win: fix error reporting when WTF-8 conversion fails in fs paths

[dylan-conway]

Summary

Two Windows fs functions mishandle failures of the internal UTF-16 to WTF-8 conversion (uv_utf16_to_wtf8()). That function reports failure by returning a negative UV error code (UV_ENOMEM when its allocation fails) and does not set the thread's Win32 last error — but the surrounding code assumed the "-1 with GetLastError()" convention.

• uv_fs_realpath reported success with a NULL path pointer. fs__realpath() only treated -1 as failure, so a negative UV code from the conversion fell through to SET_REQ_RESULT(req, 0): the request completed with result == 0 while req->ptr was still NULL. Callers that trust the documented contract (success implies a valid path pointer) crash dereferencing it. Fixed by treating any negative return as failure and propagating the UV error code for the conversion case. Also capture GetLastError() before CloseHandle() so a succeeding close cannot clobber the failure code.
• uv_fs_readlink (and the lstat reparse-point path) reported a stale error. fs__readlink_handle() mixed two failure conventions: most paths set the last error and return -1, but the Linux-symlink branch returned UV_ENOMEM directly and the tail returned the conversion result verbatim. All three callers report failures via GetLastError(), so a conversion failure surfaced as whatever unrelated error happened to be set, instead of UV_ENOMEM. Fixed by normalizing the helper to always set the last error and return -1 (ERROR_OUTOFMEMORY translates back to UV_ENOMEM).

Both bugs are only reachable when malloc fails inside the conversion, so they are out-of-memory-only in practice. No behavior change on any success path. Both bugs also exist in upstream libuv v1.x.

Test plan

• Full build on Windows (MSVC/clang) with no new warnings
• uv_run_tests_a fs_realpath passes
• uv_run_tests_a fs_readlink, fs_symlink, fs_symlink_dir, fs_symlink_junction pass
• uv_run_tests_a fs_stat_root, fs_lstat_windows_store_apps, fs_unlink_readonly pass (cover the other fs__readlink_handle callers)
• Verified uv_utf16_to_wtf8() can only fail with UV_ENOMEM in these call paths (the UV_ENOBUFS path requires a caller-supplied buffer; these sites pass a NULL target so the buffer is sized exactly by uv_utf16_length_as_wtf8())

[coderabbitai]

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Run ID: 8ff0f830-dfca-4f8a-825d-93ecf52fd0d8

📥 Commits

Reviewing files that changed from the base of the PR and between 439a54b and 2baf2df.

📒 Files selected for processing (1)

• src/win/fs.c

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Walkthrough

In src/win/fs.c, error propagation in the readlink and realpath paths is corrected. Allocation and UTF-16 conversion failures in readlink now call SetLastError(ERROR_OUTOFMEMORY) and return -1. fs__realpath captures the converter result and Win32 error before closing the handle, then routes r == -1 to Win32 error handling and r < 0 to SET_REQ_UV_ERROR.

Changes

Win32 fs error propagation

Layer / File(s)	Summary
readlink: malloc and conversion error mapping src/win/fs.c	Malloc failure on the Linux symlink path and UTF-16→WTF-8 conversion failure now both invoke SetLastError(ERROR_OUTOFMEMORY) and return -1 instead of returning UV error codes directly.
fs__realpath: split Win32 vs UV error outcomes src/win/fs.c	Introduces DWORD error and ssize_t r locals; captures GetLastError() before closing the handle; dispatches r == -1 to Win32 error handling and r < 0 to SET_REQ_UV_ERROR(req, r, ERROR_OUTOFMEMORY).

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the Windows fs error-reporting fix around WTF-8 conversion failures.
Description check	✅ Passed	The description matches the changes and explains the real Windows fs failure cases being fixed.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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