Safe-output integrity checks block indefinitely when a captured artifact becomes a FIFO #68786

Summary

Review of #68732 at commit 40c71f95ad05d3f2da56bc19e38e3f041495f7e9 found a P2 behavior issue: replacing an already-captured regular transport file with a FIFO can hang the safe-output handler instead of producing an integrity error.

Location

actions/setup/js/transport_integrity.cjs:33–39,52–59

Reproduction

1. Create a regular patch file and capture it with createTransportIntegrity().captureOptional(path).
2. Unlink the file and create a FIFO at the same path, with no writer attached.
3. Call assertUnchanged().

The synchronous openSync(O_RDONLY | O_NOFOLLOW) blocks before fstatSync() can reject the nonregular file. A bounded child-process reproduction timed out and required termination.

Expected behavior

Reject the replaced artifact promptly with a transport-integrity failure, without blocking the Node.js event loop until the job timeout.

Suggested fix and coverage

Use a nonblocking descriptor open followed by descriptor-based regular-file validation, preserving no-follow and identity checks. Add bounded child-process tests for FIFO inputs both at capture and after capture. Ensure cleanup still runs after rejection.

Found during behavioral review of #68732; no fix was applied.