`agent_uid_gid` returns `None` for all 13 agents on every sync, and the
tmpfiles caller answers that `None` by writing `d /run/hive-agent/<name>
0777 root root` instead of `0751 <uid> <gid>` — a world-writable socket
dir, which docs/trust-boundary/boundary.md spells out as letting anything
that can reach the path unlink an agent's socket and bind its own.
Which failure fires could not be determined, because the read used
`.ok()?` and collapsed every io::Error into the same `None` a missing
user produces. None of the three causes the doc comment enumerated (not
built yet, unparseable, missing user) fits 13 long-lived containers whose
agent user demonstrably exists — from inside one:
srw-rw-rw- atlas atlas /run/hive-agent/atlas/agent.sock
So the live cause was outside the documented set and unidentifiable. Both
`None` arms now log, with the path and the error.
Splitting the pure parser out to make it testable surfaced a second,
narrower bug. The old scan used `?` on the field reads, and those are
only reached once the name matches — so an unusable row *for the wanted
user* returned `None` from the whole function instead of skipping, hiding
a usable entry below it. (Rows for other users were always skipped fine:
`split(':')` always yields at least one item, so the first `?` could not
fire.) It now skips unusable rows and keeps looking.
Does not pre-empt #3047, which removes the lookup entirely and stays
blocked on #3998; this only makes the lookup honest about failing while
it exists.
Closes#4197.
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