matrix_sdk/client/builder/

mod.rs

// Copyright 2022 The Matrix.org Foundation C.I.C.
// Copyright 2022 Kévin Commaille
//
// 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.

mod homeserver_config;

#[cfg(feature = "experimental-search")]
use std::collections::HashMap;
#[cfg(feature = "sqlite")]
use std::path::Path;
#[cfg(any(feature = "experimental-search", feature = "sqlite"))]
use std::path::PathBuf;
use std::{
    collections::BTreeSet,
    fmt,
    sync::{Arc, RwLock as StdRwLock},
};

#[cfg(feature = "sqlite")]
use futures_util::try_join;
use homeserver_config::*;
#[cfg(feature = "e2e-encryption")]
use matrix_sdk_base::crypto::DecryptionSettings;
#[cfg(feature = "experimental-x509-identity-verification")]
use matrix_sdk_base::crypto::x509::{RawX509Signer, RawX509Verifier};
#[cfg(feature = "e2e-encryption")]
use matrix_sdk_base::crypto::{CollectStrategy, TrustRequirement};
use matrix_sdk_base::{
    BaseClient, DmRoomDefinition, ThreadingSupport, store::StoreConfig, ttl::TtlValue,
};
use matrix_sdk_common::cross_process_lock::CrossProcessLockConfig;
#[cfg(feature = "sqlite")]
use matrix_sdk_sqlite::SqliteStoreConfig;
#[cfg(not(target_family = "wasm"))]
use reqwest::Certificate;
use ruma::{
    OwnedServerName, ServerName,
    api::{MatrixVersion, SupportedVersions, error::FromHttpResponseError},
    presence::PresenceState,
};
use thiserror::Error;
#[cfg(feature = "experimental-search")]
use tokio::sync::Mutex;
use tokio::sync::OnceCell;
use tracing::{Span, debug, field::debug, instrument};

use super::{Client, ClientInner};
#[cfg(feature = "e2e-encryption")]
use crate::encryption::EncryptionSettings;
#[cfg(not(target_family = "wasm"))]
use crate::http_client::HttpSettings;
#[cfg(feature = "experimental-search")]
use crate::search_index::SearchIndex;
#[cfg(feature = "experimental-search")]
use crate::search_index::SearchIndexStoreKind;
use crate::{
    HttpError, IdParseError,
    authentication::AuthCtx,
    client::caches::CachedValue::{Cached, NotSet},
    config::RequestConfig,
    error::RumaApiError,
    http_client::HttpClient,
    media::{DefaultMediaFetcher, MediaFetcher},
    send_queue::SendQueueData,
    sliding_sync::VersionBuilder as SlidingSyncVersionBuilder,
};

/// Builder that allows creating and configuring various parts of a [`Client`].
///
/// When setting the `StateStore` it is up to the user to open/connect
/// the storage backend before client creation.
///
/// # Examples
///
/// ```
/// use matrix_sdk::Client;
/// // To pass all the request through mitmproxy set the proxy and disable SSL
/// // verification
///
/// let client_builder = Client::builder()
///     .proxy("http://localhost:8080")
///     .disable_ssl_verification();
/// ```
///
/// # Example for using a custom http client
///
/// Note: setting a custom http client will ignore `user_agent`, `proxy`, and
/// `disable_ssl_verification` - you'd need to set these yourself if you want
/// them.
///
/// ```
/// use std::sync::Arc;
///
/// use matrix_sdk::Client;
///
/// // setting up a custom http client
/// let reqwest_builder = reqwest::ClientBuilder::new()
///     .https_only(true)
///     .no_proxy()
///     .user_agent("MyApp/v3.0");
///
/// let client_builder =
///     Client::builder().http_client(reqwest_builder.build()?);
/// # anyhow::Ok(())
/// ```
#[must_use]
#[derive(Clone, Debug)]
pub struct ClientBuilder {
    homeserver_cfg: Option<HomeserverConfig>,
    sliding_sync_version_builder: SlidingSyncVersionBuilder,
    http_cfg: Option<HttpConfig>,
    store_config: BuilderStoreConfig,
    request_config: RequestConfig,
    respect_login_well_known: bool,
    well_known_lookup_disabled: bool,
    server_versions: Option<BTreeSet<MatrixVersion>>,
    handle_refresh_tokens: bool,
    base_client: Option<BaseClient>,
    #[cfg(feature = "e2e-encryption")]
    encryption_settings: EncryptionSettings,
    #[cfg(feature = "e2e-encryption")]
    room_key_recipient_strategy: CollectStrategy,
    #[cfg(feature = "e2e-encryption")]
    decryption_settings: DecryptionSettings,
    #[cfg(feature = "e2e-encryption")]
    enable_share_history_on_invite: bool,
    cross_process_lock_config: CrossProcessLockConfig,
    threading_support: ThreadingSupport,
    #[cfg(feature = "experimental-search")]
    search_index_store_kind: SearchIndexStoreKind,
    #[cfg(feature = "experimental-x509-identity-verification")]
    x509_signer: Option<Arc<dyn RawX509Signer>>,
    #[cfg(feature = "experimental-x509-identity-verification")]
    x509_verifier: Option<Arc<dyn RawX509Verifier>>,
    dm_room_definition: DmRoomDefinition,
    media_fetcher: Arc<dyn MediaFetcher>,
}

impl ClientBuilder {
    const DEFAULT_CROSS_PROCESS_STORE_LOCKS_HOLDER_NAME: &str = "main";

    pub(crate) fn new() -> Self {
        Self {
            homeserver_cfg: None,
            sliding_sync_version_builder: SlidingSyncVersionBuilder::Native,
            http_cfg: None,
            store_config: BuilderStoreConfig::Custom(StoreConfig::new(
                CrossProcessLockConfig::multi_process(
                    Self::DEFAULT_CROSS_PROCESS_STORE_LOCKS_HOLDER_NAME,
                ),
            )),
            request_config: Default::default(),
            respect_login_well_known: true,
            well_known_lookup_disabled: false,
            server_versions: None,
            handle_refresh_tokens: false,
            base_client: None,
            #[cfg(feature = "e2e-encryption")]
            encryption_settings: Default::default(),
            #[cfg(feature = "e2e-encryption")]
            room_key_recipient_strategy: Default::default(),
            #[cfg(feature = "e2e-encryption")]
            decryption_settings: DecryptionSettings {
                sender_device_trust_requirement: TrustRequirement::Untrusted,
            },
            #[cfg(feature = "e2e-encryption")]
            enable_share_history_on_invite: true,
            cross_process_lock_config: CrossProcessLockConfig::MultiProcess {
                holder_name: Self::DEFAULT_CROSS_PROCESS_STORE_LOCKS_HOLDER_NAME.to_owned(),
            },
            threading_support: ThreadingSupport::Disabled,
            #[cfg(feature = "experimental-search")]
            search_index_store_kind: SearchIndexStoreKind::InMemory,
            #[cfg(feature = "experimental-x509-identity-verification")]
            x509_signer: None,
            #[cfg(feature = "experimental-x509-identity-verification")]
            x509_verifier: None,
            dm_room_definition: DmRoomDefinition::MatrixSpec,
            media_fetcher: Arc::new(DefaultMediaFetcher),
        }
    }

