test_runner: avoid hanging on incomplete v8 frames

[thisalihassan]

The test runner could hang while draining child stdout when the buffered data started with bytes that matched the V8 serializer header (0xff 0x0f) but did not contain a complete serialized frame.

In that case #processRawBuffer() made no progress, and #drainRawBuffer() kept calling it in a loop forever.

This change makes the deserializer recover from that end-of-stream case by flushing the buffered bytes as stdout and continuing to resync. It also preserves the ordering of a trailing partial V8 header split across chunks.

Closes #62693

Assisted-by: Claude Opus

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[aduh95]

We can get a much more useful error output by using partialDeepStrictEqual here

Suggested change

	assert(reported.every((event) => event.type === 'test:stdout'));
	assert.partialDeepStrictEqual(reported, Array.from({ length: reported.length }, () => ({ type: 'test:stdout' }));

[aduh95]

If we're dealing with a single-byte UTF-8 char, we don't need the intermediate view I think

Suggested change

	message: TypedArrayPrototypeSubarray(bufferHead, 0, 1).toString('utf-8'),
	message: StringFromCharCode(bufferHead[0]),

[thisalihassan]

it changes the output semantics here StringFromCharCode(0xff) gives "ÿ" but Buffer.from([0xff]).toString('utf8') gives "�" which matches the current stdout path so I’m keeping the UTF-8 decode

[aduh95]

String.fromCodePoint then – but I'm surprised we would get non-ASCII chars here

[avivkeller]

Can you add a proper PR description?

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.79%. Comparing base (0fea430) to head (825b3e3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/runner.js	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62704      +/-   ##
==========================================
- Coverage   89.80%   89.79%   -0.02%     
==========================================
  Files         699      699              
  Lines      216363   216416      +53     
  Branches    41370    41370              
==========================================
+ Hits       194309   194333      +24     
- Misses      14140    14187      +47     
+ Partials     7914     7896      -18     

Files with missing lines	Coverage Δ
lib/internal/test_runner/runner.js	93.68% <95.55%> (+0.03%)	⬆️

... and 36 files with indirect coverage changes

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