Only call waitpid for processes that Mono tracks, rather than all child processes owned by the calling process. (mono#6202)

waitpid(-1,...) reaps all zombie child processes of the calling process (see `man waitpid`). After a child process is reaped, it can no longer be queried. Because this reaping step is invoked via the `SIGCHLD` signal, all zombie children of the calling process are reaped every time any child process terminates, regardless of whether that child process was tracked by Mono.

The impact of this bug is that apps using Mono that spawn their own child processes (outside of Mono's `w32process` API) cannot query the exit state of its children. For example, a call to `waitpid(my_child_pid,...)` will return `ECHILD` for "no such child" because Mono has already cleaned up the child the moment `SIGCHLD` was raised.

Author: PeteLewisUnity

mono/metadata/w32process-unix.c

@@ -1401,34 +1401,35 @@ process_add_sigchld_handler (void)
 void
 mono_w32process_signal_finished (void)
 {
-	int status;
-	int pid;
-	Process *process;
+	mono_coop_mutex_lock (&processes_mutex);
+
+	for (Process* process = processes; process; process = process->next) {
+		int status = -1;
+		int pid;
 
-	do {
 		do {
-			pid = waitpid (-1, &status, WNOHANG);
+			pid = waitpid (process->pid, &status, WNOHANG);
 		} while (pid == -1 && errno == EINTR);
 
-		if (pid <= 0)
-			break;
-
-		mono_coop_mutex_lock (&processes_mutex);
+		// possible values of 'pid':
+		//  process->pid : the status changed for this child
+		//  0            : status unchanged for this PID
+		//  ECHILD       : process has been reaped elsewhere (or never existed)
+		//  EINVAL       : invalid PID or other argument
 
-		for (process = processes; process; process = process->next) {
-			if (process->pid != pid)
-				continue;
-			if (process->signalled)
-				continue;
+		// Therefore, we ignore status unchanged (nothing to do) and error
+		// events (process is cleaned up later).
+		if (pid <= 0)
+			continue;
+		if (process->signalled)
+			continue;
 
-			process->signalled = TRUE;
-			process->status = status;
-			mono_coop_sem_post (&process->exit_sem);
-			break;
-		}
+		process->signalled = TRUE;
+		process->status = status;
+		mono_coop_sem_post (&process->exit_sem);
+	}
 
-		mono_coop_mutex_unlock (&processes_mutex);
-	} while (1);
+	mono_coop_mutex_unlock (&processes_mutex);
 }
 
 static gboolean