Crypto: key reshare does not work if there was a device change

[manuroe]

Steps to reproduce:

• Alice and Bob with devices Alice1 and Bob1 are in an encrypted room
• Alice sends a message
• Bob logs in with device Bob2
• If Bob1 makes a reshare request to Alice1 for the key of the message, Alice1 will reject it with the error [MXMegolmEncryption] reshareKey: ERROR: Never shared megolm key with this device

There is an issue in the implementation of the crypto store at https://github.com/matrix-org/matrix-ios-sdk/blob/v0.19.8/MatrixSDK/Crypto/Data/Store/MXRealmCryptoStore/MXRealmCryptoStore.m#L646.

With this "delete all" action, all existing references to those MXRealmDeviceInfo objects are set to null by RealmDB.
So, a MXRealmSharedOutboundSession object that depends on it will be reset like this:

MXRealmSharedOutboundSession {
		roomId = !ROOwNxCLjNICmOoaMy:localhost:8480;
		sessionId = xQVT9qTT4yVurxg8bgHqz9rHz1So/jetZ4FgLlf1C6Q;
		device = (null);
		messageIndex = 0;
	}

Next time we will try to check if we already shared a session toBob1 by querying MXRealmSharedOutboundSession objects by device id at this line, we will find nothing.

[manuroe]

A test to highlight the issue:

- (void)testReshareAfterADeviceUpdate
{
    [matrixSDKTestsE2EData doE2ETestWithAliceAndBobInARoom:self cryptedBob:YES warnOnUnknowDevices:NO readyToTest:^(MXSession *aliceSession, MXSession *bobSession, NSString *roomId, XCTestExpectation *expectation) {

        aliceSessionToClose = aliceSession;
        bobSessionToClose = bobSession;

        NSString *messageFromAlice = @"Hello I'm Alice!";

        MXRoom *roomFromBobPOV = [bobSession roomWithRoomId:roomId];
        MXRoom *roomFromAlicePOV = [aliceSession roomWithRoomId:roomId];
        __block NSString *messageFromAliceEventId;

        XCTAssert(roomFromBobPOV.summary.isEncrypted);
        XCTAssert(roomFromAlicePOV.summary.isEncrypted);

        [roomFromBobPOV liveTimeline:^(MXEventTimeline *liveTimeline) {

            [liveTimeline listenToEventsOfTypes:@[kMXEventTypeStringRoomMessage] onEvent:^(MXEvent *event, MXTimelineDirection direction, MXRoomState *roomState) {

                XCTAssertEqual(0, [self checkEncryptedEvent:event roomId:roomId clearMessage:messageFromAlice senderSession:aliceSession]);
                
                // Alice must be able to indicate it already shared the megolm session to bob
                NSNumber *index = [aliceSession.crypto.store messageIndexForSharedDeviceInRoomWithId:roomId
                                                                                           sessionId:event.wireContent[@"session_id"]
                                                                                              userId:bobSession.myUserId
                                                                                            deviceId:bobSession.myDeviceId];
                XCTAssertNotNil(index);
                XCTAssertEqual(index.integerValue, 0);
                
                
                // Simulate a device change. We only store the bob device again here
                MXDeviceInfo *bobDevice = [aliceSession.crypto.store deviceWithDeviceId:bobSession.myDeviceId forUser:bobSession.myUserId];
                [aliceSession.crypto.store storeDevicesForUser:bobSession.myUserId devices:@{
                    bobSession.myDeviceId: bobDevice
                }];
                
                
                // Alice must still be able to indicate it already shared the megolm session to bob
                index = [aliceSession.crypto.store messageIndexForSharedDeviceInRoomWithId:roomId
                                                                                 sessionId:event.wireContent[@"session_id"]
                                                                                    userId:bobSession.myUserId
                                                                                  deviceId:bobSession.myDeviceId];
                XCTAssertNotNil(index);                 // <- THIS FAILS
                XCTAssertEqual(index.integerValue, 0);

                [expectation fulfill];
            }];
        }];

        [roomFromAlicePOV sendTextMessage:messageFromAlice success:^(NSString *eventId) {
            messageFromAliceEventId = eventId;
        } failure:^(NSError *error) {
            XCTFail(@"Cannot set up intial test conditions - error: %@", error);
            [expectation fulfill];
        }];
    }];
}