &in_reply_to.event {
            debug!("Replied-to event is already being fetched");
            return Ok(());
        }
        if let TimelineDetails::Ready(_) = &in_reply_to.event {
            debug!("Replied-to event has already been fetched");
            return Ok(());
        }

        let internal_id = item.internal_id.to_owned();
        let item = item.clone();
        let event = fetch_replied_to_event(
            state_guard,
            &self.state,
            index,
            &item,
            internal_id,
            &msglike,
            &in_reply_to.event_id,
            self.room(),
        )
        .await?;

        // We need to be sure to have the latest position of the event as it
        // might have changed while waiting for the request.
        let mut state = self.state.write().await;
        let (index, item) = rfind_event_by_id(&state.items, &remote_item.event_id)
            .ok_or(Error::EventNotInTimeline(TimelineEventItemId::EventId(event_id.to_owned())))?;

        // Check the state of the event again, it might have been redacted while
        // the request was in-flight.
        let TimelineItemContent::MsgLike(MsgLikeContent {
            kind: MsgLikeKind::Message(message),
            thread_root,
            in_reply_to,
            thread_summary,
        }) = item.content().clone()
        else {
            info!("Event is no longer a message (redacted?)");
            return Ok(());
        };
        let Some(in_reply_to) = in_reply_to else {
            warn!("Event no longer has a reply (bug?)");
            return Ok(());
        };

        // Now that we've received the content of the replied-to event, replace
        // the replied-to content in the item with it.
        trace!("Updating in-reply-to details");
        let internal_id = item.internal_id.to_owned();
        let mut item = item.clone();
        item.set_content(TimelineItemContent::MsgLike(MsgLikeContent {
            kind: MsgLikeKind::Message(message),
            thread_root,
            in_reply_to: Some(InReplyToDetails { event_id: in_reply_to.event_id, event }),
            thread_summary,
        }));
        state.items.replace(index, TimelineItem::new(item, internal_id));

        Ok(())
    }

    /// Returns the thread that should be used for a read receipt based on the
    /// current focus of the timeline and the receipt type.
    ///
    /// A `SendReceiptType::FullyRead` will always use
    /// `ReceiptThread::Unthreaded`
    pub(super) fn infer_thread_for_read_receipt(
        &self,
        receipt_type: &SendReceiptType,
    ) -> ReceiptThread {
        if matches!(receipt_type, SendReceiptType::FullyRead) {
            ReceiptThread::Unthreaded
        } else {
            self.focus.receipt_thread()
        }
    }

    /// Decide whether a read receipt should be sent, and which event it should
    /// target.
    ///
    /// The returned event may differ from `event_id`: a read receipt should not
    /// point at one of the user's own events (see the Matrix spec's
    /// [Receipts module]), so if `event_id` is one of theirs, the latest unread
    /// event before it that is targeted instead.
    ///
    /// - When there's no such earlier event and `is_marking_room_as_read` is
    ///   `false`, [`SendReceiptDecision::DoNotSend`] is returned.
    /// - When `is_marking_room_as_read` is `true`, the receipt falls back to
    ///   `event_id` itself if it is explicitly unread, so that the homeserver
    ///   still recomputes the push/badge count.
    ///
    /// [Receipts module]: https://spec.matrix.org/latest/client-server-api/#receipts
    pub(super) async fn should_send_receipt(
        &self,
        receipt_type: &SendReceiptType,
        receipt_thread: &ReceiptThread,
        event_id: &EventId,
        is_marking_room_as_read: bool,
    ) -> SendReceiptDecision {
        let own_user_id = self.room().own_user_id();
        let state = self.state.read().await;
        let room = self.room();
        let all_remote_events = state.items.all_remote_events();

        // Resolve the event the receipt should target, redirecting away from
        // the user's own events for read receipts.
        let target_event_id = match receipt_type {
            SendReceiptType::Read | SendReceiptType::ReadPrivate => {
                let is_own_event = all_remote_events
                    .get_by_event_id(event_id)
                    .and_then(|event_meta| event_meta.sender.as_deref())
                    == Some(own_user_id);

                if is_own_event {
                    let filter_out_thread_events = match self.focus() {
                        TimelineFocusKind::Thread { .. } | TimelineFocusKind::Event { .. } => false,
                        TimelineFocusKind::Live { hide_threaded_events, .. } => {
                            *hide_threaded_events
                        }
                        TimelineFocusKind::PinnedEvents { .. } => true,
                    };