    /// Sets a [`MediaFetcher`] that will be used to get media from the media
    /// server.
    pub fn media_fetcher(mut self, media_fetcher: Arc<dyn MediaFetcher>) -> Self {
        self.media_fetcher = media_fetcher.clone();
        self
    }

    /// Sets the definition the [`Client`] will use to check if a room is a DM.
    ///
    /// By default this is [`DmRoomDefinition::MatrixSpec`].
    pub fn dm_room_definition(mut self, dm_room_definition: DmRoomDefinition) -> Self {
        self.dm_room_definition = dm_room_definition;
        self
    }

    /// Set the homeserver URL to use.
    ///
    /// The following methods are mutually exclusive: [`Self::homeserver_url`],
    /// [`Self::server_name`] [`Self::insecure_server_name_no_tls`],
    /// [`Self::server_name_or_homeserver_url`].
    /// If you set more than one, then whatever was set last will be used.
    ///
    /// This is the only one of them that never performs a
    /// `.well-known/matrix/client` lookup, so it is the one to use together
    /// with [`Self::disable_well_known_lookup`].
    pub fn homeserver_url(mut self, url: impl AsRef<str>) -> Self {
        self.homeserver_cfg = Some(HomeserverConfig::HomeserverUrl(url.as_ref().to_owned()));
        self
    }

    /// Set the server name to discover the homeserver from.
    ///
    /// We assume we can connect in HTTPS to that server. If that's not the
    /// case, prefer using [`Self::insecure_server_name_no_tls`].
    ///
    /// The following methods are mutually exclusive: [`Self::homeserver_url`],
    /// [`Self::server_name`] [`Self::insecure_server_name_no_tls`],
    /// [`Self::server_name_or_homeserver_url`].
    /// If you set more than one, then whatever was set last will be used.
    ///
    /// This performs a `.well-known/matrix/client` lookup, and is therefore
    /// incompatible with [`Self::disable_well_known_lookup`]: [`Self::build`]
    /// then fails with [`ClientBuildError::WellKnownLookupDisabled`].
    pub fn server_name(mut self, server_name: &ServerName) -> Self {
        self.homeserver_cfg = Some(HomeserverConfig::ServerName {
            server: server_name.to_owned(),
            // Assume HTTPS if not specified.
            protocol: UrlScheme::Https,
        });
        self
    }

    /// Set the server name to discover the homeserver from, assuming an HTTP
    /// (not secured) scheme. This also relaxes OAuth 2.0 discovery checks to
    /// allow HTTP schemes.
    ///
    /// The following methods are mutually exclusive: [`Self::homeserver_url`],
    /// [`Self::server_name`] [`Self::insecure_server_name_no_tls`],
    /// [`Self::server_name_or_homeserver_url`].
    /// If you set more than one, then whatever was set last will be used.
    ///
    /// This performs a `.well-known/matrix/client` lookup, and is therefore
    /// incompatible with [`Self::disable_well_known_lookup`]: [`Self::build`]
    /// then fails with [`ClientBuildError::WellKnownLookupDisabled`].
    pub fn insecure_server_name_no_tls(mut self, server_name: &ServerName) -> Self {
        self.homeserver_cfg = Some(HomeserverConfig::ServerName {
            server: server_name.to_owned(),
            protocol: UrlScheme::Http,
        });
        self
    }

    /// Set the server name to discover the homeserver from, falling back to
    /// using it as a homeserver URL if discovery fails. When falling back to a
    /// homeserver URL, a check is made to ensure that the server exists (unlike
    /// [`Self::homeserver_url`], so you can guarantee that the client is ready
    /// to use.
    ///
    /// The following methods are mutually exclusive: [`Self::homeserver_url`],
    /// [`Self::server_name`] [`Self::insecure_server_name_no_tls`],
    /// [`Self::server_name_or_homeserver_url`].
    /// If you set more than one, then whatever was set last will be used.
    ///
    /// With [`Self::disable_well_known_lookup`], the discovery step is skipped
    /// and only the homeserver URL check is performed, so a homeserver URL
    /// still works while a delegating server name fails with
    /// [`ClientBuildError::InvalidServerName`].
    pub fn server_name_or_homeserver_url(mut self, server_name_or_url: impl AsRef<str>) -> Self {
        self.homeserver_cfg = Some(HomeserverConfig::ServerNameOrHomeserverUrl(
            server_name_or_url.as_ref().to_owned(),
        ));
        self
    }

    /// Set sliding sync to a specific version.
    pub fn sliding_sync_version_builder(
        mut self,
        version_builder: SlidingSyncVersionBuilder,
    ) -> Self {
        self.sliding_sync_version_builder = version_builder;
        self
    }

