quic: fix reader backpressure deadlock on idle connections

src/quic/streams.cc

@@ -1512,6 +1512,8 @@ void Stream::EndWriting() {
 void Stream::EntryRead(size_t amount) {
   // Called when the JS consumer reads data from the inbound DataQueue.
   // Extend the flow control window so the sender can transmit more.
+  if (session().is_destroyed()) return;
+  Session::SendPendingDataScope send_scope(&session());
   session().ExtendStreamOffset(id(), amount);
   session().ExtendOffset(amount);
 }

test/parallel/test-quic-stream-read-after-blocked.mjs

@@ -0,0 +1,62 @@
+// Flags: --experimental-quic --experimental-stream-iter --no-warnings
+
+// Test: a sender blocked on flow control is resumed once the consumer
+// starts reading. This confirms that we do flush MAX_STREAM_DATA
+// frames as expected when the consumer reads.
+
+import { hasQuic, skip, mustCall, mustCallAtLeast } from '../common/index.mjs';
+import { setTimeout as delay } from 'node:timers/promises';
+import assert from 'node:assert';
+
+const { strictEqual, deepStrictEqual } = assert;
+
+if (!hasQuic) {
+  skip('QUIC is not enabled');
+}
+
+const { listen, connect } = await import('../common/quic.mjs');
+const { bytes } = await import('stream/iter');
+
+// Larger than the default 256 KB stream flow-control window:
+const size = 1024 * 1024;
+const expected = new Uint8Array(size);
+for (let i = 0; i < size; i++) expected[i] = i & 0xff;
+
+const senderBlocked = Promise.withResolvers();
+const done = Promise.withResolvers();
+
+const serverEndpoint = await listen(mustCall((serverSession) => {
+  serverSession.onstream = mustCall(async (stream) => {
+    stream.onblocked = mustCallAtLeast(() => senderBlocked.resolve(stream), 1);
+    stream.setBody(expected);
+    await stream.closed;
+  });
+}));
+
+const clientSession = await connect(serverEndpoint.address);
+await clientSession.opened;
+
+// Write a byte to open the stream:
+const stream = await clientSession.createBidirectionalStream();
+await stream.writer.write(new Uint8Array([1]));
+
+// Wait until the sender has filled the window and blocked.
+const serverStream = await senderBlocked.promise;
+
+// Poll until everything has been acked, so the stream is fully idle:
+while (serverStream.stats.maxOffsetAcknowledged !== serverStream.stats.bytesSent) {
+  await delay(1);
+}
+
+// Try to read:
+const received = await bytes(stream);
+strictEqual(received.byteLength, expected.byteLength);
+deepStrictEqual(received, expected);
+
+stream.writer.endSync();
+await stream.closed;
+clientSession.close();
+done.resolve();
+
+await Promise.all([done.promise, clientSession.closed]);
+await serverEndpoint.close();