PlainNettyTransport.close: use quietPeriod=0 in shutdownGracefully

[codexcoder21]

Summary

PlainNettyTransport.close() currently calls workerGroup.shutdownGracefully() and bossGroup.shutdownGracefully() with no arguments. That falls through to Netty's defaults of quietPeriod = 2 SECONDS, timeout = 15 SECONDS. After PR #412 chained the returned futures via CompletableFuture.allOf(...).thenApply { }, the caller's close() future no longer resolves until those shutdownGracefully futures resolve — which means every close() blocks for the full 2-second quiet period, even when there is no traffic in flight to drain.

This patch passes an explicit quietPeriod = 0 to both calls. The event loop terminates as soon as its run-loop notices the shutdown flag, the returned future resolves in tens of milliseconds, and the caller-can-wait correctness contract introduced in #412 is preserved.

Why the current behaviour is wrong for typical callers

The quiet period is the time shutdownGracefully keeps the event loop alive AFTER its last task finished, in case more work is submitted. Once close() has been called on this transport:

• Every existing channel has had .close() invoked on it.
• closed = true is set, so listen() and dial() throw Libp2pException.
• There is by construction no code path that submits new work to either workerGroup or bossGroup.

Waiting 2 seconds for "more work that isn't coming" is pure latency. The timeout argument (which I keep at 5 seconds) is the meaningful upper bound — it caps how long shutdownGracefully will wait for the loop to actually exit once shutdown has been requested.

Cumulative impact downstream

The 2-second per-close latency compounds badly for callers that drive many short-lived host lifecycles back-to-back, e.g. test fixtures that create-and-tear-down a host per iteration. A consumer @Timeout(30) test of 20 host create/close cycles times out after 12-13 iterations (each ~2 s of quiet-period wait plus iteration overhead), even though every individual close() is doing exactly what it was asked to do.

That is what surfaced as 12 timed-out tests in CodexCoder21Organization/UrlResolver run 7f19e875, once a downstream classpath change started loading post-#412 classes instead of the pre-#412 classes for the first time. The full investigation is in the report on the run, but the short version is: every test that creates many UrlProtocol2 instances in a loop hits the cumulative quiet-period budget exhaustion this PR fixes.

Regression test

PlainNettyTransportShutdownTimingTest exercises the contract directly. Both tests pass with this PR's fix and FAIL on develop HEAD:

Test	On develop	With fix
single close should resolve well below the default 2 second quiet period	FAIL: returns in ~2023 ms	PASS: returns in ~30 ms
20 sequential listen-then-close cycles complete in under 5 seconds	FAIL: 41,146 ms total (~2,057 ms / iter)	PASS: < 5 s total

The 1-second budget on the single-close test is deliberately conservative against any future change that might restore a smaller-but-nonzero quiet period — even a hypothetical 1 s default would fail this test decisively.

Verification

$ ./gradlew :libp2p:test --tests "io.libp2p.transport.PlainNettyTransportShutdownTimingTest"
...
BUILD SUCCESSFUL

(Pre-fix: 2 tests, 2 failed — matching the timing data above.)

Risk

Minimal. The change tightens shutdown latency without changing what state shutdown leaves behind. Any caller that genuinely relies on a 2-second quiet period to absorb in-flight tasks should already be calling close() only after explicitly draining its traffic — and if it does have an unflushed task at close time, the 5-second timeout argument still gives the loop time to drain it before the future completes.

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