Avoid creating zombie processes by waiting for the terminated process.

runtime/bin/process_macos.cc

Index: runtime/bin/process_macos.cc
diff --git a/runtime/bin/process_macos.cc b/runtime/bin/process_macos.cc
index 2b6425b2a4611dac518f8c8b0384e60851ea0af0..df4a7dc5b103959573eef96016568c0ac40ec1fb 100644
--- a/runtime/bin/process_macos.cc
+++ b/runtime/bin/process_macos.cc
@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+#include "bin/process.h"
+
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
@@ -11,8 +13,6 @@
 #include <unistd.h>
 
 #include "bin/fdutils.h"
-#include "bin/process.h"
-#include "bin/set.h"
 
 
 class ProcessInfo {
@@ -95,6 +95,11 @@ void ExitHandler(int process_signal, siginfo_t* siginfo, void* tmp) {
   }
   ProcessInfo* process = LookupProcess(siginfo->si_pid);
   if (process != NULL) {
+    int status = 0;
+    int wait_result = waitpid(siginfo->si_pid, &status, WNOHANG);
+    if (wait_result == -1 || wait_result == 0) {
+      perror("ExitHandler waitpid failed");
+    }
     intptr_t message[2] = { siginfo->si_pid, siginfo->si_status };
     intptr_t result =
         FDUtils::WriteToBlocking(process->fd(), &message, sizeof(message));