    /// Set up the store configuration for an SQLite store.
    #[cfg(feature = "sqlite")]
    pub fn sqlite_store(mut self, path: impl AsRef<Path>, passphrase: Option<&str>) -> Self {
        let sqlite_store_config = SqliteStoreConfig::new(path).passphrase(passphrase);
        self.store_config =
            BuilderStoreConfig::Sqlite { config: sqlite_store_config, cache_path: None };

        self
    }

    /// Set up the store configuration for an SQLite store with cached data
    /// separated out from state/crypto data.
    #[cfg(feature = "sqlite")]
    pub fn sqlite_store_with_cache_path(
        mut self,
        path: impl AsRef<Path>,
        cache_path: impl AsRef<Path>,
        passphrase: Option<&str>,
    ) -> Self {
        let sqlite_store_config = SqliteStoreConfig::new(path).passphrase(passphrase);
        self.store_config = BuilderStoreConfig::Sqlite {
            config: sqlite_store_config,
            cache_path: Some(cache_path.as_ref().to_owned()),
        };

        self
    }

    /// Set up the store configuration for an SQLite store with a store config,
    /// and with an optional cache data separated out from state/crypto data.
    #[cfg(feature = "sqlite")]
    pub fn sqlite_store_with_config_and_cache_path(
        mut self,
        config: SqliteStoreConfig,
        cache_path: Option<impl AsRef<Path>>,
    ) -> Self {
        self.store_config = BuilderStoreConfig::Sqlite {
            config,
            cache_path: cache_path.map(|cache_path| cache_path.as_ref().to_owned()),
        };

        self
    }

    /// Set up the store configuration for a IndexedDB store.
    #[cfg(feature = "indexeddb")]
    pub fn indexeddb_store(mut self, name: &str, passphrase: Option<&str>) -> Self {
        self.store_config = BuilderStoreConfig::IndexedDb {
            name: name.to_owned(),
            passphrase: passphrase.map(ToOwned::to_owned),
        };
        self
    }

    /// Set up the store configuration.
    ///
    /// The easiest way to get a [`StoreConfig`] is to use the
    /// `make_store_config` method from one of the store crates.
    ///
    /// # Arguments
    ///
    /// * `store_config` - The configuration of the store.
    ///
    /// # Examples
    ///
    /// ```
    /// # use matrix_sdk_base::store::MemoryStore;
    /// # use matrix_sdk_common::cross_process_lock::CrossProcessLockConfig;
    /// # let custom_state_store = MemoryStore::new();
    /// use matrix_sdk::{Client, config::StoreConfig};
    ///
    /// let store_config = StoreConfig::new(CrossProcessLockConfig::MultiProcess {
    ///     holder_name: "cross-process-store-locks-holder-name".to_owned(),
    /// })
    /// .state_store(custom_state_store);
    /// let client_builder = Client::builder().store_config(store_config);
    /// ```
    pub fn store_config(mut self, store_config: StoreConfig) -> Self {
        self.store_config = BuilderStoreConfig::Custom(store_config);
        self
    }

    /// Update the client's homeserver URL with the discovery information
    /// present in the login response, if any.
    pub fn respect_login_well_known(mut self, value: bool) -> Self {
        self.respect_login_well_known = value;
        self
    }

    /// Disable all the `/.well-known/matrix/client` lookups, both the one
    /// performed by [`Self::build`] to discover the homeserver, and all the
    /// ones performed later by the built [`Client`].
    ///
    /// Some deployments must not emit any request to the well-known URI of
    /// their domain. When disabled, [`Client::tile_server`] returns `None`,
    /// [`Client::well_known_rtc_transports`] returns an empty list, and
    /// [`Client::discover_rtc_transports`] doesn't fall back to the well-known
    /// `m.rtc_foci`, relying only on the MSC4143 discovery endpoint.
    ///
    /// # Interaction with the homeserver setters
    ///
    /// The homeserver must then be resolvable without a well-known lookup:
    ///
    /// * [`Self::homeserver_url`] works, and is the recommended choice.
    /// * [`Self::server_name`] and [`Self::insecure_server_name_no_tls`] can
    ///   *only* be resolved through the well-known, so [`Self::build`] fails
    ///   with [`ClientBuildError::WellKnownLookupDisabled`]. Assuming that the
    ///   server name is also the homeserver would silently talk to the wrong
    ///   host for any deployment that delegates.
    /// * [`Self::server_name_or_homeserver_url`] skips the well-known step and
    ///   goes straight to checking whether the value points at a homeserver, so
    ///   it works when given a homeserver URL, and fails with
    ///   [`ClientBuildError::InvalidServerName`] otherwise.
    ///
    /// [`Client::discover_rtc_transports`]: crate::Client::discover_rtc_transports
    /// [`Client::tile_server`]: crate::Client::tile_server
    /// [`Client::well_known_rtc_transports`]: crate::Client::well_known_rtc_transports
    pub fn disable_well_known_lookup(mut self, disable: bool) -> Self {
        self.well_known_lookup_disabled = disable;
        self
    }

    /// Set the default timeout, fail and retry behavior for all HTTP requests.
    pub fn request_config(mut self, request_config: RequestConfig) -> Self {
        self.request_config = request_config;
        self
    }

    /// Set the proxy through which all the HTTP requests should go.
    ///
    /// Note, only HTTP proxies are supported.
    ///
    /// # Arguments
    ///
    /// * `proxy` - The HTTP URL of the proxy.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use matrix_sdk::Client;
    ///
    /// let client_config = Client::builder().proxy("http://localhost:8080");
    /// ```
    #[cfg(not(target_family = "wasm"))]
    pub fn proxy(mut self, proxy: impl AsRef<str>) -> Self {
        self.http_settings().proxy = Some(proxy.as_ref().to_owned());
        self
    }

    /// Disable SSL verification for the HTTP requests.
    #[cfg(not(target_family = "wasm"))]
    pub fn disable_ssl_verification(mut self) -> Self {
        self.http_settings().disable_ssl_verification = true;
        self
    }

    /// Set a custom HTTP user agent for the client.
    #[cfg(not(target_family = "wasm"))]
    pub fn user_agent(mut self, user_agent: impl AsRef<str>) -> Self {
        self.http_settings().user_agent = Some(user_agent.as_ref().to_owned());
        self
    }

    /// Add the given list of certificates to the certificate store of the HTTP
    /// client.
    ///
    /// These additional certificates will be trusted and considered when
    /// establishing a HTTP request.
    ///
    /// Internally this will call the
    /// [`reqwest::ClientBuilder::add_root_certificate()`] method.
    #[cfg(not(target_family = "wasm"))]
    pub fn add_root_certificates(mut self, certificates: Vec<Certificate>) -> Self {
        self.http_settings().additional_root_certificates = certificates;
        self
    }

    /// Don't trust any system root certificates, only trust the certificates
    /// provided through
    /// [`add_root_certificates`][ClientBuilder::add_root_certificates].
    #[cfg(not(target_family = "wasm"))]
    pub fn disable_built_in_root_certificates(mut self) -> Self {
        self.http_settings().disable_built_in_root_certificates = true;
        self
    }

