[ronag]

Buffer.byteLength() can abort when flattening a string triggers a GC that
lazily deoptimizes its optimized caller. V8 does not support that
deoptimization after a value-returning fast API call, including when the
call takes its slow fallback.

Register byteLengthUtf8 as a regular callback and remove its fast callback.
The existing SIMD path for large two-byte strings remains. This removes
the fast API optimization for this binding and avoids shared result state
or startup changes.

Add a GC regression and coverage for flat, Latin-1, cons, sliced, two-byte,
lone-surrogate and external strings before and after optimization.

Validation on Linux x64, release build of main:

• Base 502c117: the new GC regression aborts in 3/3 runs.
• Candidate 7f67aa5: the same regression passes in 10/10 runs.
• All 502 buffer, HTTP and string-decoder tests pass.
• ESLint, cpplint, import, C++ formatting and whitespace checks pass.

Fixes: #66870

AI assistance: Codex helped prepare the change and regression tests.

[Copilot]

🔵 Needs a closer look

The regression depends on V8 optimization and GC-triggered lazy deoptimization, requiring human runtime verification of pre-fix failure, post-fix success, and stability across supported CI builds.

No open issues 🎉

What changed in this PR

This PR addresses the Buffer.byteLength() process abort reported in #66870 by replacing its UTF-8 fast API binding with a regular callback, allowing garbage-collection-triggered deoptimization.

Changes:

• Removes the fast callback and its registration while retaining the existing SIMD path for large two-byte strings.
• Adds a regression test for garbage collection during string flattening.
• Adds string-representation coverage before and after optimization.

File	Description
test/​parallel/​test-buffer-bytelength-strings.js	Checks UTF-8 byte lengths across string representations before and after optimization.
test/​parallel/​test-buffer-bytelength-gc.js	Exercises the GC-triggered deoptimization that caused the abort.
src/​node_buffer.cc	Registers a regular UTF-8 byte-length callback and removes the fast API implementation.

_{Review effort: Balanced}

[ronag]

Investigating how to possibly re-enable the optimization. But this is pretty bad and should be fixed ASAP.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.68%. Comparing base (502c117) to head (7f67aa5).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #67134      +/-   ##
==========================================
+ Coverage   90.66%   90.68%   +0.02%     
==========================================
  Files         797      797              
  Lines      278791   278743      -48     
  Branches    53620    53602      -18     
==========================================
+ Hits       252768   252791      +23     
+ Misses      16376    16306      -70     
+ Partials     9647     9646       -1     

Files with missing lines	Coverage Δ
src/node_buffer.cc	74.08% <100.00%> (+2.41%)	⬆️

... and 29 files with indirect coverage changes

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