Linux: Use _exit() instead of exit() in the child after fork() in failure conditions.

base/process_util_posix.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <sys/resource.h>
@@ skipping 423 matching lines @@
   if (pipe(pipe_fd) < 0)
     return false;
 
   switch (pid = fork()) {
     case -1:  // error
       close(pipe_fd[0]);
       close(pipe_fd[1]);
       return false;
     case 0:  // child
       {
+        // Obscure fork() rule: in the child, if you don't end up doing exec*(),
+        // you call _exit() instead of exit(). This is because _exit() does not
+        // call any previously-registered (in the parent) exit handlers, which
+        // might do things like block waiting for threads that don't even exist
+        // in the child.
         int dev_null = open("/dev/null", O_WRONLY);
         if (dev_null < 0)
           _exit(127);
 
         InjectiveMultimap fd_shuffle;
         fd_shuffle.push_back(InjectionArc(pipe_fd[1], STDOUT_FILENO, true));
         fd_shuffle.push_back(InjectionArc(dev_null, STDERR_FILENO, true));
         fd_shuffle.push_back(InjectionArc(dev_null, STDIN_FILENO, true));
 
         if (!ShuffleFileDescriptors(fd_shuffle))
@@ skipping 89 matching lines @@
                       const ProcessFilter* filter) {
   bool exited_cleanly =
       WaitForProcessesToExit(executable_name, wait_milliseconds,
                              filter);
   if (!exited_cleanly)
     KillProcesses(executable_name, exit_code, filter);
   return exited_cleanly;
 }
 
 }  // namespace base