    /// Specify a [`reqwest::Client`] instance to handle sending requests and
    /// receiving responses.
    ///
    /// This method is mutually exclusive with
    /// [`proxy()`][ClientBuilder::proxy],
    /// [`disable_ssl_verification`][ClientBuilder::disable_ssl_verification],
    /// [`add_root_certificates`][ClientBuilder::add_root_certificates],
    /// [`disable_built_in_root_certificates`][ClientBuilder::disable_built_in_root_certificates],
    /// and [`user_agent()`][ClientBuilder::user_agent].
    pub fn http_client(mut self, client: reqwest::Client) -> Self {
        self.http_cfg = Some(HttpConfig::Custom(client));
        self
    }

    /// Specify the Matrix versions supported by the homeserver manually, rather
    /// than `build()` doing it using a `get_supported_versions` request.
    ///
    /// This is helpful for test code that doesn't care to mock that endpoint.
    pub fn server_versions(mut self, value: impl IntoIterator<Item = MatrixVersion>) -> Self {
        self.server_versions = Some(value.into_iter().collect());
        self
    }

    #[cfg(not(target_family = "wasm"))]
    fn http_settings(&mut self) -> &mut HttpSettings {
        self.http_cfg.get_or_insert_with(Default::default).settings()
    }

    /// Handle [refreshing access tokens] automatically.
    ///
    /// By default, the `Client` forwards any error and doesn't handle errors
    /// with the access token, which means that
    /// [`Client::refresh_access_token()`] needs to be called manually to
    /// refresh access tokens.
    ///
    /// Enabling this setting means that the `Client` will try to refresh the
    /// token automatically, which means that:
    ///
    /// * If refreshing the token fails, the error is forwarded, so any endpoint
    ///   can return [`HttpError::RefreshToken`]. If an [`UnknownToken`] error
    ///   is encountered, it means that the user needs to be logged in again.
    ///
    /// * The access token and refresh token need to be watched for changes,
    ///   using the authentication API's `session_tokens_stream()` for example,
    ///   to be able to [restore the session] later.
    ///
    /// [refreshing access tokens]: https://spec.matrix.org/v1.3/client-server-api/#refreshing-access-tokens
    /// [`UnknownToken`]: ruma::api::error::ErrorKind::UnknownToken
    /// [restore the session]: Client::restore_session
    pub fn handle_refresh_tokens(mut self) -> Self {
        self.handle_refresh_tokens = true;
        self
    }

    /// Public for test only
    #[doc(hidden)]
    pub fn base_client(mut self, base_client: BaseClient) -> Self {
        self.base_client = Some(base_client);
        self
    }

    /// Enables specific encryption settings that will persist throughout the
    /// entire lifetime of the `Client`.
    #[cfg(feature = "e2e-encryption")]
    pub fn with_encryption_settings(mut self, settings: EncryptionSettings) -> Self {
        self.encryption_settings = settings;
        self
    }

    /// Set the strategy to be used for picking recipient devices, when sending
    /// an encrypted message.
    #[cfg(feature = "e2e-encryption")]
    pub fn with_room_key_recipient_strategy(mut self, strategy: CollectStrategy) -> Self {
        self.room_key_recipient_strategy = strategy;
        self
    }

    /// Set the trust requirement to be used when decrypting events.
    #[cfg(feature = "e2e-encryption")]
    pub fn with_decryption_settings(mut self, decryption_settings: DecryptionSettings) -> Self {
        self.decryption_settings = decryption_settings;
        self
    }

    /// Whether to enable the experimental support for sending and receiving
    /// encrypted room history on invite, per [MSC4268].
    ///
    /// This setting is now enabled by default, but can be disabled via this
    /// method.
    ///
    /// [MSC4268]: https://github.com/matrix-org/matrix-spec-proposals/pull/4268
    #[cfg(feature = "e2e-encryption")]
    pub fn with_enable_share_history_on_invite(
        mut self,
        enable_share_history_on_invite: bool,
    ) -> Self {
        self.enable_share_history_on_invite = enable_share_history_on_invite;
        self
    }

    /// Set the cross-process store locks holder name.
    ///
    /// The SDK provides cross-process store locks (see
    /// [`matrix_sdk_common::cross_process_lock::CrossProcessLock`]). The
    /// `holder_name` will be the value used for all cross-process store locks
    /// used by the `Client` being built.
    ///
    /// If 2 concurrent `Client`s are running in 2 different process, this
    /// method must be called with different `hold_name` values.
    pub fn cross_process_store_config(
        mut self,
        cross_process_store_config: CrossProcessLockConfig,
    ) -> Self {
        self.cross_process_lock_config = cross_process_store_config;
        self
    }

    /// Whether the threads feature is enabled throuoghout the SDK.
    /// This will affect how timelines are setup, how read receipts are sent
    /// and how room unreads are computed.
    pub fn with_threading_support(mut self, threading_support: ThreadingSupport) -> Self {
        self.threading_support = threading_support;
        self
    }

    /// The base directory in which each room's index directory will be stored.
    #[cfg(feature = "experimental-search")]
    pub fn search_index_store(mut self, kind: SearchIndexStoreKind) -> Self {
        self.search_index_store_kind = kind;
        self
    }

    /// The signer we will use to sign master signing keys and outgoing secret
    /// requests.
    #[cfg(feature = "experimental-x509-identity-verification")]
    pub fn with_x509_signer(mut self, x509_signer: Option<Arc<dyn RawX509Signer>>) -> Self {
        self.x509_signer = x509_signer;
        self
    }

    /// The verifier we will use to verify master signing keys and incoming
    /// secret requests.
    #[cfg(feature = "experimental-x509-identity-verification")]
    pub fn with_x509_verifier(mut self, x509_verifier: Option<Arc<dyn RawX509Verifier>>) -> Self {
        self.x509_verifier = x509_verifier;
        self
    }

    /// Create a [`Client`] with the options set on this builder.
    ///
    /// # Errors
    ///
    /// This method can fail for two general reasons:
    ///
    /// * Invalid input: a missing or invalid homeserver URL or invalid proxy
    ///   URL
    /// * HTTP error: If you supplied a user ID instead of a homeserver URL, a
    ///   server discovery request is made which can fail; if you didn't set
    ///   [`server_versions(false)`][Self::server_versions], that amounts to
    ///   another request that can fail
    #[instrument(skip_all, target = "matrix_sdk::client", fields(homeserver))]
    pub async fn build(self) -> Result<Client, ClientBuildError> {
        debug!("Starting to build the Client");

        let homeserver_cfg = self.homeserver_cfg.ok_or(ClientBuildError::MissingHomeserver)?;
        Span::current().record("homeserver", debug(&homeserver_cfg));

