A test runner that could not create a scratch file

One denied mktemp stopped every commit. Toggle the two conditions to see where the failure lands.

conditions system temp dir denies the write apply the fallback

Why TMPDIR is not the workaround

$ TMPDIR=/unwritable mktemp
/var/folders/7v/.../T/tmp.iGOVlVqii5   <-- TMPDIR ignored

$ TMPDIR=/unwritable mktemp "$TMPDIR/x.XXXXXX"
mktemp: mkstemp failed: Operation not permitted   <-- honoured

macOS mktemp with no TEMPLATE ignores TMPDIR entirely. That is why setting it looks like a fix and proves nothing, and why the template is the actual repair rather than a style preference.

The part that is almost funny

Two of the tests this runner executes were written to catch a denied mktemp silently zeroing a count. Both used a bare mktemp -d themselves, so both died of the condition they exist to detect. They pass in CI, whose temp dir works, and fail in exactly the environment the bug lives in.

Order of preference: TMPDIR, then /tmp, then a gitignored tests/.tmp/ beside the runner. A named failure if all three are unusable, never a silent one.