fix(logger): prevent "write after end" error during shutdown

lib/winston/logger.js

@@ -79,7 +79,7 @@
    * @param {!Object} options - TODO: add param description.
    * @returns {undefined}
    */
   configure({
     silent,
     format,
     defaultMeta,
@@ -227,7 +227,7 @@
    *
    */
   /* eslint-enable valid-jsdoc */
   log(level, msg, ...splat) {
     // eslint-disable-line max-params
     // Optimize for the hotpath of logging JSON literals
     if (arguments.length === 1) {
@@ -349,15 +349,49 @@
    */
   _final(callback) {
     const transports = this.transports.slice();
-    asyncForEach(
-      transports,
-      (transport, next) => {
-        if (!transport || transport.finished) return setImmediate(next);
-        transport.once('finish', next);
-        transport.end();
-      },
-      callback
-    );
+
+    // We need to ensure the readable buffer is fully drained before ending
+    // transports. The pipe from Logger to transports is asynchronous - data
+    // pushed via _transform() goes to the readable buffer, and the pipe reads
+    // from it to write to transports. If we call transport.end() while there's
+    // still data in the readable buffer, the pipe will try to write to an
+    // ending transport, causing "write after end" errors.
+    //
+    // We use an event-driven approach: listen for 'data' events which fire
+    // as the pipe consumes data from our readable buffer. When the buffer
+    // empties, we can safely end transports.
+
+    const endTransports = () => {
+      asyncForEach(
+        transports,
+        (transport, next) => {
+          if (!transport || transport.finished) return setImmediate(next);
+          transport.once('finish', next);
+          transport.end();
+        },
+        callback
+      );
+    };
+
+    const isBufferEmpty = () => {
+      return !this._readableState || this._readableState.length === 0;
+    };
+
+    // If buffer is already empty, proceed immediately
+    if (isBufferEmpty()) {
+      setImmediate(endTransports);
+      return;
+    }
+
+    // Listen for data consumption - 'data' events fire as pipe reads from buffer
+    const onData = () => {
+      if (isBufferEmpty()) {
+        this.removeListener('data', onData);
+        endTransports();
+      }
+    };
+
+    this.on('data', onData);
   }
 
   /**