        #[cfg_attr(target_family = "wasm", allow(clippy::infallible_destructuring_match))]
        let inner_http_client = match self.http_cfg.unwrap_or_default() {
            #[cfg(not(target_family = "wasm"))]
            HttpConfig::Settings(mut settings) => {
                settings.timeout = self.request_config.timeout;
                settings.make_client()?
            }
            HttpConfig::Custom(c) => c,
        };

        let base_client = if let Some(base_client) = self.base_client {
            base_client
        } else {
            #[allow(unused_mut)]
            let mut client = BaseClient::new(
                build_store_config(self.store_config, &self.cross_process_lock_config).await?,
                self.threading_support,
                self.dm_room_definition,
            );

            #[cfg(feature = "e2e-encryption")]
            {
                client.room_key_recipient_strategy = self.room_key_recipient_strategy;
                client.decryption_settings = self.decryption_settings;
            }

            #[cfg(feature = "experimental-x509-identity-verification")]
            client.set_x509_signer(self.x509_signer);
            #[cfg(feature = "experimental-x509-identity-verification")]
            client.set_x509_verifier(self.x509_verifier);

            client
        };

        let http_client = HttpClient::new(inner_http_client.clone(), self.request_config);

        #[allow(unused_variables)]
        let HomeserverDiscoveryResult { server, homeserver, supported_versions, well_known } =
            homeserver_cfg.discover(&http_client, self.well_known_lookup_disabled).await?;

        let sliding_sync_version = {
            let supported_versions = match supported_versions {
                Some(versions) => Some(versions),
                None if self.sliding_sync_version_builder.needs_get_supported_versions() => {
                    Some(get_supported_versions(&homeserver, &http_client).await?)
                }
                None => None,
            };

            let version = self.sliding_sync_version_builder.build(
                supported_versions.map(|response| response.as_supported_versions()).as_ref(),
            )?;

            tracing::info!(?version, "selected sliding sync version");

            version
        };

        let allow_insecure_oauth = homeserver.scheme() == "http";
        let auth_ctx = Arc::new(AuthCtx::new(self.handle_refresh_tokens, allow_insecure_oauth));

        // Enable the send queue by default.
        let send_queue = Arc::new(SendQueueData::new(true));

        let supported_versions = match self.server_versions {
            Some(versions) => Cached(TtlValue::without_expiry(SupportedVersions {
                versions,
                features: Default::default(),
            })),
            None => NotSet,
        };
        let well_known = match well_known {
            Some(well_known) => Cached(TtlValue::new(Some(well_known.into()))),
            None => NotSet,
        };

        let event_cache = OnceCell::new();
        let latest_events = OnceCell::new();
        let thread_subscriptions_catchup = OnceCell::new();

        #[cfg(feature = "experimental-search")]
        let search_index =
            SearchIndex::new(Arc::new(Mutex::new(HashMap::new())), self.search_index_store_kind);

        let inner = ClientInner::new(
            auth_ctx,
            server,
            homeserver,
            sliding_sync_version,
            Arc::new(StdRwLock::new(PresenceState::Online)),
            http_client,
            base_client,
            supported_versions,
            well_known,
            self.respect_login_well_known,
            self.well_known_lookup_disabled,
            event_cache,
            send_queue,
            latest_events,
            #[cfg(feature = "e2e-encryption")]
            self.encryption_settings,
            #[cfg(feature = "e2e-encryption")]
            self.enable_share_history_on_invite,
            self.cross_process_lock_config,
            #[cfg(feature = "experimental-search")]
            search_index,
            thread_subscriptions_catchup,
            self.media_fetcher.clone(),
        )
        .await;

        debug!("Done building the Client");

        Ok(Client { inner })
    }
}

/// Creates a server name from a user supplied string. The string is first
/// sanitized by removing whitespace, the http(s) scheme and any trailing
/// slashes before being parsed.
pub fn sanitize_server_name(s: &str) -> crate::Result<OwnedServerName, IdParseError> {
    ServerName::parse(
        s.trim().trim_start_matches("http://").trim_start_matches("https://").trim_end_matches('/'),
    )
}

#[allow(clippy::unused_async, unused)] // False positive when building with !sqlite & !indexeddb
async fn build_store_config(
    builder_config: BuilderStoreConfig,
    cross_process_store_config: &CrossProcessLockConfig,
) -> Result<StoreConfig, ClientBuildError> {
    #[allow(clippy::infallible_destructuring_match)]
    let store_config = match builder_config {
        #[cfg(feature = "sqlite")]
        BuilderStoreConfig::Sqlite { config, cache_path } => {
            let config_with_cache_path = if let Some(ref cache_path) = cache_path {
                config.clone().path(cache_path)
            } else {
                config.clone()
            };

            #[cfg(feature = "e2e-encryption")]
            let (state_store, event_cache_store, media_store, crypto_store) = try_join!(
                matrix_sdk_sqlite::SqliteStateStore::open_with_config(&config),
                matrix_sdk_sqlite::SqliteEventCacheStore::open_with_config(&config_with_cache_path),
                matrix_sdk_sqlite::SqliteMediaStore::open_with_config(&config_with_cache_path),
                matrix_sdk_sqlite::SqliteCryptoStore::open_with_config(&config),
            )?;
            #[cfg(not(feature = "e2e-encryption"))]
            let (state_store, event_cache_store, media_store) = try_join!(
                matrix_sdk_sqlite::SqliteStateStore::open_with_config(&config),
                matrix_sdk_sqlite::SqliteEventCacheStore::open_with_config(&config_with_cache_path),
                matrix_sdk_sqlite::SqliteMediaStore::open_with_config(&config),
            )?;
            let store_config = StoreConfig::new(cross_process_store_config.clone())
                .state_store(state_store)
                .event_cache_store(event_cache_store)
                .media_store(media_store);

