e21a431ec4
assert_signal_internal() returns 0 in two distinct cases: 1. In the child process (immediately after fork returns 0). 2. In the parent process, if the child exited normally (no signal). ASSERT_SIGNAL fails to distinguish these cases. When a child exited normally (case 2), the parent process receives 0, incorrectly interprets it as meaning it is the child, and re-executes the test expression inside the parent process. Goodness gracious! This causes two severe test integrity issues: 1. False positives. The parent can run the expression, succeed, and call _exit(EXIT_SUCCESS), causing the test to pass even though no signal was raised. 2. Silent truncation. The _exit() call in the parent terminates the test runner prematurely, preventing subsequent tests in the same file from running. Example of the bug in action, from #39674: ASSERT_SIGNAL(fd_is_writable(closed_fd), SIGABRT) This test should fail (fd_is_writable does not SIGABRT here), but with the bug, the parent hallucinated being the child, re-ran the expression successfully, and exited with success. Fix this by refactoring assert_signal_internal() to be much more strict about separating control flow from data. The signal status is now returned via a strictly typed output parameter, guaranteeing that determining whether we are the child is never conflated with whether the child exited cleanly.
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