fix(http2): replenish connection receive window when stream closes with unconsumed data

[mosuem]

Note

This PR was generated by an AI coding agent (Jetski) on behalf of @mosuem.

Summary

Per RFC 9113 Section 5.2 (Flow Control) and Section 6.9 (WINDOW_UPDATE), a receiver that receives a flow-controlled DATA frame must always account for its contribution against the connection flow-control window, even when the stream terminates abnormally or its incoming messages are discarded before the application consumes them.

Previously:

1. ConnectionMessageQueueIn.removeStreamMessageQueue(streamId) discarded buffered DataMessages in _stream2pendingMessages[streamId] without calling _windowUpdateHandler.dataProcessed(ignoredBytes). If a stream was paused (or had not yet been listened to) when DATA frames arrived and was subsequently terminated (stream.terminate() or RST_STREAM), those bytes were permanently leaked from the connection-level receive window. Once 65,535 bytes (or connectionWindowSize) accumulated across terminated streams, all future streams on the connection deadlocked waiting for connection window credit.
2. StreamMessageQueueIn.cancel() only completed _onCancelCompleter without draining _pendingMessages, updating bufferIndicator, or running onCloseCheck(). Cancelling stream.incomingMessages while paused left bufferIndicator.wouldBuffer == true, stranding buffered DATA in ConnectionMessageQueueIn and keeping the stream's active slot open indefinitely.
3. StreamHandler._terminateStream was a no-op when stream.state == StreamState.Closed, so calling stream.terminate() on a stream whose remote end had already sent END_STREAM while the local listener was paused failed to clean up the stream or replenish the connection window.

Changes

• lib/src/flowcontrol/connection_queues.dart: In ConnectionMessageQueueIn.removeStreamMessageQueue, sum the byte lengths of any undelivered DataMessages and call _windowUpdateHandler.dataProcessed(ignoredBytes) when !wasTerminated. Also terminate any remaining stream message queues in ConnectionMessageQueueIn.onTerminated.
• lib/src/flowcontrol/stream_queues.dart: Override StreamMessageQueueIn.cancel() to drain _pendingMessages, update bufferIndicator so ConnectionMessageQueueIn flushes any held messages and replenishes the connection window, and run onCloseCheck().
• lib/src/streams/stream_handler.dart: Allow _terminateStream to clean up StreamState.Closed streams whose incomingQueue has not yet closed.

Test Verification (Fails Before $\rightarrow$ Passes After)

Added two regression tests in test/flow_control_test.dart, driving the raw peer from #1999:

• terminating a stream with unconsumed buffered DATA replenishes the connection receive window
• cancelling incomingMessages on a paused stream replenishes the connection window and releases the active stream slot

Wherever a test needs to know that the client has processed the frames sent so far (or finished reacting to a cancel), it uses the peer's PING round trip (peer.barrier()) rather than a wall-clock sleep.

Before fix (tests on master):

00:05 +0 -1: terminating a stream with unconsumed buffered DATA replenishes the connection receive window [E]
  TimeoutException after 0:00:05.000000: Future not completed
  test/flow_control_test.dart 126:7  _RawPeer.until
  test/flow_control_test.dart 427:5  main.<fn>
00:10 +0 -2: cancelling incomingMessages on a paused stream replenishes the connection window and releases the active stream slot [E]
  TimeoutException after 0:00:05.000000: Future not completed
  test/flow_control_test.dart 126:7  _RawPeer.until
  test/flow_control_test.dart 469:5  main.<fn>

After fix:

00:00 +9: All tests passed!

[github-actions]

PR Health

Coverage ⚠️

File	Coverage
pkgs/http2/lib/src/flowcontrol/connection_queues.dart	💔 93 % ⬇️ 0 %
pkgs/http2/lib/src/flowcontrol/stream_queues.dart	💚 90 % ⬆️ 1 %
pkgs/http2/lib/src/streams/stream_handler.dart	💔 91 % ⬇️ 1 %

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Package	Change	Current Version	New Version	Needed Version	Looking good?
http2	Non-Breaking	3.1.0	3.2.0-wip	3.2.0-wip	✔️

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