                    let previous_event = all_remote_events
                        .iter()
                        .rev()
                        // Only consider the events that precede the requested one.
                        .skip_while(|event_meta| event_meta.event_id != *event_id)
                        .skip(1)
                        // Never point a read receipt at one of the user's own events.
                        .filter(|event_meta| event_meta.sender.as_deref() != Some(own_user_id))
                        .find_map(|event_meta| {
                            if !filter_out_thread_events {
                                Some(event_meta.event_id.clone())
                            } else if event_meta.thread_root_id.is_none() {
                                if let Some(TimelineEventItemId::EventId(aggregated_event_id)) =
                                    state.meta.aggregations.is_aggregation_of(
                                        &TimelineEventItemId::EventId(event_meta.event_id.clone()),
                                    )
                                    && let Some(target_meta) =
                                        all_remote_events.get_by_event_id(aggregated_event_id)
                                    && target_meta.thread_root_id.is_some()
                                {
                                    None
                                } else {
                                    Some(event_meta.event_id.clone())
                                }
                            } else {
                                None
                            }
                        });

                    match previous_event {
                        Some(event_id) => event_id,
                        // Nothing from another user to point at. When marking
                        // the room as read, fall back to the user's own event
                        // so the homeserver still recomputes its push/badge
                        // count; otherwise there's nothing to send.
                        None if is_marking_room_as_read => event_id.to_owned(),
                        None => return SendReceiptDecision::DoNotSend,
                    }
                } else {
                    event_id.to_owned()
                }
            }

            _ => event_id.to_owned(),
        };

        // Find the real receipt the homeserver already knows about.
        let previous_event_id = match receipt_type {
            SendReceiptType::Read => state
                .meta
                .user_receipt(
                    own_user_id,
                    ReceiptType::Read,
                    receipt_thread.clone(),
                    room,
                    all_remote_events,
                    read_receipts::ImplicitReadReceipts::Exclude,
                )
                .await
                .map(|(event_id, _)| event_id),

            // Implicit read receipts are saved as public read receipts, so get
            // the latest. It also doesn't make sense to have a private read
            // receipt behind a public one.
            SendReceiptType::ReadPrivate => state
                .latest_user_read_receipt(
                    own_user_id,
                    receipt_thread.clone(),
                    room,
                    read_receipts::ImplicitReadReceipts::Exclude,
                )
                .await
                .map(|(event_id, _)| event_id),

            SendReceiptType::FullyRead => self.room_data_provider.load_fully_read_marker().await,

            _ => None,
        };

        // Don't send anything if the resolved event isn't more recent than
        // that.
        if let Some(previous_event_id) = previous_event_id {
            trace!(%previous_event_id, "found a previous receipt");
            if let Some(relative_pos) = TimelineMetadata::compare_events_positions(
                &previous_event_id,
                &target_event_id,
                all_remote_events,
            ) && relative_pos != RelativePosition::After
            {
                return SendReceiptDecision::DoNotSend;
            }
        }

        // No previous receipt was found (or it's an unknown one): let the
        // server handle it.
        SendReceiptDecision::SendTo(target_event_id)
    }

    /// Returns the latest event identifier, even if it's not visible, or if
    /// it's folded into another timeline item.
    pub(crate) async fn latest_event_id(&self) -> Option<OwnedEventId> {
        let state = self.state.read().await;
        let filter_out_thread_events = match self.focus() {
            TimelineFocusKind::Thread { .. } => false,
            TimelineFocusKind::Live { hide_threaded_events, .. } => *hide_threaded_events,
            TimelineFocusKind::Event { .. } => {
                // For event-focused timelines, filtering is handled in the
                // event cache layer.
                false
            }
            TimelineFocusKind::PinnedEvents { .. } => true,
        };

        state
            .items
            .all_remote_events()
            .iter()
            .rev()
            .filter_map(|event_meta| {
                if !filter_out_thread_events {
                    // For an unthreaded timeline, the last event is always the
                    // latest event.
                    Some(event_meta.event_id.clone())
                } else if event_meta.thread_root_id.is_none() {
                    // For the main-thread timeline, only non-threaded events
                    // are valid candidates for the latest event.
                    //
                    // But! An event could be an aggregation that relate to an
                    // in-thread event. In this case, it's not a valid latest
                    // event.
                    if let Some(TimelineEventItemId::EventId(target_event_id)) =
                        state.meta.aggregations.is_aggregation_of(&TimelineEventItemId::EventId(
                            event_meta.event_id.clone(),
                        ))
                        && let Some(target_meta) =
                            state.items.all_remote_events().get_by_event_id(target_event_id)
                        && target_meta.thread_root_id.is_some()
                    {
                        // This event is an aggregation of an in-thread event,
                        // so skip it.
                        None
                    } else {
                        // Not in a thread, and not the aggregation of an
                        // in-thread event, so it's a valid candidate for the
                        // latest event.
                        Some(event_meta.event_id.clone())
                    }
                } else {
                    // An in-thread event, when we're filtering out threaded
                    // events, is never a valid candidate for the latest event.
                    None
                }
            })
            .next()
    }

