Discharges the standing fix recorded in `docs/agent-handoff.md` §1 and
assigned to this lane. Every gate invocation now gets a fresh,
disk-backed `TMPDIR` at `<gate-root>/tmp/<pid>`, exported once so every
stage and every process they spawn inherits it, reaped by the same exit
trap as the ambient root. **A gate run no longer needs a `TMPDIR=`
override.**
**A CHILD OF `/tmp` WOULD NOT HAVE WORKED**, which is why the obvious
cheaper fix was not taken. The hazard is an ANCESTOR marker: project
detection walks upward, so a fresh subdirectory of `/tmp` inherits
`/tmp`'s ancestors and the same stray `.git`. The directory had to move
somewhere the gate already owns.
**`SUN_LEN` shaped the layout, and the fix's own gate run is what found
it.** A Unix socket path cannot exceed 108 bytes, and the suites bind
sockets INSIDE `TMPDIR`. The first placement --- `$TARGET/gate-tmp/$STAMP-$$`
--- produced a 114-byte socket path and failed SIX daemon and attach
tests with "path must be shorter than SUN_LEN". It hangs off the gate
root (36 bytes) rather than the per-worktree target (60) now, with a
short name: 47 bytes, leaving 61 for fixtures. Running the real gate
rather than only the witnesses is what caught this.
**A startup guard turns that failure class into a named one.** Six
socket failures deep in a suite name a LIMIT, not a CAUSE; the guard
fails immediately with the path, its length, and what to shorten.
**Its reserve is measured, not round, and the first value was wrong in
the more embarrassing direction.** The longest suffix a fixture appends
is `/.tmpXXXXXXX/test.sock`, 21 bytes, so 30 leaves ~40% headroom. An
earlier "generous" 45 FIRED ON THE GATE'S OWN BEHAVIOUR TESTS: they run
the gate inside the gate, so their root sits under the outer run's
TMPDIR and the nested path reaches 71 bytes. A guard that rejects a
legitimate configuration is worse than the failure it prevents, because
it fires on every run instead of a rare one. Verified both directions:
still catches an 87-byte root, silent on the real one and on the nested
tests.
**Two witnesses, each mutation-checked.** `M-G-1` removes the export ->
the propagation row alone; `M-G-2` stops the reaping -> the cleanup row
alone. Propagation is observed in a SPAWNED CHILD --- the self-test's
first step reports its own `$TMPDIR` into its log --- because asserting
the variable inside the script would only prove the script can set a
variable. The cleanup row runs against the self-test, which FAILS on
purpose, so it also pins that the trap fires on the failure path, which
is the path a leak would actually happen on.
One witness of mine needed correcting twice, both times because it
asserted something adjacent to the contract: first `!starts_with("/tmp/")`,
which tested where the FIXTURE put its root and failed on correct code;
then `contains("/gate-tmp/")`, stale after the directory was shortened.
It now asserts the exact parent, `<root>/tmp`.
**Proved against the live hazard:** `/tmp/.git` is still present on this
machine, and `m4_24_bare_string_glob_stays_relative` --- one of the two
tests it reddened --- passes with no override.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016bqGA6s9tTUFzYpbeW3tai
Levi Neuwirth
d84df23aa7