doc(event cache): Document the redecryptor

crates/matrix-sdk/src/event_cache/redecryptor.rs

@@ -12,14 +12,63 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//! The REDECRYPTOR is a layer that handles redecryption of events in case we
-//! couldn't decrypt them imediatelly
+//! The Redecryptor (Rd) is a layer and long-running background task which
+//! handles redecryption of events in case we couldn't decrypt them imediatelly.
+//!
+//! Rd listens to the OlmMachine for received room keys. If a new room key has
+//! been received it attempts to find any UTDs in the [`EventCache`]. If Rd
+//! decrypts any UTDs from the event cache it will replace the events in the
+//! cache and send out new [`RoomEventCacheUpdates`] to any of its listeners.
+//!
+//! There's an additional gotcha, the [`OlmMachine`] might get recreated by
+//! calls to [`BaseClient::regenerate_olm()`]. When this happens we will receive
+//! a `None` on the room keys stream and we need to re-listen to it.
+//!
+//! Another gotcha is that room keys might be received on another process if the
+//! Client is operating on a iOS device. A separate process is used in this case
+//! to receive push notifications. In this case the room key will be received
+//! and Rd won't get notified about it. To work around this decryption requests
+//! can be explicitly sent to Rd.
+//!
+//!
+//!                              ┌─────────────┐
+//!                              │             │
+//!                  ┌───────────┤  Timeline   │◄────────────┐
+//!                  │           │             │             │
+//!                  │           └─────▲───────┘             │
+//!                  │                 │                     │
+//!                  │                 │                     │
+//!                  │                 │                     │
+//!              Decryption            │                 Redecryptor
+//!                request             │                   report
+//!                  │       RoomEventCacheUpdates           │         
+//!                  │                 │                     │
+//!                  │                 │                     │
+//!                  │      ┌──────────┴────────────┐        │
+//!                  │      │                       │        │
+//!                  └──────►      Redecryptor      │────────┘
+//!                         │                       │
+//!                         └───────────▲───────────┘
+//!                                     │
+//!                                     │
+//!                                     │
+//!                         Received room keys stream
+//!                                     │
+//!                                     │
+//!                                     │
+//!                             ┌───────┴──────┐
+//!                             │              │
+//!                             │  OlmMachine  │
+//!                             │              │
+//!                             └──────────────┘
 
 use std::{collections::BTreeSet, pin::Pin, sync::Weak};
 
 use as_variant::as_variant;
 use futures_core::Stream;
 use futures_util::{StreamExt, pin_mut};
+#[cfg(doc)]
+use matrix_sdk_base::{BaseClient, crypto::OlmMachine};
 use matrix_sdk_base::{
     crypto::{store::types::RoomKeyInfo, types::events::room::encrypted::EncryptedEvent},
     deserialized_responses::{DecryptedRoomEvent, TimelineEvent, TimelineEventKind},
@@ -44,6 +93,13 @@ pub struct DecryptionRetryRequest {
 type SessionId<'a> = &'a str;
 
 impl EventCache {
+    /// Retrieve a set of events that we weren't able to decrypt.
+    ///
+    /// # Arguments
+    ///
+    /// * `room_id` - The ID of the room where the events were sent to.
+    /// * `session_id` - The unique ID of the room key that was used to encrypt
+    ///   the event.
     async fn get_utds(
         &self,
         room_id: &RoomId,
@@ -70,6 +126,17 @@ impl EventCache {
         Ok(events.into_iter().filter_map(map_timeline_event).collect())
     }
 
+    /// Handle a chunk of events that we were previously unable to decrypt but
+    /// have now successfully decrypted.
+    ///
+    /// This function will replace the existing UTD events in memory and the
+    /// store and send out a [`RoomEventCacheUpdate`] for the newly
+    /// decrypted events.
+    ///
+    /// # Arguments
+    ///
+    /// * `room_id` - The ID of the room where the events were sent to.
+    /// * `events` - A chunk of events that were successfully decrypted.
     #[instrument(skip_all, fields(room_id))]
     async fn on_resolved_utds(
         &self,
@@ -110,12 +177,17 @@ impl EventCache {
         Ok(())
     }
 
