stream: fix UTF-8 character corruption in fast-utf8-stream

[mcollina]

Summary

Fix releaseWritingBuf() to correctly handle partial writes that split multi-byte UTF-8 characters.

The previous implementation incorrectly converted byte counts to character counts by using:

n = Buffer.from(writingBuf).subarray(0, n).toString().length;

When n bytes cuts through a multi-byte character, the incomplete UTF-8 sequence becomes U+FFFD (replacement character) via .toString(), which has a different .length than the original character. This caused:

• 3-byte characters (CJK) to be silently dropped
• 4-byte characters (emoji) to leave lone surrogates in the buffer

The fix backs up from the byte position to find a valid UTF-8 character boundary by checking for continuation bytes (pattern 10xxxxxx), then decodes the properly-aligned bytes to get the correct character count.

Also fixes a typo where this._asyncDrainScheduled was used instead of the private field this.#asyncDrainScheduled.

Fixes: #61744

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (f77a709) to head (4f5cf65).
⚠️ Report is 160 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61745      +/-   ##
==========================================
- Coverage   89.74%   89.74%   -0.01%     
==========================================
  Files         674      675       +1     
  Lines      204360   204537     +177     
  Branches    39265    39308      +43     
==========================================
+ Hits       183412   183569     +157     
- Misses      13251    13257       +6     
- Partials     7697     7711      +14     

Files with missing lines	Coverage Δ
lib/internal/streams/fast-utf8-stream.js	74.57% <100.00%> (+0.33%)	⬆️

... and 44 files with indirect coverage changes

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[mcollina]

Manual CI: https://ci.nodejs.org/job/node-test-pull-request/71336/

(no idea why request-ci did not work here)

[mcollina]

@nodejs/build can someone help to see why CI is not starting?

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/61745
✔  Done loading data for nodejs/node/pull/61745
----------------------------------- PR info ------------------------------------
Title      stream: fix UTF-8 character corruption in fast-utf8-stream (#61745)
Author     Matteo Collina <matteo.collina@gmail.com> (@mcollina)
Branch     mcollina:fix-utf8-stream-partial-write -> nodejs:main
Labels     stream, needs-ci, commit-queue-squash
Commits    1
 - stream: fix UTF-8 character corruption in fast-utf8-stream
Committers 1
 - Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/61745
Fixes: https://github.com/nodejs/node/issues/61744
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/61745
Fixes: https://github.com/nodejs/node/issues/61744
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Sun, 08 Feb 2026 20:39:59 GMT
   ✔  Approvals: 4
   ✔  - Yagiz Nizipli (@anonrig) (TSC): https://github.com/nodejs/node/pull/61745#pullrequestreview-3770760306
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/61745#pullrequestreview-3774539778
   ✔  - Colin Ihrig (@cjihrig): https://github.com/nodejs/node/pull/61745#pullrequestreview-3778727542
   ✔  - Paolo Insogna (@ShogunPanda) (TSC): https://github.com/nodejs/node/pull/61745#pullrequestreview-3859183492
   ✔  Last GitHub CI successful
   ℹ  Last Full PR CI on 2026-02-13T22:03:07Z: https://ci.nodejs.org/job/node-test-pull-request/71336/
- Querying data for job/node-test-pull-request/71336/
✔  Build data downloaded
   ✘  1 failure(s) on the last Jenkins CI run
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   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/22433955375