stream: fix Writable.toWeb() hang on synchronous drain

A race condition in the Writable.toWeb() adapter caused the stream
to hang if the underlying Node.js Writable emitted a 'drain' event
synchronously during a write() call. This often happened when
highWaterMark was set to 0.

By checking writableNeedDrain immediately after a backpressured write,
the adapter now correctly detects if the stream has already drained,
resolving the backpressure promise instead of waiting indefinitely
for an event that has already occurred.

Fixes: #61145

Author: Han5991

lib/internal/webstreams/adapters.js

@@ -238,6 +238,9 @@ function newWritableStreamFromStreamWritable(streamWritable, options = kEmptyObj
         options[kValidateChunk]?.(chunk);
         if (streamWritable.writableNeedDrain || !streamWritable.write(chunk)) {
           backpressurePromise = PromiseWithResolvers();
+          if (!streamWritable.writableNeedDrain) {
+            backpressurePromise.resolve();
+          }
           return SafePromisePrototypeFinally(
             backpressurePromise.promise, () => {
               backpressurePromise = undefined;

test/parallel/test-whatwg-webstreams-adapters-to-writablestream.js

@@ -197,3 +197,26 @@ class TestWritable extends Writable {
     assert.strictEqual(chunk, arrayBuffer);
   }));
 }
+
+{
+  // Test that the stream doesn't hang when the underlying Writable
+  // emits 'drain' synchronously during write().
+  // Fixes: https://github.com/nodejs/node/issues/61145
+  const writable = new Writable({
+    write(chunk, encoding, callback) {
+      callback();
+    },
+  });
+
+  // Force synchronous 'drain' emission during write()
+  // to simulate a stream that doesn't have Node.js's built-in kSync protection.
+  writable.write = function(chunk) {
+    this.emit('drain');
+    return false;
+  };
+
+  const writableStream = newWritableStreamFromStreamWritable(writable);
+  const writer = writableStream.getWriter();
+  writer.write(new Uint8Array([1, 2, 3])).then(common.mustCall());
+  writer.write(new Uint8Array([4, 5, 6])).then(common.mustCall());
+}