+    /// Attempt to decrypt a single event.
     async fn decrypt_event(
         &self,
         room_id: &RoomId,
         event: &Raw<EncryptedEvent>,
     ) -> Option<DecryptedRoomEvent> {
         let client = self.inner.client().ok()?;
+        // TODO: Do we need to use the `Room` object to decrypt these events so we can
+        // calculate if the event should count as a notification, i.e. get the push
+        // actions. I thing we do, what happens if the room can't be found? We fallback
+        // to this?
         let machine = client.olm_machine().await;
         let machine = machine.as_ref()?;
 
@@ -138,7 +210,10 @@ impl EventCache {
     ) -> Result<(), EventCacheError> {
         trace!("Retrying to decrypt");
 
+        // Get all the relevant UTDs.
         let events = self.get_utds(room_id, session_id).await?;
+
+        // Let's attempt to decrypt them them.
         let mut decrypted_events = Vec::with_capacity(events.len());
 
         for (event_id, event) in events {
@@ -149,6 +224,8 @@ impl EventCache {
             }
         }
 
+        // Replace the events and notify listeners that UTDs have been replaced with
+        // decrypted events.
         self.on_resolved_utds(room_id, decrypted_events).await?;
 
         Ok(())
@@ -167,6 +244,10 @@ impl Drop for Redecryptor {
 }
 
 impl Redecryptor {
+    /// Create a new [`Redecryptor`].
+    ///
+    /// This creates a task that listens to various streams and attempts to
+    /// redecrypt UTDs that can be found inside the [`EventCache`].
     pub(super) fn new(cache: Weak<EventCacheInner>) -> Self {
         let (request_decryption_sender, receiver) = tokio::sync::mpsc::unbounded_channel();
 
@@ -186,6 +267,10 @@ impl Redecryptor {
         });
     }
 
+    /// (Re)-subscribe to the room key stream from the [`OlmMachine`].
+    ///
+    /// This needs to happen any time this stream returns a `None` meaning that
+    /// the sending part of the stream has been dropped.
     async fn subscribe_to_room_key_stream(
         cache: &Weak<EventCacheInner>,
     ) -> Option<impl Stream<Item = Result<Vec<RoomKeyInfo>, BroadcastStreamRecvError>>> {
@@ -212,6 +297,8 @@ impl Redecryptor {
 
         loop {
             tokio::select! {
+                // An explicit request, presumably from the timeline, has been received to decrypt
+                // events that were encrypted with a certain room key.
                 Some(request) = decryption_request_stream.next() => {
                         let Some(cache) = Self::upgrade_event_cache(cache) else {
                             break false;
@@ -224,6 +311,8 @@ impl Redecryptor {
                                 .inspect_err(|e| warn!("Error redecrypting after an explicit request was received {e:?}"));
                         }
                 }
+                // The room key stream from the OlmMachine. Needs to be recreated every time we
+                // receive a `None` from the stream.
                 room_keys = room_key_stream.next() => {
                     match room_keys {
                         Some(Ok(room_keys)) => {
@@ -241,6 +330,8 @@ impl Redecryptor {
                         Some(Err(_)) => {
                             todo!("Handle lagging here, how?")
                         },
+                        // The stream got closed, this could mean that our OlmMachine got
+                        // regenerated, let's return true and try to recreate the stream.
                         None => {
                             break true
                         }
@@ -255,6 +346,9 @@ impl Redecryptor {
         cache: Weak<EventCacheInner>,
         decryption_request_stream: UnboundedReceiverStream<DecryptionRetryRequest>,
     ) {
+        // We pin the decryption request stream here since that one doesn't need to be
+        // recreated and we don't want to miss messages coming from the stream
+        // while recreating it unnecessarily.
         pin_mut!(decryption_request_stream);
 
         while Self::redecryption_loop(&cache, &mut decryption_request_stream).await {