            #[cfg(feature = "e2e-encryption")]
            let store_config = store_config.crypto_store(crypto_store);

            store_config
        }

        #[cfg(feature = "indexeddb")]
        BuilderStoreConfig::IndexedDb { name, passphrase } => {
            build_indexeddb_store_config(
                &name,
                passphrase.as_deref(),
                cross_process_store_config.clone(),
            )
            .await?
        }

        BuilderStoreConfig::Custom(config) => config,
    };
    Ok(store_config)
}

// The indexeddb stores only implement `IntoStateStore` and `IntoCryptoStore` on
// wasm32, so this only compiles there.
#[cfg(all(target_family = "wasm", feature = "indexeddb"))]
async fn build_indexeddb_store_config(
    name: &str,
    passphrase: Option<&str>,
    cross_process_store_config: CrossProcessLockConfig,
) -> Result<StoreConfig, ClientBuildError> {
    let stores = matrix_sdk_indexeddb::IndexeddbStores::open(name, passphrase).await?;
    let store_config = StoreConfig::new(cross_process_store_config)
        .state_store(stores.state)
        .event_cache_store(stores.event_cache)
        .media_store(stores.media);

    #[cfg(feature = "e2e-encryption")]
    let store_config = store_config.crypto_store(stores.crypto);

    Ok(store_config)
}

#[cfg(all(not(target_family = "wasm"), feature = "indexeddb"))]
#[allow(clippy::unused_async)]
async fn build_indexeddb_store_config(
    _name: &str,
    _passphrase: Option<&str>,
    _cross_process_store_config: CrossProcessLockConfig,
) -> Result<StoreConfig, ClientBuildError> {
    panic!("the IndexedDB is only available on the 'wasm32' arch")
}

#[derive(Clone, Debug)]
enum HttpConfig {
    #[cfg(not(target_family = "wasm"))]
    Settings(HttpSettings),
    Custom(reqwest::Client),
}

#[cfg(not(target_family = "wasm"))]
impl HttpConfig {
    fn settings(&mut self) -> &mut HttpSettings {
        match self {
            Self::Settings(s) => s,
            Self::Custom(_) => {
                *self = Self::default();
                match self {
                    Self::Settings(s) => s,
                    Self::Custom(_) => unreachable!(),
                }
            }
        }
    }
}

impl Default for HttpConfig {
    fn default() -> Self {
        #[cfg(not(target_family = "wasm"))]
        return Self::Settings(HttpSettings::default());

        #[cfg(target_family = "wasm")]
        return Self::Custom(reqwest::Client::new());
    }
}

#[derive(Clone)]
enum BuilderStoreConfig {
    #[cfg(feature = "sqlite")]
    Sqlite {
        config: SqliteStoreConfig,
        cache_path: Option<PathBuf>,
    },
    #[cfg(feature = "indexeddb")]
    IndexedDb {
        name: String,
        passphrase: Option<String>,
    },
    Custom(StoreConfig),
}

#[cfg(not(tarpaulin_include))]
impl fmt::Debug for BuilderStoreConfig {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        #[allow(clippy::infallible_destructuring_match)]
        match self {
            #[cfg(feature = "sqlite")]
            Self::Sqlite { config, cache_path, .. } => f
                .debug_struct("Sqlite")
                .field("config", config)
                .field("cache_path", cache_path)
                .finish_non_exhaustive(),

            #[cfg(feature = "indexeddb")]
            Self::IndexedDb { name, .. } => {
                f.debug_struct("IndexedDb").field("name", name).finish_non_exhaustive()
            }

            Self::Custom(store_config) => f.debug_tuple("Custom").field(store_config).finish(),
        }
    }
}

/// Errors that can happen in [`ClientBuilder::build`].
#[derive(Debug, Error)]
pub enum ClientBuildError {
    /// No homeserver or user ID was configured
    #[error("No homeserver or user ID was configured")]
    MissingHomeserver,

    /// The supplied server name was invalid.
    #[error("The supplied server name is invalid")]
    InvalidServerName,

    /// Resolving the homeserver requires a `.well-known/matrix/client` lookup,
    /// but those were disabled with
    /// [`ClientBuilder::disable_well_known_lookup`].
    #[error(
        "Homeserver discovery requires a .well-known lookup, which was disabled; \
         use `ClientBuilder::homeserver_url` instead"
    )]
    WellKnownLookupDisabled,

    /// Error looking up the .well-known endpoint on auto-discovery
    #[error("Error looking up the .well-known endpoint on auto-discovery")]
    AutoDiscovery(Box<FromHttpResponseError<RumaApiError>>),

    /// Error when building the sliding sync version.
    #[error(transparent)]
    SlidingSyncVersion(#[from] crate::sliding_sync::VersionBuilderError),

    /// An error encountered when trying to parse the homeserver url.
    #[error(transparent)]
    Url(#[from] url::ParseError),

    /// Error doing an HTTP request.
    #[error(transparent)]
    Http(#[from] HttpError),

    /// Error opening the indexeddb store.
    #[cfg(feature = "indexeddb")]
    #[error(transparent)]
    IndexeddbStore(#[from] matrix_sdk_indexeddb::OpenStoreError),

    /// Error opening the sqlite store.
    #[cfg(feature = "sqlite")]
    #[error(transparent)]
    SqliteStore(#[from] matrix_sdk_sqlite::OpenStoreError),
}

// The http mocking library is not supported for wasm32
#[cfg(all(test, not(target_family = "wasm")))]
pub(crate) mod tests {
    use std::{future, iter, net::SocketAddr, sync::Mutex as StdMutex};

    use assert_matches::assert_matches;
    use assert_matches2::assert_let;
    use matrix_sdk_test::{async_test, test_json};
    use reqwest::dns::{Addrs, Name, Resolve, Resolving};
    use serde_json::{Value as JsonValue, json_internal};
    use url::Url;
    use wiremock::{
        Mock, MockServer, ResponseTemplate,
        matchers::{header, method, path},
    };

    use super::*;
    use crate::sliding_sync::Version as SlidingSyncVersion;

    #[test]
    fn test_sanitize_server_name() {
        assert_eq!(sanitize_server_name("matrix.org").unwrap().as_str(), "matrix.org");
        assert_eq!(sanitize_server_name("https://matrix.org").unwrap().as_str(), "matrix.org");
        assert_eq!(sanitize_server_name("http://matrix.org").unwrap().as_str(), "matrix.org");
        assert_eq!(
            sanitize_server_name("https://matrix.server.org").unwrap().as_str(),
            "matrix.server.org"
        );
        assert_eq!(
            sanitize_server_name("https://matrix.server.org/").unwrap().as_str(),
            "matrix.server.org"
        );
        assert_eq!(
            sanitize_server_name("  https://matrix.server.org// ").unwrap().as_str(),
            "matrix.server.org"
        );
        assert_matches!(sanitize_server_name("https://matrix.server.org/something"), Err(_))
    }

    // Note: Due to a limitation of the http mocking library the following tests all
    // supply an http:// url, to `server_name_or_homeserver_url` rather than the plain server name,
    // otherwise  the builder will prepend https:// and the request will fail. In practice, this
    // isn't a problem as the builder first strips the scheme and then checks if the
    // name is a valid server name, so it is a close enough approximation.