    #[instrument(skip(self), fields(room_id = ?self.room().room_id()))]
    pub(super) async fn retry_event_decryption(&self, session_ids: Option<BTreeSet<String>>) {
        let (utds, decrypted) = self.compute_redecryption_candidates().await;

        let request = DecryptionRetryRequest {
            room_id: self.room().room_id().to_owned(),
            utd_session_ids: utds,
            refresh_info_session_ids: decrypted,
        };

        self.room().client().event_cache().request_decryption(request);
    }

    /// Combine the global (event cache) pagination status with the local state
    /// of the timeline.
    ///
    /// This only changes the global pagination status of this room, in one
    /// case: if the timeline has a skip count greater than 0, it will ensure
    /// that the pagination status says that we haven't reached the timeline
    /// start yet.
    pub(super) async fn map_pagination_status(&self, status: PaginationStatus) -> PaginationStatus {
        match status {
            PaginationStatus::Idle { hit_timeline_start } => {
                if hit_timeline_start {
                    let state = self.state.read().await;
                    // If the skip count is greater than 0, it means that a
                    // subsequent pagination could return more items, so pretend
                    // we didn't get the information that the timeline start was
                    // hit.
                    if state.meta.subscriber_skip_count.get() > 0 {
                        return PaginationStatus::Idle { hit_timeline_start: false };
                    }
                }
            }
            PaginationStatus::Paginating => {}
        }

        // You're perfect, just the way you are.
        status
    }
}

impl<P: RoomDataProvider> TimelineController<P> {
    /// Returns the timeline focus of the [`TimelineController`].
    pub(super) fn focus(&self) -> &TimelineFocusKind {
        &self.focus
    }

    /// Find an event by ID in this timeline, along with its related events.
    ///
    /// The related events can be filtered by relation type.
    pub(in crate::timeline) async fn find_event_with_relations(
        &self,
        event_id: &EventId,
        filter: Option<Vec<RelationType>>,
    ) -> Result<(TimelineEvent, Vec<TimelineEvent>), Error> {
        self.room_data_provider
            .load_or_fetch_event_with_relations(event_id, filter)
            .await
            .map_err(Into::into)
    }
}

#[allow(clippy::too_many_arguments)]
async fn fetch_replied_to_event<P: RoomDataProvider>(
    mut state_guard: RwLockWriteGuard<'_, TimelineState<P>>,
    state_lock: &RwLock<TimelineState<P>>,
    index: usize,
    item: &EventTimelineItem,
    internal_id: TimelineUniqueId,
    msglike: &MsgLikeContent,
    in_reply_to: &EventId,
    room: &Room,
) -> Result<TimelineDetails<Box<EmbeddedEvent>>, Error> {
    if let Some((_, item)) = rfind_event_by_id(&state_guard.items, in_reply_to) {
        let details = TimelineDetails::Ready(Box::new(EmbeddedEvent::from_timeline_item(&item)));
        trace!("Found replied-to event locally");
        return Ok(details);
    }

    // Replace the item with a new timeline item that has the fetching status of
    // the replied-to event to pending.
    trace!("Setting in-reply-to details to pending");
    let in_reply_to_details =
        InReplyToDetails { event_id: in_reply_to.to_owned(), event: TimelineDetails::Pending };

    let event_item = item
        .with_content(TimelineItemContent::MsgLike(msglike.with_in_reply_to(in_reply_to_details)));

    let new_timeline_item = TimelineItem::new(event_item, internal_id);
    state_guard.items.replace(index, new_timeline_item);

    // Don't hold the state lock while the network request is made.
    drop(state_guard);

    trace!("Fetching replied-to event");
    let res = match room.load_or_fetch_event(in_reply_to, None).await {
        Ok(timeline_event) => {
            let state = state_lock.read().await;

            let replied_to_item =
                EmbeddedEvent::try_from_timeline_event(timeline_event, room, &state.meta).await?;

            if let Some(item) = replied_to_item {
                TimelineDetails::Ready(Box::new(item))
            } else {
                // The replied-to item is an aggregation, not a standalone item.
                return Err(Error::UnsupportedEvent);
            }
        }

        Err(e) => TimelineDetails::Error(Arc::new(e)),
    };

    Ok(res)
}