Fix #16344: graceful HTTP/2 GOAWAY migration to eliminate request failure window

[devjue]

What is the purpose of the change

Fixes #16344.

Eliminate the single-request failure window that occurs every time a Dubbo Triple Consumer receives an HTTP/2 GOAWAY(errorCode=0, lastStreamId=MAX_INT) frame from gateways / reverse proxies / providers that gracefully rotate connections (e.g. max_requests_per_connection, idle_timeout, hot-restart drain).

Previously, AbstractNettyConnectionClient#onGoaway nulled the channel reference immediately and NettyConnectionHandler#onGoAway then scheduled a reconnect, leaving a brief window in which AbstractClusterInvoker#checkInvokers throws RpcException before FailoverCluster's retry loop — so retries=N did NOT mitigate the issue. This PR implements graceful migration: keep the old channel serving until a new channel is ready, then atomically swap.

Brief changelog

• NettyConnectionHandler#onGoAway: keep old channel alive; schedule attemptGracefulMigration on connectivityExecutor after GRACEFUL_RECONNECT_DELAY_MS = 200ms; one retry on failure, then fall back to AbstractNettyConnectionClient#onGoaway.
• NettyConnectionClient#initBootstrap: closeFuture listener now calls compareAndClearNettyChannel(ch) instead of clearNettyChannel() — prevents the old channel's close listener from wiping a freshly swapped-in new channel.
• AbstractNettyConnectionClient: add compareAndClearNettyChannel, plus package-private getConnectivityExecutor() / getReconnectDuration() accessors.
• NettyChannel#getChannelIfPresent: lookup-only API used by channelInactive to avoid allocating a transient NettyChannel purely for logging.
• TripleGoAwayHandler: log errorCode and lastStreamId so the GOAWAY trigger is observable.

Verifying this change

Validated against a local Triple demo with sustained 1 QPS traffic:

• 26 GOAWAY frames received → 100% graceful migration success → zero No provider available errors and zero request failures.
• Channel rotation chain (first 18 migrations):
  :45956 → :45110 → :53222 → :58366 → :56372 → :55436 → :55188 → :52926 → :50682 → :48682 → :47626 → :44014 → :43116 → :45044 → :48602 → :51880 → :50992 → :51672
• No regression in existing dubbo-remoting-netty4 and dubbo-rpc-triple unit tests.

Does this pull request potentially affect one of the following parts

• Dependencies (does it add or upgrade a dependency)
• The public API
• The runtime per-connection behavior (HTTP/2 GOAWAY handling on the Consumer side)
• The persistence format of the configurations
• The default values of configurations
• The threading model (uses existing connectivityExecutor; does NOT introduce new threads)
• The serialization protocol
• The compatibility with previous versions

Backward-compatible: defaults are unchanged; the only observable difference is that GOAWAY frames no longer cause a request failure window.

Documentation

• Does this pull request introduce a new feature? No (bug fix)
• If yes, how is the feature documented? Not applicable

Happy to backport to 3.2 once this lands on 3.3.

[uuuyuqi]

@chickenlj PTAL

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.90476% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.82%. Comparing base (866860d) to head (02e4fa9).

Files with missing lines	Patch %	Lines
...oting/transport/netty4/NettyConnectionHandler.java	51.85%	10 Missing and 3 partials ⚠️
.../dubbo/remoting/transport/netty4/NettyChannel.java	33.33%	1 Missing and 1 partial ⚠️
...ransport/netty4/AbstractNettyConnectionClient.java	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16345      +/-   ##
============================================
+ Coverage     60.80%   60.82%   +0.01%     
- Complexity       15    11771   +11756     
============================================
  Files          1953     1953              
  Lines         89208    89239      +31     
  Branches      13458    13459       +1     
============================================
+ Hits          54245    54276      +31     
+ Misses        29397    29395       -2     
- Partials       5566     5568       +2     

Flag	Coverage Δ
integration-tests-java21	32.14% <40.47%> (+0.03%)	⬆️
integration-tests-java8	32.30% <40.47%> (+<0.01%)	⬆️
samples-tests-java21	32.17% <14.28%> (-0.01%)	⬇️
samples-tests-java8	29.82% <11.90%> (+0.10%)	⬆️
unit-tests-java11	59.05% <57.14%> (+0.02%)	⬆️
unit-tests-java17	58.48% <11.90%> (-0.04%)	⬇️
unit-tests-java21	58.50% <11.90%> (+<0.01%)	⬆️
unit-tests-java25	58.45% <11.90%> (+<0.01%)	⬆️
unit-tests-java8	59.03% <11.90%> (-0.03%)	⬇️

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