Log errno and thread ID on CreateIOUring failure (#14520)

[archang19]

Summary:

CreateIOUring() silently returns nullptr on failure, discarding the errno
from io_uring_queue_init. This makes it impossible to diagnose why
io_uring initialization fails on specific threads (e.g. ENOMEM from
memlock limits, EINVAL from unsupported flags, EMFILE from fd exhaustion).

Add a fprintf(stderr, ...) that logs strerror, errno, and pthread thread
ID when io_uring_queue_init fails, so failures are diagnosable from logs
without needing to reproduce.

Reviewed By: xingbowang

Differential Revision: D98526792

[meta-codesync]

@archang19 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D98526792.

[github-actions]

✅ clang-tidy: No findings on changed lines

Completed in 66.1s.

[github-actions]

✅ Claude Code Review

Auto-triggered after CI passed — reviewing commit b2ccf9e

---

Code Review: PR #14520 — Log errno and thread ID on CreateIOUring failure

PR: 2 files changed, 8 insertions, 1 deletion
Author: archang19 | Reviewed By: xingbowang

---

Summary

This PR improves diagnostic logging when io_uring_queue_init fails in CreateIOUring(). It adds errno details, human-readable error string, and thread ID to the failure message, and removes a less-informative fprintf from the ReadAsync() caller. The change is correct, focused, and follows existing codebase patterns.

---

Findings

F1: Startup probe will print error to stderr on systems where io_uring fails at runtime

Severity: Medium
File: env/io_posix.h:349 (new fprintf) / env/fs_posix.cc:1345 (caller)

The PosixFileSystem constructor (fs_posix.cc:1338-1352) calls CreateIOUring() as a one-time probe to test io_uring support. With this PR, systems where io_uring is compiled in (ROCKSDB_IOURING_PRESENT) but fails at runtime will now print an error to stderr during normal startup:

CreateIOUring failed: Function not implemented (errno=38), thread=140123456789

This can happen in Docker containers with restricted syscalls, systems with low RLIMIT_MEMLOCK, or older kernels with liburing installed. The old code had no stderr output for this probe path.

Impact: Users may interpret this as an error when it's actually expected behavior (graceful fallback to pread). One-time per process, so log volume is minimal.

Suggestion: Consider wording that indicates fallback is normal, e.g., "CreateIOUring failed (falling back to pread): ...".

---

F2: pthread_t cast assumes glibc type definition

Severity: Low
File: env/io_posix.h:351

static_cast<unsigned long>(pthread_self()) is safe on Linux glibc where pthread_t is unsigned long. On musl libc it's a struct pointer. Since ROCKSDB_IOURING_PRESENT is Linux-only and targets glibc, this is safe in practice. No action needed now.

---

F3: Correctness, error handling, and memory safety — all verified clean

• -ret correctly converts negative errno return ✓
• errnoStr(-ret).c_str() temporary lifetime is safe per C++ standard ✓
• fprintf(stderr) is thread-safe per POSIX ✓
• Memory: new_io_uring properly deleted before returning nullptr ✓
• errnoStr() heap allocation acceptable on cold path ✓

---

F4: Include adds transitive dependency

Severity: Low — #include "util/string_util.h" inside ROCKSDB_IOURING_PRESENT guard is correct. string_util.h is lightweight and widely included.

F5: No new tests needed — diagnostic-only change on cold path with no behavioral changes.

---

Verdict

The PR is a net improvement. The only substantive concern is F1 — the startup probe will now emit a stderr message on systems where io_uring fails at runtime, which was previously silent. Consider whether the message should clarify that fallback behavior is normal.

---

ℹ️ About this response

Generated by Claude Code.
Review methodology: claude_md/code_review.md

Limitations:

• Claude may miss context from files not in the diff
• Large PRs may be truncated
• Always apply human judgment to AI suggestions

Commands:

• /claude-review [context] — Request a code review
• /claude-query <question> — Ask about the PR or codebase

[meta-codesync]

This pull request has been merged in 2af3820.