feat: add automatic channel joining for bot accounts

Bot accounts need to be channel members to access channel information.
This update detects 403 permission errors and automatically joins the bot
to the target channel if it's not already a member.

Changes:
- Add isChannelMember() method to check bot membership status
- Add joinChannel() method to automatically join bot to channels
- Update bridge startup to detect 403 errors and auto-join target channel
- Add comprehensive logging for bot membership operations
- Improve error messages for permission-related issues

Fixes issue where bot accounts could not access target channels they
weren't members of, resulting in 403 Forbidden errors on startup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: cln-io

src/bridge.ts

@@ -98,12 +98,41 @@ export class MattermostBridge {
 
       // Resolve target channel info
       console.log(`${this.LOG_PREFIX} ${emoji('🔍')}[${this.config.right.name}] Looking up channel (${this.targetChannelId})[${this.targetChannelId}]`.trim());
-      this.targetChannelInfo = await this.rightClient.getChannelById(this.targetChannelId);
-      
+
+      // Check if we can access the channel, and if not (403 error), try to join it first
+      try {
+        this.targetChannelInfo = await this.rightClient.getChannelById(this.targetChannelId);
+      } catch (error: any) {
+        if (error.response?.status === 403) {
+          console.log(`${this.LOG_PREFIX} ${emoji('🔒')}[${this.config.right.name}] Permission denied - checking if bot needs to join channel...`.trim());
+
+          // Check if bot is a member
+          const isMember = await this.rightClient.isChannelMember(this.targetChannelId);
+
+          if (!isMember) {
+            console.log(`${this.LOG_PREFIX} ${emoji('🤖')}[${this.config.right.name}] Bot is not a member - attempting to join channel [${this.targetChannelId}]...`.trim());
+            const joined = await this.rightClient.joinChannel(this.targetChannelId);
+
+            if (!joined) {
+              throw new Error(`Failed to join target channel '${this.targetChannelId}' on ${this.config.right.name}. Bot may need to be manually added to the channel.`);
+            }
+
+            // Try to get channel info again after joining
+            this.targetChannelInfo = await this.rightClient.getChannelById(this.targetChannelId);
+          } else {
+            // Bot is a member but still got 403 - something else is wrong
+            throw error;
+          }
+        } else {
+          // Not a 403 error, re-throw
+          throw error;
+        }
+      }
+
       if (!this.targetChannelInfo) {
         throw new Error(`Target channel '${this.targetChannelId}' not found on ${this.config.right.name}`);
       }
-      
+
       console.log(`${this.LOG_PREFIX} ${emoji('✅')}[${this.config.right.name}] Found channel: (${this.targetChannelInfo.name})[${this.targetChannelId}]`.trim());
       console.log('');
 

src/mattermost-client.ts

@@ -251,7 +251,7 @@ export class MattermostClient {
     try {
       const response = await this.api.get(`/channels/${channelId}`);
       const channel = response.data;
-      
+
       const channelInfo: ChannelInfo = {
         id: channel.id,
         name: channel.name,
@@ -261,7 +261,7 @@ export class MattermostClient {
 
       // Cache the result
       this.channelCache.set(channelId, channelInfo);
-      
+
       return channelInfo;
     } catch (error: any) {
       if (error.response?.status === 404) {
@@ -270,11 +270,45 @@ export class MattermostClient {
         this.channelCache.set(channelId, null as any);
         return null;
       }
+      if (error.response?.status === 403) {
+        console.warn(`${this.LOG_PREFIX} ${emoji('🔒')}[${this.config.name}] Permission denied accessing channel (unknown)[${channelId}] - bot may not be a member`.trim());
+        throw error;
+      }
       console.error(`${this.LOG_PREFIX} ${emoji('❌')}[${this.config.name}] Error getting channel (unknown)[${channelId}]:`.trim(), error.response?.data || error.message);
       throw error;
     }
   }
 
+  async isChannelMember(channelId: string, userId?: string): Promise<boolean> {
+    try {
+      const userIdToCheck = userId || this.userId;
+      const response = await this.api.get(`/channels/${channelId}/members/${userIdToCheck}`);
+      console.log(`${this.LOG_PREFIX} ${emoji('✅')}[${this.config.name}] User (${userIdToCheck}) is a member of channel [${channelId}]`.trim());
+      return true;
+    } catch (error: any) {
+      if (error.response?.status === 404 || error.response?.status === 403) {
+        console.log(`${this.LOG_PREFIX} ${emoji('❌')}[${this.config.name}] User (${userId || this.userId}) is NOT a member of channel [${channelId}]`.trim());
+        return false;
+      }
+      console.error(`${this.LOG_PREFIX} ${emoji('❌')}[${this.config.name}] Error checking channel membership:`.trim(), error.response?.data || error.message);
+      throw error;
+    }
+  }
+
+  async joinChannel(channelId: string): Promise<boolean> {
+    try {
+      console.log(`${this.LOG_PREFIX} ${emoji('🚪')}[${this.config.name}] Attempting to join channel [${channelId}]...`.trim());
+      await this.api.post(`/channels/${channelId}/members`, {
+        user_id: this.userId
+      });
+      console.log(`${this.LOG_PREFIX} ${emoji('✅')}[${this.config.name}] Successfully joined channel [${channelId}]`.trim());
+      return true;
+    } catch (error: any) {
+      console.error(`${this.LOG_PREFIX} ${emoji('❌')}[${this.config.name}] Failed to join channel [${channelId}]:`.trim(), error.response?.data || error.message);
+      return false;
+    }
+  }
+
   async getChannelByName(channelName: string): Promise<Channel | null> {
     try {
       // First get teams