    #[async_test]
    async fn test_discovery_invalid_server() {
        // Given a new client builder.
        let mut builder = ClientBuilder::new();

        // When building a client with an invalid server name.
        builder = builder.server_name_or_homeserver_url("⚠️ This won't work 🚫");
        let error = builder.build().await.unwrap_err();

        // Then the operation should fail due to the invalid server name.
        assert_matches!(error, ClientBuildError::InvalidServerName);
    }

    #[async_test]
    async fn test_discovery_no_server() {
        // Given a new client builder.
        let mut builder = ClientBuilder::new();

        // When building a client with a valid server name that doesn't exist.
        builder = builder.server_name_or_homeserver_url("localhost:3456");
        let error = builder.build().await.unwrap_err();

        // Then the operation should fail with an HTTP error.
        println!("{error}");
        assert_matches!(error, ClientBuildError::Http(_));
    }

    #[async_test]
    async fn test_discovery_web_server() {
        // Given a random web server that isn't a Matrix homeserver or hosting the
        // well-known file for one.
        let server = MockServer::start().await;
        let mut builder = ClientBuilder::new();

        // When building a client with the server's URL.
        builder = builder.server_name_or_homeserver_url(server.uri());
        let error = builder.build().await.unwrap_err();

        // Then the operation should fail with a server discovery error.
        assert_let!(ClientBuildError::AutoDiscovery(e) = error);
        assert_matches!(*e, FromHttpResponseError::Server(_));
    }

    #[async_test]
    async fn test_discovery_direct_legacy() {
        // Given a homeserver without a well-known file.
        let homeserver = make_mock_homeserver().await;
        let mut builder = ClientBuilder::new();

        // When building a client with the server's URL.
        builder = builder.server_name_or_homeserver_url(homeserver.uri());
        let _client = builder.build().await.unwrap();

        // Then a client should be built with native support for sliding sync.
        assert!(_client.sliding_sync_version().is_native());
    }

    #[async_test]
    async fn test_discovery_well_known_parse_error() {
        // Given a base server with a well-known file that has errors.
        let server = MockServer::start().await;
        let homeserver = make_mock_homeserver().await;
        let mut builder = ClientBuilder::new();

        let well_known = make_well_known_json(&homeserver.uri());
        let bad_json = well_known.to_string().replace(',', "");
        Mock::given(method("GET"))
            .and(path("/.well-known/matrix/client"))
            .respond_with(ResponseTemplate::new(200).set_body_json(bad_json))
            .mount(&server)
            .await;

        // When building a client with the base server.
        builder = builder.server_name_or_homeserver_url(server.uri());
        let error = builder.build().await.unwrap_err();

        // Then the operation should fail due to the well-known file's contents.
        assert_let!(ClientBuildError::AutoDiscovery(e) = error);
        assert_matches!(*e, FromHttpResponseError::Deserialization(_));
    }

    #[async_test]
    async fn test_discovery_well_known_legacy() {
        // Given a base server with a well-known file that points to a homeserver that
        // doesn't support sliding sync.
        let server = MockServer::start().await;
        let homeserver = make_mock_homeserver().await;
        let mut builder = ClientBuilder::new();

        Mock::given(method("GET"))
            .and(path("/.well-known/matrix/client"))
            .respond_with(
                ResponseTemplate::new(200).set_body_json(make_well_known_json(&homeserver.uri())),
            )
            .mount(&server)
            .await;

        // When building a client with the base server.
        builder = builder.server_name_or_homeserver_url(server.uri());
        let client = builder.build().await.unwrap();

        // Then a client should be built with native support for sliding sync.
        // It's native support because it's the default. Nothing is checked here.
        assert!(client.sliding_sync_version().is_native());
    }

    #[async_test]
    async fn test_discovery_server_name_with_well_known_lookup_disabled() {
        // Given a new client builder configured with a server name only.
        let builder = ClientBuilder::new()
            .server_name(&ServerName::parse("example.org").unwrap())
            .disable_well_known_lookup(true);

        // When building it. Note that no mock server is involved: the whole point is
        // that not a single request is made.
        let error = builder.build().await.unwrap_err();

        // Then the operation should fail, rather than assume that the server name is
        // also the homeserver.
        assert_matches!(error, ClientBuildError::WellKnownLookupDisabled);

        // And the same goes for its insecure counterpart.
        let error = ClientBuilder::new()
            .insecure_server_name_no_tls(&ServerName::parse("example.org").unwrap())
            .disable_well_known_lookup(true)
            .build()
            .await
            .unwrap_err();

        assert_matches!(error, ClientBuildError::WellKnownLookupDisabled);
    }

    #[async_test]
    async fn test_discovery_server_name_or_url_with_well_known_lookup_disabled() {
        // Given a homeserver that also serves a well-known file, which must never be
        // requested. `MockServer` verifies the expectation when it is dropped.
        let homeserver = make_mock_homeserver().await;
        Mock::given(method("GET"))
            .and(path("/.well-known/matrix/client"))
            .respond_with(
                ResponseTemplate::new(200).set_body_json(make_well_known_json(&homeserver.uri())),
            )
            .named("well-known mock")
            .expect(0)
            .mount(&homeserver)
            .await;

        // When building a client with its URL.
        let client = ClientBuilder::new()
            .server_name_or_homeserver_url(homeserver.uri())
            .disable_well_known_lookup(true)
            .build()
            .await
            .unwrap();

        // Then the homeserver should have been resolved through the
        // `/_matrix/client/versions` check alone.
        assert_eq!(client.homeserver().as_str().trim_end_matches('/'), homeserver.uri());
    }

