Track T: c-conformance shards time out under full parallel load (false REDs)

• Track: T (test infra)
• Found: 2026-07-13, during a Track A/C session — it produced THREE false REDs in one night and cost several full-matrix reruns.

Symptom

tools/testmgr.py --tier full intermittently reports

  FAIL     test-c-conformance-arm32#shard3/6   conformance   39.7s  tools/run_c_conformance.sh
  FAIL     test-c-conformance-aarch64#shard3/6 conformance   44.5s  tools/run_c_conformance.sh

and the same job run on its own passes in 8.2 s:

tools/testmgr.py --tier full --job 'test-c-conformance-arm32#shard3/6'
  PASS ... 8.2s

So the shard takes ~5x longer under the full 16-way parallel matrix than alone, and crosses the timeout. #shard3 is the usual victim; test-aarch64#…sysopen and test-c-conformance-aarch64#shard1 have flaked the same way (exit=124).

Not a compiler regression — verified

Suspecting my own change (anonymous bit-fields, which stops those structs being opaque and so does MORE layout work), I timed the same shard with the PINNED pre-change compiler: 8.2 s, identical. The slowdown is contention, not codegen.

Why it matters

A false RED is worse than a slow test: it burns a full-matrix rerun (~5 min each), and it trains the reader to shrug at REDs — exactly the reflex that lets a real regression through. Three of four full runs went RED this way tonight.

Suggested fixes (Track T's call)

• Scale the per-job timeout by the concurrency actually in flight, or give the conformance jobs (which fork 220 compiles + 220 qemu runs each) a larger budget than a unit test.
• Or lower the parallel cap for the conformance family specifically — they are already internally parallel, so stacking 6 shards x N targets against 16 slots oversubscribes.
• Either way, distinguish TIMEOUT from FAIL in the report. exit=124 currently reads exactly like a wrong-output failure, and the log tail shows a successful build followed by nothing, which is genuinely ambiguous (that ambiguity already cost one bogus regression ticket: regression-test-aarch64-test-cross-sysopen-family).

Log

• 2026-07-15 — resolved, commit ab3a5b2a.