ffi: refresh cached string buffers on every call

[trivikr]

Fixes: #65050

Native code can mutate temporary string storage during an FFI call. Rewrite cached buffers on every conversion so a later call with the same JavaScript string receives a fresh copy of its UTF-8 bytes.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (a576f1c) to head (8ef6d08).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65051   +/-   ##
=======================================
  Coverage   90.30%   90.31%           
=======================================
  Files         760      760           
  Lines      248526   248521    -5     
  Branches    46886    46896   +10     
=======================================
+ Hits       224439   224445    +6     
+ Misses      15515    15502   -13     
- Partials     8572     8574    +2     

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	95.54% <ø> (+0.76%)	⬆️

... and 28 files with indirect coverage changes

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[mcollina]

lgtm

[trivikr]

Converted to draft as test-ffi-fast-buffer is failing on ci.nodejs.org/job/node-test-commit-aix/64314

---
duration_ms: 1355.201
exitcode: 1
severity: fail
stack: |-
  Test failure: 'fast FFI refreshes cached temporary string buffers'
  Location: test/ffi/test-ffi-fast-buffer.js:99:1
  AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:

  '' !== 'yello'

      at TestContext.<anonymous> (/home/iojs/build/workspace/node-test-commit-aix/nodes/aix72-power9/test/ffi/test-ffi-fast-buffer.js:108:12)
      at Test.runInAsyncScope (node:async_hooks:227:14)
      at Test.run (node:internal/test_runner/test:1397:25)
      at Test.processPendingSubtests (node:internal/test_runner/test:969:18)
      at Test.postRun (node:internal/test_runner/test:1537:19)
      at Test.run (node:internal/test_runner/test:1462:12)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
      at async Test.processPendingSubtests (node:internal/test_runner/test:969:7) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: '',
    expected: 'yello',
    operator: 'strictEqual',
    diff: 'simple'
  }

  (node:38469944) internal/test/binding: These APIs are for internal testing only. Do not use them.
  (Use `node --trace-warnings ...` to show where the warning was created)
  (node:38469944) ExperimentalWarning: FFI is an experimental feature and might change at any time
...

[trivikr]

There was a bug in test.

AIX does not support Node’s PPC64 fast-call trampoline, so it exposes the test’s invalid assumption: the pointer returned by overwrite_string() points into temporary storage owned by Node’s generic FFI invocation. That storage is only valid during the native call. AIX frees/reuses the storage immediately, yielding ''.

Fast-path platforms retain the cached Buffer, so "yello" remains readable.

It was fixed in 89cc17f by observing the mutation during the native call instead of returning a pointer to temporary storage.

I'll re-request review after the CI is successful.

[trivikr]

The ffi contract explicitly says JavaScript strings are copied to temporary NUL-terminated UTF-8 storage “for the duration of the call”

string values are copied to temporary NUL-terminated UTF-8 strings for the duration of the call.

The fast path happens to retain reusable buffers longer, but that is an implementation detail, not a lifetime guarantee.
The test should return the observed byte by value as done in 8ef6d08 and not the pointer.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75761/