Resizable ArrayBuffer Fixups

[jasnell]

Multiple improvements to resizable ArrayBuffer handling

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[ask-bonk]

Overall this is a solid set of fixes for resizable ArrayBuffer handling. The temp-buffer approach in the BYOB read path correctly prevents SIGSEGV from decommitted pages, and eagerly copying data on the write path eliminates TOCTOU races with resizable/shared buffers. Two minor items:

1. Dead code in buffer.c++ (inline comment below).
2. src/workerd/api/streams/README.md line 425 still shows the old Write struct pattern with jsg::V8Ref<v8::ArrayBuffer> ownBytes — worth updating to kj::Array<kj::byte> ownBytes to match the new implementation.

[ask-bonk]

This check is dead code: since effectiveSize = kj::min(src.size(), ...), effectiveSize > 0 (which passed line 255) implies src.size() > 0. Consider removing it to avoid confusing future readers.

Suggested change

if (src.size() == 0) continue;

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

Summary: Multiple fixes for resizable ArrayBuffer handling — temp-buffer reads for BYOB to prevent SIGSEGV, eager data copying on writes to eliminate TOCTOU with resizable/shared buffers, and bounds-clamping in Buffer.concat.

Issues found (ranked by severity):

1. (Low) Dead code in buffer.c++:261 — if (src.size() == 0) continue; is unreachable after the effectiveSize == 0 guard above it. Posted as an inline suggestion to remove it.

2. (Low) Stale documentation — src/workerd/api/streams/README.md line 425 still documents the Write struct with jsg::V8Ref<v8::ArrayBuffer> ownBytes, which should be updated to reflect the new kj::Array<kj::byte> ownBytes pattern. Noted in the review body since the file isn't in the diff.

github run

[jasnell]

@AlbertSPedersen can I ask you to acknowledge the CLA here in a comment please

[AlbertSPedersen]

I have read the CLA Document and I hereby sign the CLA

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.58%. Comparing base (686673f) to head (46d1bea).

Files with missing lines	Patch %	Lines
src/workerd/api/streams/internal.c++	70.83%	11 Missing and 3 partials ⚠️
src/workerd/api/node/buffer.c++	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6214      +/-   ##
==========================================
- Coverage   70.60%   70.58%   -0.03%     
==========================================
  Files         413      413              
  Lines      109994   110034      +40     
  Branches    18121    18129       +8     
==========================================
+ Hits        77660    77666       +6     
- Misses      21527    21554      +27     
- Partials    10807    10814       +7     

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