---
id: "bug-1791"
status: "fixed"
status_updated: "2026-09-24"
status_note: ""
component: "controller"
severity: "unknown"
timestamp: "2026-09-09T08:50:58.000Z"
last_seen: "2026-09-19T09:00:00Z"
occurrences: 1
files: ["src/controller/testsuite/interruptcall.c"]
tags: ["pthread", "mutex", "EPERM", "cross-thread-unlock", "ownership-handoff", "xce-test-cli-ux", "task-1.5", "interruptcall.c", "design-review-caught-by-test", "posix-ub", "sem_t", "semaphore-migration"]
related: []
legacy: true
---

# bug-1791: controller_unit_test: interruptcall.c:142: xcetest_worker_main: Assertion \`unlock_rc == 0' failed…

## Description
controller_unit_test: interruptcall.c:142: xcetest_worker_main: Assertion \`unlock_rc == 0' failed (pthread_mutex_unlock returned EPERM)

## Environment
_Not recorded (migrated from buglog.json)._

## Reproduction
_Not recorded (migrated from buglog.json)._

## Root cause
xce-test-cli-ux task 1.5's per-connection lock registry initialized each connection's pthread_mutex_t with PTHREAD_MUTEX_ERRORCHECK, intended to catch double-unlock/never-unlocked regressions. But this design's own ownership-handoff protocol (design.md/research.md's 'per-connection lock' decision) REQUIRES a legitimate cross-thread release on the abandonment path: the calling (REPL) thread acquires the lock via pthread_mutex_trylock(), but on abandonment the still-running detached WORKER thread is the one that must release it later, once fn() finishes -- ownership of 'release the lock' passes to the worker at the same moment ownership of the heap call block does. PTHREAD_MUTEX_ERRORCHECK (and _RECURSIVE) mutexes track a single owning thread and reject pthread_mutex_unlock() from any other thread with EPERM, which fired immediately on the very first same-key-serialization test.

## Fix
Switched the per-connection lock registry (xcetest_conn_lock_for() in interruptcall.c) to initialize each mutex with the DEFAULT type (pthread_mutex_init(&node->mutex, NULL)) instead of PTHREAD_MUTEX_ERRORCHECK. glibc default (PTHREAD_MUTEX_NORMAL-equivalent) mutexes do not track a per-thread owner, so a worker thread may validly unlock a mutex the REPL thread locked. Kept the assert(pthread_mutex_unlock(...) == 0) safety-net checks at every release site (computed outside the assert() expression itself, so the unlock call is never skipped under NDEBUG) as a lower-cost guard against genuine corruption, without relying on error-checking-mutex semantics that conflict with the intentional cross-thread handoff. SUPERSEDED 2026-09-09 (task 1.5 remediation round 1, independent review): an external reviewer correctly pointed out that POSIX classifies non-owning-thread pthread_mutex_unlock() as undefined behavior for PTHREAD_MUTEX_NORMAL/DEFAULT too, not just ERRORCHECK/_RECURSIVE -- NORMAL is merely *permitted* not to detect the violation, not declared well-defined for it. The DEFAULT-mutex fix above was therefore empirically safe on XCE's glibc/NPTL-exclusive Nix build but relied on non-portable, POSIX-UB behavior, under-documented as such. Replaced the pthread_mutex_t per-connection lock with a POSIX semaphore (sem_t, binary, sem_init(&sem, 0, 1)) throughout: sem_trywait() in place of pthread_mutex_trylock() (EAGAIN in place of EBUSY) in the interruptible poll loop, and sem_post() in place of pthread_mutex_unlock() at every release site (both the caller-normal-completion path and the worker-abandoned-cleanup cross-thread path). sem_t has no ownership concept, so this cross-thread release is now fully POSIX-defined with no UB reliance at all, at the same code complexity. All 1285 controller_unit_test tests (including the 3 task-1.5-specific tests) continued to pass unchanged after the migration.

## Verification
_Not recorded (migrated from buglog.json)._

## References
_Not recorded (migrated from buglog.json)._