Fix a race condition that causes Emacs to mess up glib.
The symptom is a diagnostic "GLib-WARNING **: In call to g_spawn_sync(), exit status of a child process was requested but SIGCHLD action was set to SIG_IGN and ECHILD was received by waitpid(), so exit status can't be returned." The diagnostic is partly wrong, as the SIGCHLD action is not set to SIG_IGN. The real bug is a race condition between Emacs and glib: Emacs does a waitpid (-1, ...) and reaps glib's subprocess by mistake, so that glib can't find it. Work around the bug by invoking waitpid only on subprocesses that Emacs itself creates. * process.c (create_process, record_child_status_change): Don't use special value -1 in pid field, as the caller now must know the pid rather than having the callee infer it. The inference was sometimes incorrect anyway, due to another race. (create_process): Set new 'alive' member if child is created. (process_status_retrieved): New function. (record_child_status_change): Use it. Accept negative 1st argument, which means to wait for the processes that Emacs already knows about. Move special-case code for DOS_NT (which lacks WNOHANG) here, from caller. Keep track of processes that have already been waited for, by testing and clearing new 'alive' member. (CAN_HANDLE_MULTIPLE_CHILDREN): Remove, as record_child_status_change now does this internally. (handle_child_signal): Let record_child_status_change do all the work, since we do not want to reap all exited child processes, only the child processes that Emacs itself created. * process.h (Lisp_Process): New boolean member 'alive'. Fixes: debbugs:8855
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