Fix ETXTBSY flake in process command_for spawn test

[Mikola Lysenko (mikolalysenko)]

Failing run

The coverage job in CI run 37999238426 failed on 2026-10-09 in utils::process::tests::command_for_spawns_the_resolved_path_directly_on_unix:

spawn the shim: Os { code: 26, kind: ExecutableFileBusy, message: "Text file busy" }

Root cause

This is the known ETXTBSY fork/exec race. The test writes a shell shim, chmods it and spawns it at once. fs::write closes our handle, but other test threads in the same process fork children. A child forked between our open() and its own exec() inherits the shim's write fd, and Linux refuses to exec a file while any process holds it open for writing. Writing to a temp file and renaming it would not help, because the inherited fd still refers to the same inode.

Fix

The test now spawns through a test-local output_of_fresh_shim helper. It retries ErrorKind::ExecutableFileBusy only, with a 10 s deadline and 20 ms sleeps, the same pattern as sanity_exec in update/download.rs and the CLI's exec_freshly_written fixture. Any other spawn error still panics, and the test's assertions are unchanged.

I checked the other core tests that write executables. The only other test that writes and runs a shim is a_hung_probe_answers_none_within_its_budget. It expects None, so an ETXTBSY there can't fail it, and I left it alone.

Verification

• cargo test -p socket-patch-core --lib utils::process: 25/25 serial runs and 10/10 runs with --test-threads=64 passed.
• Full cargo test -p socket-patch-core --lib passed once (6164 tests).
• cargo clippy --locked --workspace --all-features -- -D warnings is clean.

🤖 Generated with Claude Code

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Note

Low Risk
Test-only change in utils::process tests; no production spawn or resolver logic is modified.

Overview
Fixes intermittent CI failure in command_for_spawns_the_resolved_path_directly_on_unix when parallel test threads hit Linux ETXTBSY (“Text file busy”) after writing and immediately exec’ing a temp shim.

Adds a Unix-only test helper output_of_fresh_shim that retries spawn on ErrorKind::ExecutableFileBusy only (10s cap, 20ms backoff), matching sanity_exec / CLI exec_freshly_written. The test now spawns the resolved shim through that helper; assertions and production command_for behavior are unchanged.

^{Reviewed by Cursor Bugbot for commit 256d857. Configure here.}