buffer: fix unaligned UTF-16LE decoding

[mcollina]

When Buffer.prototype.toString() decoded unaligned UTF-16LE input with an odd byte length, the temporary destination was sized for complete code units while the copy included the trailing byte.

Copy only complete code units, matching the aligned and big-endian paths. Add coverage for stack-backed and heap-backed temporary buffers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (fe4a42b) to head (ccf1c73).
⚠️ Report is 61 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65905      +/-   ##
==========================================
- Coverage   90.18%   90.17%   -0.02%     
==========================================
  Files         771      771              
  Lines      264911   265097     +186     
  Branches    50309    50353      +44     
==========================================
+ Hits       238919   239046     +127     
- Misses      16966    17005      +39     
- Partials     9026     9046      +20     

Files with missing lines	Coverage Δ
src/string_bytes.cc	75.75% <100.00%> (ø)

... and 30 files with indirect coverage changes

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[gurgunday]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77189/

[nodejs-github-bot]

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Full Commit Queue output

- Loading data for nodejs/node/pull/65905
✔  Done loading data for nodejs/node/pull/65905
----------------------------------- PR info ------------------------------------
Title      buffer: fix unaligned UTF-16LE decoding (#65905)
Author     Matteo Collina <matteo.collina@gmail.com> (@mcollina)
Branch     mcollina:fix-buffer-utf16le-odd-length -> nodejs:main
Labels     buffer, c++, author ready, needs-ci, commit-queue
Commits    1
 - buffer: fix unaligned UTF-16LE decoding
Committers 1
 - Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/65905
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/65905
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Tue, 08 Sep 2026 06:44:52 GMT
   ✔  Approvals: 3
   ✔  - Gürgün Dayıoğlu (@gurgunday): https://github.com/nodejs/node/pull/65905#pullrequestreview-5141170565
   ✔  - Filip Skokan (@panva) (TSC): https://github.com/nodejs/node/pull/65905#pullrequestreview-5141847462
   ✔  - Robert Nagy (@ronag) (TSC): https://github.com/nodejs/node/pull/65905#pullrequestreview-5146517461
   ✔  Last GitHub CI successful
   ℹ  Last Full PR CI on 2026-09-08T11:42:34Z: https://ci.nodejs.org/job/node-test-pull-request/77189/
- Querying data for job/node-test-pull-request/77189/
✔  Build data downloaded
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  No git cherry-pick in progress
   ✔  No git am in progress
   ✔  No git rebase in progress
--------------------------------------------------------------------------------
- Bringing origin/main up to date...
From https://github.com/nodejs/node
 * branch                  main       -> FETCH_HEAD
✔  origin/main is now up-to-date
- Downloading patch for 65905
From https://github.com/nodejs/node
 * branch                  refs/pull/65905/merge -> FETCH_HEAD
✔  Fetched commits as e026733cd068..ccf1c73a2663
--------------------------------------------------------------------------------
[main ae513b71eb] buffer: fix unaligned UTF-16LE decoding
 Author: Matteo Collina <hello@matteocollina.com>
 Date: Mon Sep 7 18:37:50 2026 +0200
 2 files changed, 9 insertions(+), 2 deletions(-)
   ✔  Patches applied
--------------------------------------------------------------------------------
--------------------------------- New Message ----------------------------------
buffer: fix unaligned UTF-16LE decoding

For odd-length input, the destination only has room for complete code
units. Copy those units and ignore the trailing byte. This matches the
aligned and big-endian paths.

Assisted-by: pi
Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/65905
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
--------------------------------------------------------------------------------
[main 120fd7a52d] buffer: fix unaligned UTF-16LE decoding
 Author: Matteo Collina <hello@matteocollina.com>
 Date: Mon Sep 7 18:37:50 2026 +0200
 2 files changed, 9 insertions(+), 2 deletions(-)
  ✔  120fd7a52d55f9bced960d3b5bed614b55c6eda9
     ✔  0:0      Assisted-by is a trailer                  assisted-by-is-trailer
     ✔  0:0      no Co-authored-by metadata                co-authored-by-is-trailer
     ✔  0:0      skipping fixes-url                        fixes-url
     ✔  0:0      blank line after title                    line-after-title
     ✔  0:0      line-lengths are valid                    line-length
     ✔  0:0      metadata is at end of message             metadata-end
     ✔  7:8      PR-URL is valid.                          pr-url
     ✔  0:0      reviewers are valid                       reviewers
     ✔  0:0      has valid Signed-off-by                   signed-off-by
     ✔  0:0      valid subsystems                          subsystem
     ✔  0:0      Title is formatted correctly.             title-format
     ✔  0:0      Title is <= 50 columns.                   title-length
--------------------------------------------------------------------------------
The following commits are landable on origin/main
- 120fd7a52d buffer: fix unaligned UTF-16LE decoding
--------------------------------------------------------------------------------
- Removing temporary files
✔  Temporary files removed.
To finish landing:
1. Run: 
   git push origin main
2. Post "Landed in 120fd7a52d55" in https://github.com/nodejs/node/pull/65905
   gh pr comment https://github.com/nodejs/node/pull/65905 --body "Landed in 120fd7a52d55"
   gh pr close https://github.com/nodejs/node/pull/65905
gh: Server Error (HTTP 502)

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[nodejs-github-bot]

Landed in b805fb5