Summary

test_model_rts aborts on macOS in subprocess_helper::wait_if_parent():

test_model_rts: FAILED: void subprocess_helper::wait_if_parent()/145: waitpid(_child_pid, &status, 0) > 0
test_model_rts: process aborting

The waitpid call fails with EINTR (errno 4, confirmed under a debugger).

Root cause

In test/model/test/common/subprocess.cpp:

• subprocess_helper::subprocess_helper() installs a SIGCHLD handler with sigaction and a zeroed struct sigaction, so sa_flags does not include SA_RESTART.
• handler_sigchld() itself calls wait(NULL), so the handler reaps the child.
• wait_if_parent() then calls waitpid(_child_pid, ...) and asserts the return value is positive.

The parent's waitpid therefore races the signal handler. Without SA_RESTART it is interrupted by SIGCHLD and returns EINTR; if the handler wins the race and reaps first, it returns ECHILD. Neither is a real failure, but the assertion treats both as fatal.

Solution

Retry the wait on EINTR, and accept ECHILD since the handler legitimately reaps the child:

void
subprocess_helper::wait_if_parent()
{
    if (parent()) {
        int status;
        pid_t pid;

        /*
         * The SIGCHLD handler reaps the child itself, and it is installed without SA_RESTART, so
         * this wait can either be interrupted by the signal or find the child already gone.
         */
        while ((pid = waitpid(_child_pid, &status, 0)) < 0 && errno == EINTR)
            ;
        testutil_assert(pid > 0 || errno == ECHILD);
    }
}

This also needs #include <cerrno> in the same file, and SIGCHLD added to dist/s_string.ok for the new comment.

Notes

This was found while working on WT-15324 (test/model fails on macOS). It is currently latent: on macOS every test/model binary aborts earlier in decode_utf8, so test_model_rts never reaches this code. Fixing WT-15324 unmasks it, which is why it is split out here and should land first.

Definition of Done

• test_model_rts no longer aborts in wait_if_parent() on macOS.
• The existing test/model suite still passes on Linux in Evergreen.
• dist/s_all (or the targeted style checks) passes on the changed files.