Add debug logs to encryption prep

This extra debug logs may help isolate the cause of
https://github.com/vector-im/element-web/issues/16194.

These changes also fix a related (but most likely different) failure mode: if a
failure occurred in the `encryptionPreparation` async task, we would skip trying
to prepare in all future attempts for that room. This change ensures prep
failures are logged and we resume prep attempts on the next call from the
application.

src/crypto/algorithms/megolm.js

@@ -264,11 +264,14 @@ MegolmEncryption.prototype._ensureOutboundSession = async function(
         await Promise.all([
             (async () => {
                 // share keys with devices that we already have a session for
+                logger.debug(`Sharing keys with existing Olm sessions in ${this._roomId}`);
                 await this._shareKeyWithOlmSessions(
                     session, key, payload, olmSessions,
                 );
+                logger.debug(`Shared keys with existing Olm sessions in ${this._roomId}`);
             })(),
             (async () => {
+                logger.debug(`Sharing keys with new Olm sessions in ${this._roomId}`);
                 const errorDevices = [];
 
                 // meanwhile, establish olm sessions for devices that we don't
@@ -319,8 +322,10 @@ MegolmEncryption.prototype._ensureOutboundSession = async function(
                 } else {
                     await this._notifyFailedOlmDevices(session, key, errorDevices);
                 }
+                logger.debug(`Shared keys with new Olm sessions in ${this._roomId}`);
             })(),
             (async () => {
+                logger.debug(`Notifying blocked devices in ${this._roomId}`);
                 // also, notify blocked devices that they're blocked
                 const blockedMap = {};
                 for (const [userId, userBlockedDevices] of Object.entries(blocked)) {
@@ -336,6 +341,7 @@ MegolmEncryption.prototype._ensureOutboundSession = async function(
                 }
 
                 await this._notifyBlockedDevices(session, blockedMap);
+                logger.debug(`Notified blocked devices in ${this._roomId}`);
             })(),
         ]);
     };
@@ -348,6 +354,11 @@ MegolmEncryption.prototype._ensureOutboundSession = async function(
     // first wait for the previous share to complete
     const prom = this._setupPromise.then(prepareSession);
 
+    // Ensure any failures are logged for debugging
+    prom.catch(e => {
+        logger.error(`Failed to ensure outbound session in ${this._roomId}`, e);
+    });
+
     // _setupPromise resolves to `session` whether or not the share succeeds
     this._setupPromise = prom.then(returnSession, returnSession);
 
@@ -840,24 +851,41 @@ MegolmEncryption.prototype.prepareToEncrypt = function(room) {
         // We're already preparing something, so don't do anything else.
         // FIXME: check if we need to restart
         // (https://github.com/matrix-org/matrix-js-sdk/issues/1255)
+        const elapsedTime = Date.now() - this.encryptionPreparationMetadata.startTime
+        logger.debug(
+            `Already started preparing to encrypt for ${this._roomId} ` +
+            `${elapsedTime} ms ago, skipping`,
+        );
         return;
     }
 
     logger.debug(`Preparing to encrypt events for ${this._roomId}`);
 
+    this.encryptionPreparationMetadata = {
+        startTime: Date.now(),
+    };
     this.encryptionPreparation = (async () => {
-        const [devicesInRoom, blocked] = await this._getDevicesInRoom(room);
+        try {
+            logger.debug(`Getting devices in ${this._roomId}`);
+            const [devicesInRoom, blocked] = await this._getDevicesInRoom(room);
 
-        if (this._crypto.getGlobalErrorOnUnknownDevices()) {
+            if (this._crypto.getGlobalErrorOnUnknownDevices()) {
-            // Drop unknown devices for now.  When the message gets sent, we'll
+                // Drop unknown devices for now.  When the message gets sent, we'll
-            // throw an error, but we'll still be prepared to send to the known
+                // throw an error, but we'll still be prepared to send to the known
-            // devices.
+                // devices.
-            this._removeUnknownDevices(devicesInRoom);
+                this._removeUnknownDevices(devicesInRoom);
+            }
+
+            logger.debug(`Ensuring outbound session in ${this._roomId}`);
+            await this._ensureOutboundSession(devicesInRoom, blocked, true);
+
+            logger.debug(`Ready to encrypt events for ${this._roomId}`);
+        } catch (e) {
+            logger.error(`Failed to prepare to encrypt events for ${this._roomId}`, e);
+        } finally {
+            delete this.encryptionPreparationMetadata;
+            delete this.encryptionPreparation;
         }
-
-        await this._ensureOutboundSession(devicesInRoom, blocked, true);
-
-        delete this.encryptionPreparation;
     })();
 };