Cache inbound group sessions when decrypting

MatrixSDK/Crypto/MXOlmDevice.m

@@ -29,6 +29,8 @@
 #import "MXKeyProvider.h"
 #import "MXRawDataKey.h"
 
+NSInteger const kMXInboundGroupSessionCacheSize = 100;
+
 @interface MXOlmDevice () <OLMKitPickleKeyDelegate>
 {
     // The OLMKit utility instance.
@@ -53,8 +55,10 @@ @interface MXOlmDevice () <OLMKitPickleKeyDelegate>
 // The store where crypto data is saved.
 @property (nonatomic, readonly) id<MXCryptoStore> store;
 
-@end
+// Cache to avoid refetching unchanged sessions from the crypto store
+@property (nonatomic, strong) MXLRUCache *inboundGroupSessionCache;
 
+@end
 
 @implementation MXOlmDevice
 @synthesize store;
@@ -98,6 +102,8 @@ - (instancetype)initWithStore:(id<MXCryptoStore>)theStore
 
         _deviceCurve25519Key = olmAccount.identityKeys[@"curve25519"];
         _deviceEd25519Key = olmAccount.identityKeys[@"ed25519"];
+
+        _inboundGroupSessionCache = [[MXLRUCache alloc] initWithCapacity:kMXInboundGroupSessionCacheSize];
     }
     return self;
 }
@@ -394,7 +400,7 @@ - (BOOL)addInboundGroupSession:(NSString*)sessionId
         session.sharedHistory = sharedHistory;
     }
 
-    [store storeInboundGroupSessions:@[session]];
+    [self storeInboundGroupSessions:@[session]];
 
     return YES;
 }
@@ -441,7 +447,7 @@ - (BOOL)addInboundGroupSession:(NSString*)sessionId
         [sessions addObject:session];
     }
 
-    [store storeInboundGroupSessions:sessions];
+    [self storeInboundGroupSessions:sessions];
 
     return sessions;
 }
@@ -462,7 +468,7 @@ - (MXDecryptionResult *)decryptGroupMessage:(NSString *)body
 
     MXDecryptionResult *result;
 
-    [store performSessionOperationWithGroupSessionWithId:sessionId senderKey:senderKey block:^(MXOlmInboundGroupSession *session) {
+    [self performGroupSessionOperationWithSessionId:sessionId senderKey:senderKey block:^(MXOlmInboundGroupSession *session) {
 
         *error = [self checkInboundGroupSession:session roomId:roomId];
         if (*error)
@@ -521,6 +527,41 @@ - (MXDecryptionResult *)decryptGroupMessage:(NSString *)body
     return result;
 }
 
+- (void)performGroupSessionOperationWithSessionId:(NSString*)sessionId senderKey:(NSString*)senderKey block:(void (^)(MXOlmInboundGroupSession *inboundGroupSession))block
+{
+    // Based on a feature flag megolm decryption will either fetch a group session from the store on every decryption,
+    // or (if the flag is enabled) it will use LRU cache to avoid refetching unchanged sessions.
+    //
+    // Additionally the duration of each variant is tracked in analytics (if configured and enabled by the user)
+    // to allow performance comparison
+    //
+    // LRU cache variant will eventually become the default implementation if proved stable.
+
+    BOOL enableCache = MXSDKOptions.sharedInstance.enableGroupSessionCache;
+    NSString *operation = enableCache ? @"megolm.decrypt.cache" : @"megolm.decrypt.store";
+    StopDurationTracking stopTracking = [MXSDKOptions.sharedInstance.analyticsDelegate startDurationTrackingForName:@"MXOlmDevice" operation:operation];
+
+    if (enableCache)
+    {
+        __block MXOlmInboundGroupSession *session;
+        @synchronized (self.inboundGroupSessionCache)
+        {
+            session = (MXOlmInboundGroupSession *)[self.inboundGroupSessionCache get:sessionId];
+            if (!session)
+            {
+                session = [store inboundGroupSessionWithId:sessionId andSenderKey:senderKey];
+                [self.inboundGroupSessionCache put:sessionId object:session];
+            }
+        }
+        block(session);
+    }
+    else
+    {
+        [store performSessionOperationWithGroupSessionWithId:sessionId senderKey:senderKey block:block];
+    }
+    stopTracking();
+}
+
 - (void)resetReplayAttackCheckInTimeline:(NSString*)timeline
 {
     [inboundGroupSessionMessageIndexes removeObjectForKey:timeline];
@@ -622,6 +663,21 @@ - (NSDictionary*)getInboundGroupSessionKey:(NSString*)roomId senderKey:(NSString
     return inboundGroupSessionKey;
 }
 
