_probe_remote_gateway held _remote_probe_lock only around the cache
read and cache write; the network probe itself (_http_probe per path,
each up to ~2s) ran unlocked. On a cold cache a dashboard that fans out
to N panels fired N concurrent probe sets at the (possibly dead)
gateway, each blocking up to ~6s (#5455, #2476).
Wrap the existing lock in a Condition and add an in-flight set: the
first 'leader' thread marks the base_url in-flight and runs the probe
(via extracted _run_remote_probe), while latecomers wait on the
Condition for the leader's cached result instead of probing themselves.
The leader clears the marker and notify_all()s in a finally block, so
waiters are woken even if the probe raises (no deadlock); a bounded wait
budget lets a waiter fall back to probing itself if a leader hangs.
_reset_remote_probe_cache_for_tests also clears the in-flight set.
Expire the cache from the probe's COMPLETION time, not the leader's
entry time: walking every path of a hung gateway takes
len(paths) * timeout (~6s), which exceeds the 5s TTL, so
`entry_time + TTL` would store an already-expired cache line. Waiters
woken right after would then miss the cache and each re-probe the dead
gateway — collapsing single-flight and regressing latency to worse-than-
serial. A regression test drives a walk longer than the TTL and asserts
still exactly one probe set (the shipped happy-path test kept the walk
well under the TTL and so never exercised this).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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