    #[async_test]
    async fn test_homeserver_url_never_contacts_the_server_name() {
        // Given a deployment where the server name serves a well-known that points to
        // the underlying homeserver (hosted on an unrelated domain in this test).
        let mock_server = MockServer::start().await;
        let address = *mock_server.address();
        let port = address.port();
        let server_name = format!("servername.com:{port}");
        let homeserver_name = format!("matrix.server.com:{port}");

        // Every host is resolved to the mock server and recorded.
        let resolver = Arc::new(RecordingResolver { address, hosts: StdMutex::new(Vec::new()) });
        let http_client =
            reqwest::Client::builder().dns_resolver(resolver.clone()).build().unwrap();

        Mock::given(method("GET"))
            .and(path("/.well-known/matrix/client"))
            .and(header("host", server_name.as_str()))
            .respond_with(
                ResponseTemplate::new(200)
                    .set_body_json(make_well_known_json(&format!("http://{homeserver_name}"))),
            )
            .mount(&mock_server)
            .await;

        Mock::given(method("GET"))
            .and(path("/_matrix/client/versions"))
            .and(header("host", homeserver_name.as_str()))
            .respond_with(ResponseTemplate::new(200).set_body_json(&*test_json::VERSIONS))
            .mount(&mock_server)
            .await;

        // When building a client using server_name_or_homeserver_url with the
        // homeserver's URL as the string.
        let client = ClientBuilder::new()
            .http_client(http_client)
            .server_name_or_homeserver_url(format!("http://{homeserver_name}"))
            .build()
            .await
            .unwrap();

        // Then homeserver should be the only host that was contacted while building.
        let resolved_hosts = resolver.hosts.lock().unwrap();
        assert!(!resolved_hosts.is_empty(), "The homeserver should have been contacted");
        assert!(
            resolved_hosts.iter().all(|host| host == "matrix.server.com"),
            "A connection was attempted to an unexpected host: {resolved_hosts:?}"
        );
        assert_eq!(client.homeserver(), Url::parse(&format!("http://{homeserver_name}")).unwrap());
        assert_eq!(client.server(), None);
    }

    #[async_test]
    async fn test_sliding_sync_discover_native() {
        // Given a homeserver with a `/versions` file.
        let homeserver = make_mock_homeserver().await;
        let mut builder = ClientBuilder::new();

        // When building the client with sliding sync to auto-discover the
        // native version.
        builder = builder
            .server_name_or_homeserver_url(homeserver.uri())
            .sliding_sync_version_builder(SlidingSyncVersionBuilder::DiscoverNative);

        let client = builder.build().await.unwrap();

        // Then, sliding sync has the correct native version.
        assert_matches!(client.sliding_sync_version(), SlidingSyncVersion::Native);
    }

    #[async_test]
    #[cfg(feature = "e2e-encryption")]
    async fn test_set_up_decryption_trust_requirement_cross_signed() {
        let homeserver = make_mock_homeserver().await;
        let builder = ClientBuilder::new()
            .server_name_or_homeserver_url(homeserver.uri())
            .with_decryption_settings(DecryptionSettings {
                sender_device_trust_requirement: TrustRequirement::CrossSigned,
            });

        let client = builder.build().await.unwrap();
        assert_matches!(
            client.base_client().decryption_settings.sender_device_trust_requirement,
            TrustRequirement::CrossSigned
        );
    }

    #[async_test]
    #[cfg(feature = "e2e-encryption")]
    async fn test_set_up_decryption_trust_requirement_untrusted() {
        let homeserver = make_mock_homeserver().await;

        let builder = ClientBuilder::new()
            .server_name_or_homeserver_url(homeserver.uri())
            .with_decryption_settings(DecryptionSettings {
                sender_device_trust_requirement: TrustRequirement::Untrusted,
            });

        let client = builder.build().await.unwrap();
        assert_matches!(
            client.base_client().decryption_settings.sender_device_trust_requirement,
            TrustRequirement::Untrusted
        );
    }

    #[async_test]
    async fn test_cross_process_store_locks_holder_name() {
        {
            let homeserver = make_mock_homeserver().await;
            let client =
                ClientBuilder::new().homeserver_url(homeserver.uri()).build().await.unwrap();

            assert_let!(
                CrossProcessLockConfig::MultiProcess { holder_name } =
                    client.cross_process_lock_config()
            );
            assert_eq!(holder_name, "main");
        }

        {
            let homeserver = make_mock_homeserver().await;
            let client = ClientBuilder::new()
                .homeserver_url(homeserver.uri())
                .cross_process_store_config(CrossProcessLockConfig::multi_process("foo"))
                .build()
                .await
                .unwrap();

            assert_let!(
                CrossProcessLockConfig::MultiProcess { holder_name } =
                    client.cross_process_lock_config()
            );
            assert_eq!(holder_name, "foo");
        }
    }

    /* Helper functions */

    async fn make_mock_homeserver() -> MockServer {
        let homeserver = MockServer::start().await;
        Mock::given(method("GET"))
            .and(path("/_matrix/client/versions"))
            .respond_with(ResponseTemplate::new(200).set_body_json(&*test_json::VERSIONS))
            .mount(&homeserver)
            .await;
        Mock::given(method("GET"))
            .and(path("/_matrix/client/r0/login"))
            .respond_with(ResponseTemplate::new(200).set_body_json(&*test_json::LOGIN_TYPES))
            .mount(&homeserver)
            .await;
        homeserver
    }

    fn make_well_known_json(homeserver_url: &str) -> JsonValue {
        ::serde_json::Value::Object({
            let mut object = ::serde_json::Map::new();
            let _ = object.insert(
                "m.homeserver".into(),
                json_internal!({
                    "base_url": homeserver_url
                }),
            );

            object
        })
    }

    /// A DNS resolver that records the hostname of every lookup it is asked to
    /// make, and resolves all of them to the same address.
    #[derive(Debug)]
    struct RecordingResolver {
        address: SocketAddr,
        hosts: StdMutex<Vec<String>>,
    }

    impl Resolve for RecordingResolver {
        fn resolve(&self, name: Name) -> Resolving {
            self.hosts.lock().unwrap().push(name.as_str().to_owned());

            let addrs: Addrs = Box::new(iter::once(self.address));
            Box::pin(future::ready(Ok(addrs)))
        }
    }
}