test(pkg): expose revision-store lock inode race

[Alizter]

Description

Add a Linux-only cram regression for the revision-store lock inode race.

Two dune pkg lock processes share a fresh cache. A fake git, FIFO barriers, and /proc/<pid>/fd deterministically queue the second process on the first process's lock inode, wait until the first owner has finished cleanup, and then hold the second process inside Rev_store.load_or_create. A third nonblocking flock on the canonical path must still contend with that owner.

All readiness and exit waits are bounded. Failure cleanup releases FIFO readers, kills, and reaps the spawned Dune processes. util-linux is added only to the Linux test environment to provide the external flock probe.

This is intentionally a test-only draft. The expected mutual-exclusion behavior remains in the cram, so the new test is red on main.

Current Failure

with_flock unlinks rev-store.lock before releasing its flock. The queued second process subsequently owns the unlinked original inode while the third process creates and locks a replacement inode at the canonical path, entering the revision-store critical section concurrently.

Related Issue and Motivation

#9140 added the revision-store lock to prevent concurrent processes from corrupting the shared bare repository. This test isolates an inode-lifetime hole in that locking contract before any production fix is proposed.

Verification

• The new cram failed at the intended third-contender assertion in five consecutive local runs with byte-identical diagnostics.
• nix develop path:. -c dune fmt passed.
• nix develop path:. -c dune build @check passed.
• Existing rev-store-lock-linux.t and rev-store-lock.t passed.

Checklist

• Test added.
• Changelog not applicable: test-only reproducer.
• Documentation not applicable: test-only reproducer.