fix: prevent memory leaks in socket and event handling

- Convert anonymous streamer error handler to named function for proper cleanup
- Add explicit cleanup of connection error handler in setSocketHandlers()
- Clean up socketPlain error handler after successful STARTTLS upgrade
- Add explicit nullification of socket/stream references in close()
- Add comprehensive memory leak tests for connection cycles

Fixes potential memory leaks when creating/destroying many IMAP connections.

Author: andris9

lib/imap-flow.js

@@ -372,7 +372,8 @@ class ImapFlow extends EventEmitter {
 
         this.disableBinary = !!this.options.disableBinary;
 
-        this.streamer.on('error', err => {
+        // Named error handler for proper cleanup
+        this._streamerErrorHandler = err => {
             if (['Z_BUF_ERROR', 'ECONNRESET', 'EPIPE', 'ETIMEDOUT', 'EHOSTUNREACH'].includes(err.code)) {
                 // just close the connection, usually nothing but noise
                 this.closeAfter();
@@ -381,7 +382,8 @@ class ImapFlow extends EventEmitter {
 
             this.log.error({ err, cid: this.id });
             this.emitError(err);
-        });
+        };
+        this.streamer.on('error', this._streamerErrorHandler);
 
         // Has the `connect` method already been called
         this._connectCalled = false;
@@ -772,6 +774,12 @@ class ImapFlow extends EventEmitter {
         // Clear any existing handlers first to prevent duplicates
         this.clearSocketHandlers();
 
+        // Remove temporary connection error handler if present
+        if (this._connectErrorHandler && this.socket) {
+            this.socket.removeListener('error', this._connectErrorHandler);
+            this._connectErrorHandler = null;
+        }
+
         this._socketError =
             this._socketError ||
             (err => {
@@ -828,6 +836,12 @@ class ImapFlow extends EventEmitter {
             return;
         }
 
+        // Remove temporary connection error handler if still present
+        if (this._connectErrorHandler) {
+            this.socket.removeListener('error', this._connectErrorHandler);
+            this._connectErrorHandler = null;
+        }
+
         if (this._socketError) {
             this.socket.removeListener('error', this._socketError);
             this.socket.removeListener('tlsClientError', this._socketError);
@@ -1043,7 +1057,8 @@ class ImapFlow extends EventEmitter {
             );
             this.clearSocketHandlers();
 
-            socketPlain.once('error', err => {
+            // Store error handler for cleanup after successful upgrade
+            const socketPlainErrorHandler = err => {
                 clearTimeout(this.connectTimeout);
                 clearTimeout(this.upgradeTimeout);
                 if (!this.upgrading) {
@@ -1054,7 +1069,8 @@ class ImapFlow extends EventEmitter {
                 this.upgrading = false;
                 err.tlsFailed = true;
                 reject(err);
-            });
+            };
+            socketPlain.once('error', socketPlainErrorHandler);
 
             this.upgradeTimeout = setTimeout(() => {
                 if (!this.upgrading) {
@@ -1093,6 +1109,9 @@ class ImapFlow extends EventEmitter {
                         });
                     }
 
+                    // Clean up the plain socket error handler after successful upgrade
+                    socketPlain.removeListener('error', socketPlainErrorHandler);
+
                     return resolve(true);
                 } catch (ex) {
                     this.emitError(ex);
@@ -1607,13 +1626,15 @@ class ImapFlow extends EventEmitter {
 
             this.writeSocket = this.socket;
 
-            this.socket.on('error', err => {
+            // Store connection error handler for cleanup
+            this._connectErrorHandler = err => {
                 clearTimeout(this.connectTimeout);
                 clearTimeout(this.greetingTimeout);
                 this.closeAfter();
                 this.log.error({ err, cid: this.id });
                 reject(err);
-            });
+            };
+            this.socket.on('error', this._connectErrorHandler);
         });
     }
 
@@ -1761,11 +1782,13 @@ class ImapFlow extends EventEmitter {
             // cleanup streamer
             if (this.streamer) {
                 try {
-                    // remove our listeners
+                    // remove our listeners explicitly by reference
                     if (this.socketReadable) {
                         this.streamer.removeListener('readable', this.socketReadable);
                     }
-                    this.streamer.removeAllListeners('error');
+                    if (this._streamerErrorHandler) {
+                        this.streamer.removeListener('error', this._streamerErrorHandler);
+                    }
                     if (!this.streamer.destroyed) {
                         this.streamer.destroy();
                     }
@@ -1811,6 +1834,18 @@ class ImapFlow extends EventEmitter {
                 }
             }
 
+            // Explicit nullification to help garbage collection
+            this.socket = null;
+            this.writeSocket = null;
+            this._inflate = null;
+            this._deflate = null;
+            this._streamerErrorHandler = null;
+            this._connectErrorHandler = null;
+            this._socketError = null;
+            this._socketClose = null;
+            this._socketEnd = null;
+            this._socketTimeout = null;
+
             this.log.trace({ msg: 'Connection closed', cid: this.id });
             this.emit('close');
         } catch (ex) {