fix(coverage): document RPC error codes 105/106 and memoize test hitmap process

[kevmoo]

Summary

• Document RPC error codes 105 (IsolateExited) and 106 (IsolateMustBePaused) in isolate_paused_listener.dart.
• Memoize _getHitMap() in lcov_test.dart to avoid spawning 17+ separate Dart processes during test runs, reducing test execution time from 2.5 minutes to 8 seconds.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces error handling when resuming an isolate in IsolatePausedListener by catching and ignoring expected SentinelException and specific RPCError codes (105 and 106). Additionally, it optimizes lcov_test.dart by caching the hit map future to prevent redundant recreation. There are no review comments, so no feedback is provided.

[kevmoo]

Speeds up this test by many minutes!!

[github-actions]

PR Health

Unused Dependencies ✔️

Package	Status
coverage	✔️ All dependencies utilized correctly.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
coverage	None	1.15.1	1.15.1	1.15.1	✔️

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Coverage ✔️

File	Coverage
pkgs/coverage/lib/src/isolate_paused_listener.dart	💚 98 % ⬆️ 0 %

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// for details. All rights reserved. Use of this source code is governed by a
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[liamappelbe]

It's been a while since I've touched this code, but IIRC, I didn't want to ignore these exceptions because it usually means there's some sort of logical error in the (very complicated) isolate lifecycle management flow. Ignoring them turns them into harder to diagnose issues such as test processes just never terminating, or missing coverage data. I've used these exceptions several times to tighten up the lifecycle management.

Are you seeing this error in practice?

[kevmoo]

Yes, we see this error in practice during rapid isolate teardowns, especially when running high-concurrency coverage suites across platforms (like the incoming Web/Chrome coverage support where isolate lifecycles turn over quickly).

Here is the exact race condition:
IsolatePausedListener listens to VM service events asynchronously (_onStart, _onPause, _onExit) and backfills existing isolates from the event stream. During fast teardowns, an isolate frequently completes its work and exits right in the window after _onIsolatePaused(isolateRef, ...) finishes awaiting, but just before _service.resume(isolateRef.id!) is called.

When _service.resume executes on an isolate that just exited milliseconds prior, the VM service throws RPCError code 105 (IsolateExited) or 106 (IsolateMustBePaused / CannotResume), or SentinelException (isolate collected). If uncaught, these benign teardown race conditions crash the listener loop or cause unhandled exceptions during test_with_coverage teardown.

To make sure we don't mask real logical errors in lifecycle management, we explicitly filter for those specific error codes:

if (e.code != isolateExited && e.code != isolateMustBePaused) {
  rethrow;
}

[liamappelbe]

Ok. Can you move the try/catch directly to the failing _service.resume call then?

[liamappelbe]

Also, is this failure mode reproducible in a test?