Fail fast if the template process dies during dmtcp_restart#7
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When model checking from a checkpoint, multithreaded_fork_process_source launched dmtcp_restart and then blocked on the readiness semaphore via a plain sig_semwait. If the restarted template process died before signaling readiness -- a corrupt or incompatible checkpoint, or any crash during restore -- mcmini hung forever with no diagnostic: SIGCHLD is not guaranteed to reach us (dmtcp_restart may reparent the restored process), so the semaphore was never posted and the wait never returned. Poll the readiness semaphore with a 1s timeout and, whenever it hasn't been posted, actively check whether the template process is still alive with waitid(P_PID, ..., WEXITED|WNOHANG|WNOWAIT). If it died, throw process_creation_exception with a clear message instead of blocking. WNOWAIT peeks without reaping, so the process handle's destructor is unaffected. The liveness check also runs after a successful acquire, because the SIGCHLD handler posts the same semaphore on child death -- so a post alone does not imply the template stabilized. This is a general robustness fix (independent of TSAN/DMTCP specifics): the classic fork-from-main path already detects template death via SIGCHLD; the restart path did not. It converts every restart failure from a silent hang into a surfaced error. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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In fork_process_source.cpp: "The template process ... was not synchronized with correctly:", it's possible that mtcp_restart crashed before the template process could post. (See the comment in the code.) The result was silent hanging, instead of registering the crash of mtcp_restart. This PR fixes that. It was helpful in development to support TSAN.