ffi: preserve strings during reentrant calls

[trivikr]

Fixes: #64550

Temporary buffers used to convert JavaScript strings for Fast FFI calls were
cached per library and argument index. A reentrant callback could make another
FFI call using the same argument index, overwriting or replacing a buffer still
in use by the outer native call.

This changes the cache to be scoped by FFI wrapper and active call depth. Each
reentrant invocation therefore receives separate string storage while buffers
remain reusable across non-overlapping calls.

---

Assisted-by: openai:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 66.17647% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (0992f6b) to head (414f602).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/ffi/fast-api.js	66.17%	22 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64551      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         741      741              
  Lines      242112   242153      +41     
  Branches    45605    45600       -5     
==========================================
+ Hits       218244   218266      +22     
- Misses      15360    15378      +18     
- Partials     8508     8509       +1     

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	84.38% <66.17%> (-2.97%)	⬇️

... and 26 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74996/

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75050/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/64551
✔  Done loading data for nodejs/node/pull/64551
----------------------------------- PR info ------------------------------------
Title      ffi: preserve strings during reentrant calls (#64551)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     trivikr:ffi-reentrant-callbacks -> nodejs:main
Labels     author ready, needs-ci, commit-queue-rebase, ffi
Commits    2
 - ffi: preserve strings during reentrant calls
 - test: skip FFI reentrant callback test on SmartOS
Committers 1
 - Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/64551
Fixes: https://github.com/nodejs/node/issues/64550
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/64551
Fixes: https://github.com/nodejs/node/issues/64550
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Fri, 17 Jul 2026 03:25:47 GMT
   ✔  Approvals: 2
   ✔  - Paolo Insogna (@ShogunPanda) (TSC): https://github.com/nodejs/node/pull/64551#pullrequestreview-4732245739
   ✔  - Matteo Collina (@mcollina) (TSC): https://github.com/nodejs/node/pull/64551#pullrequestreview-4761492559
   ✔  Last GitHub CI successful
   ℹ  Last Full PR CI on 2026-07-23T12:47:00Z: https://ci.nodejs.org/job/node-test-pull-request/75050/
- Querying data for job/node-test-pull-request/75050/
✔  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
   aedf2a4187..f9715fc595  main       -> origin/main
✔  origin/main is now up-to-date
main is out of sync with origin/main. Mismatched commits:
 - 3780b725a6 quic: defer server session emit until TLS ClientHello is processed
 - f9715fc595 quic: defer server session emit until TLS ClientHello is processed
--------------------------------------------------------------------------------
HEAD is now at f9715fc595 quic: defer server session emit until TLS ClientHello is processed
   ✔  Reset to origin/main
- Downloading patch for 64551
From https://github.com/nodejs/node
 * branch                  refs/pull/64551/merge -> FETCH_HEAD
✔  Fetched commits as 0992f6b87ecf..414f6029da46
--------------------------------------------------------------------------------
[main 1c79438f9e] ffi: preserve strings during reentrant calls
 Author: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
 Date: Thu Jul 16 20:20:07 2026 -0700
 3 files changed, 100 insertions(+), 27 deletions(-)
[main c38a79461b] test: skip FFI reentrant callback test on SmartOS
 Author: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
 Date: Sat Jul 18 12:31:00 2026 -0700
 2 files changed, 7 insertions(+), 4 deletions(-)
   ✔  Patches applied
There are 2 commits in the PR. Attempting autorebase.
(node:722) [DEP0190] DeprecationWarning: Passing args to a child process with shell option true can lead to security vulnerabilities, as the arguments are not escaped, only concatenated.
(Use `node --trace-deprecation ...` to show where the warning was created)
Rebasing (2/4)
Executing: git node land --amend --yes
--------------------------------- New Message ----------------------------------
ffi: preserve strings during reentrant calls
Cache temporary string conversion buffers by wrapper and active call

depth. This prevents nested FFI calls from overwriting or replacing

buffers still in use by an outer native call.
Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>

Assisted-by: openai:gpt-5.6-sol

PR-URL: #64551

Fixes: #64550

Reviewed-By: Paolo Insogna <paolo@cowtech.it>

Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
[detached HEAD 958f509712] ffi: preserve strings during reentrant calls

Author: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>

Date: Thu Jul 16 20:20:07 2026 -0700

3 files changed, 100 insertions(+), 27 deletions(-)

Rebasing (3/4)

Rebasing (4/4)

Executing: git node land --amend --yes

--------------------------------- New Message ----------------------------------

test: skip FFI reentrant callback test on SmartOS
PR-URL: #64551

Fixes: #64550

Reviewed-By: Paolo Insogna <paolo@cowtech.it>

Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
[detached HEAD b11170c7de] test: skip FFI reentrant callback test on SmartOS

Author: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>

Date: Sat Jul 18 12:31:00 2026 -0700

2 files changed, 7 insertions(+), 4 deletions(-)

Successfully rebased and updated refs/heads/main.
✔  958f5097124e4de43c52b9ff01c3bb42b92ae232

✔  0:0      Assisted-by is a trailer                  assisted-by-is-trailer

✔  0:0      no Co-authored-by metadata                co-authored-by-is-trailer

✔  8:7      Valid 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

⚠  5:0      "Signed-off-by" email does not match the commit author email signed-off-by

✔  0:0      valid subsystems                          subsystem

✔  0:0      Title is formatted correctly.             title-format

✔  0:0      Title is <= 50 columns.                   title-length

✖  b11170c7de6393ce5fe904fa76f10d856964389e

✔  0:0      no Assisted-by metadata                   assisted-by-is-trailer

✔  0:0      no Co-authored-by metadata                co-authored-by-is-trailer

✔  2:7      Valid 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

✔  1:8      PR-URL is valid.                          pr-url

✔  0:0      reviewers are valid                       reviewers

✖  0:0      Commit must have a "Signed-off-by" trailer signed-off-by

✔  0:0      valid subsystems                          subsystem

✔  0:0      Title is formatted correctly.             title-format

✔  0:0      Title is <= 50 columns.                   title-length
ℹ  Please fix the commit message and try again.

Please manually ammend the commit message, by running

git commit --amend

Once commit message is fixed, finish the landing command running

git node land --continue

https://github.com/nodejs/node/actions/runs/30012300383

[nodejs-github-bot]

Landed in 241fb9a