+- (void)storeInboundGroupSessions:(NSArray <MXOlmInboundGroupSession *>*)sessions
+{
+    [store storeInboundGroupSessions:sessions];
+    if (MXSDKOptions.sharedInstance.enableGroupSessionCache)
+    {
+        @synchronized (self.inboundGroupSessionCache)
+        {
+            for (MXOlmInboundGroupSession *session in sessions)
+            {
+                [self.inboundGroupSessionCache put:session.session.sessionIdentifier object:session];
+            }
+        }
+    }
+}
+
 
 #pragma mark - OLMKitPickleKeyDelegate
 

MatrixSDK/MXSDKOptions.h

@@ -214,6 +214,14 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nonatomic) BOOL enableCryptoV2;
 
+/**
+ Enable performance optimization where inbound group sessions are cached between decryption of events
+ rather than fetched from the store every time.
+
+ @remark The value is set randomly between YES / NO to perform a very basic A/B test
+ */
+@property (nonatomic) BOOL enableGroupSessionCache;
+
 #endif
 
 @end

MatrixSDK/MXSDKOptions.m

@@ -58,6 +58,10 @@ - (instancetype)init
         #if DEBUG
         _enableCryptoV2 = NO;
         #endif
+
+        // The value is set randomly between YES / NO to perform a very basic A/B test
+        // measured by `analytics` (if set and enabled)
+        _enableGroupSessionCache = arc4random_uniform(2) == 1;
     }
 
     return self;

MatrixSDK/MXSession.m

@@ -1406,6 +1406,7 @@ - (void)serverSyncWithServerTimeout:(NSUInteger)serverTimeout
     dispatch_group_t initialSyncDispatchGroup = dispatch_group_create();
 
     __block MXTaskProfile *syncTaskProfile;
+    __block StopDurationTracking stopDurationTracking;
     __block MXSyncResponse *syncResponse;
     __block BOOL useLiveResponse = YES;
 
@@ -1437,6 +1438,13 @@ - (void)serverSyncWithServerTimeout:(NSUInteger)serverTimeout
             BOOL isInitialSync = !self.isEventStreamInitialised;
             MXTaskProfileName taskName = isInitialSync ? MXTaskProfileNameStartupInitialSync : MXTaskProfileNameStartupIncrementalSync;
             syncTaskProfile = [MXSDKOptions.sharedInstance.profiler startMeasuringTaskWithName:taskName];
+            if (isInitialSync) {
+                // Temporarily tracking performance both by `MXSDKOptions.sharedInstance.profiler` (manually measuring time)
+                // and `MXSDKOptions.sharedInstance.analyticsDelegate` (delegating to performance monitoring tool).
+                // This ambiguity will be resolved in the future
+                NSString *operation = MXSDKOptions.sharedInstance.enableGroupSessionCache ? @"initialSync.enableGroupSessionCache" : @"initialSync.diableGroupSessionCache";
+                stopDurationTracking = [MXSDKOptions.sharedInstance.analyticsDelegate startDurationTrackingForName:@"MXSession" operation:operation];
+            }
         }
 
         NSString * streamToken = self.store.eventStreamToken;
@@ -1479,6 +1487,9 @@ - (void)serverSyncWithServerTimeout:(NSUInteger)serverTimeout
             syncTaskProfile.units = syncResponse.rooms.join.count;
 
             [MXSDKOptions.sharedInstance.profiler stopMeasuringTaskWithProfile:syncTaskProfile];
+            if (stopDurationTracking) {
+                stopDurationTracking();
+            }
         }
 
         BOOL isInitialSync = !self.isEventStreamInitialised;

MatrixSDK/Utils/MXAnalyticsDelegate.h

@@ -20,6 +20,12 @@
 #import "MXCallHangupEventContent.h"
 #import "MXTaskProfile.h"
 
+/**
+ Callback function to cancel ongoing duration tracking
+ started by `[MXAnalyticsDelegate startDurationTracking]`
+ */
+typedef void (^StopDurationTracking)(void);
+
 NS_ASSUME_NONNULL_BEGIN
 
 /**
@@ -46,6 +52,21 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)trackDuration:(NSInteger)milliseconds name:(MXTaskProfileName)name units:(NSUInteger)units;
 
+/**
+ Start tracking the duration of a task and manually cancel when finished using the return handle
+
+ @note This method is similar to `trackDuration`, but instead of passing the measured duraction
+       as a parameter, it relies on the implementation of `MXAnalyticsDelegate` to perform the
+       measurements.
+
+ @param name Name of the entity being measured (e.g. `RoomsViewController` or `Crypto`)
+ @param operation Short code identifying the type of operation measured (e.g. `viewDidLoad` or `decrypt`)
+
+
+ @return Handle that can be used to stop the performance tracking
+ */
+- (StopDurationTracking)startDurationTrackingForName:(NSString *)name operation:(NSString *)operation;
+
 /**
  Report that a call has started.
 

changelog.d/pr-1566.change

@@ -0,0 +1 @@
+Crypto: Cache inbound group sessions when decrypting