fix(remoting): stop ReconnectTimerTask when client is closed

[daguimu]

Problem

After all providers go offline, the consumer's ReconnectTimerTask keeps attempting to reconnect to the old provider IP indefinitely. In K8s environments where providers restart with new IPs, this generates continuous connection-refused errors and triggers false alerts.

Root Cause

Three issues allow the reconnect timer to run indefinitely after a client should be considered closed:

1. AbstractTimerTask.run() always reschedules the timer via reput(), even when all channels are already closed. The task becomes an orphaned timer doing nothing but consuming resources.

2. HeaderExchangeClient.isClosed() only checks HeaderExchangeChannel.closed but not the underlying Client.isClosed(). When the underlying AbstractClient/NettyClient is closed independently (e.g., by protocol destroy), the HeaderExchangeClient still reports as open.

3. ReconnectTimerTask.doTask() has no defensive check for client closure before attempting reconnection. Even if the client is closed between the run() check and the doTask() execution, reconnection proceeds.

Fix

• AbstractTimerTask.run(): When all channels report as closed, cancel the timer task instead of rescheduling. This prevents orphaned timers from running indefinitely.

• HeaderExchangeClient.isClosed(): Also check client.isClosed() so that if the underlying client is closed by any code path, the wrapper correctly reflects the closed state.

• ReconnectTimerTask.doTask(): Add a defensive check at the start — if the channel is a Client and reports as closed, cancel the timer and return immediately.

Tests Added

Change Point	Test
AbstractTimerTask auto-cancel on all channels closed	AbstractTimerTaskTest.testAutoCancelWhenAllChannelsClosed() — verifies task stops executing and cancel flag is set after channel closes
AbstractTimerTask continues when channel is open	AbstractTimerTaskTest.testTaskContinuesWhenChannelIsOpen() — regression test ensuring normal scheduling works
HeaderExchangeClient.isClosed() checks underlying client	HeaderExchangeClientTest.testIsClosedWhenUnderlyingClientClosed() — verifies isClosed() returns true when underlying client is closed
HeaderExchangeClient.isClosed() normal case	HeaderExchangeClientTest.testIsNotClosedWhenBothOpen() — regression test for normal open state
ReconnectTimerTask stops on client close	ReconnectTimerTaskTest.testStopReconnectWhenClientClosed() — verifies reconnect count stops increasing and timer is cancelled after client closes
ReconnectTimerTask continues when not closed	ReconnectTimerTaskTest.testReconnectContinuesWhenNotClosed() — regression test ensuring reconnection works for transient disconnections

Impact

Only affects HeaderExchangeClient-based connections (Dubbo protocol). The newer AbstractNettyConnectionClient (Triple protocol) already handles this correctly via NettyConnectionHandler. No behavioral change for normal reconnection scenarios — the timer still runs for transient disconnections where the client is not closed.

Fixes #15880

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.77%. Comparing base (b54059d) to head (94d98b6).
⚠️ Report is 7 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
.../exchange/support/header/HeaderExchangeClient.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16176      +/-   ##
============================================
- Coverage     60.80%   60.77%   -0.04%     
+ Complexity    11767    11752      -15     
============================================
  Files          1953     1953              
  Lines         89118    89123       +5     
  Branches      13444    13446       +2     
============================================
- Hits          54190    54165      -25     
- Misses        29367    29383      +16     
- Partials       5561     5575      +14     

Flag	Coverage Δ
integration-tests-java21	32.11% <16.66%> (-0.07%)	⬇️
integration-tests-java8	32.22% <16.66%> (-0.03%)	⬇️
samples-tests-java21	32.13% <0.00%> (-0.04%)	⬇️
samples-tests-java8	29.72% <0.00%> (-0.01%)	⬇️
unit-tests-java11	59.05% <83.33%> (+0.01%)	⬆️
unit-tests-java17	58.51% <83.33%> (+0.01%)	⬆️
unit-tests-java21	58.49% <83.33%> (-0.04%)	⬇️
unit-tests-java25	58.46% <83.33%> (-0.05%)	⬇️
unit-tests-java8	59.01% <83.33%> (-0.06%)	⬇️

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