The schedAdmin dispatch case blocks its vtask handler thread for up to 5s
waiting on a cross-process reply while holding c->sched.lock -- bounded
blocking on a vtask thread, which ha-async-io.md permits only under an
admission reservation. The real risk isn't just the wait itself: since
c->sched.lock serializes concurrent requests one at a time, a burst of N
concurrent schedAdmin calls could already occupy N vtask threads for up to
N*5s each before this fix.
Now acquires vtaskmgr_blocking_acquire(&c->vtaskmgr) before ever locking
c->sched.lock, releasing on every exit path -- the same shared admission
budget commit.c's commit-wait barrier already gates on (wired via
commit_set_pool). Admission-exhausted requests fast-reject with
{ok=false, error="server busy, try again"} rather than queuing. New test:
test_schedadmin_fast_rejects_when_admission_exhausted, using a poison
c->sched.ep sentinel to prove the fast-reject happens before any
lock/forward attempt.
Rongsong Shen
e52c68f471