Add the ability to run a callback between fork and exec.

base/process/launch.h

 // Copyright 2013 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.
 
 // This file contains functions for launching subprocesses.
 
 #ifndef BASE_PROCESS_LAUNCH_H_
 #define BASE_PROCESS_LAUNCH_H_
 
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/environment.h"
 #include "base/process/process.h"
 #include "base/process/process_handle.h"
 #include "base/strings/string_piece.h"
 
 #if defined(OS_POSIX)
 #include "base/posix/file_descriptor_shuffle.h"
 #elif defined(OS_WIN)
 #include <windows.h>
 #endif
@@ skipping 89 matching lines @@
 
 #if defined(OS_LINUX)
   // If non-zero, start the process using clone(), using flags as provided.
   int clone_flags;
 
   // By default, child processes will have the PR_SET_NO_NEW_PRIVS bit set. If
   // true, then this bit will not be set in the new child process.
   bool allow_new_privs;
 #endif  // defined(OS_LINUX)
 
+#if defined(OS_POSIX)
+  // A callback to be run in immediately prior to executing the new program in
+  // the child process. Warning: If LaunchProcess was called in the presence of
+  // multiple threads, it may not be safe for this callback to perform
+  // allocations or grab locks.
+  Closure pre_exec_hook;

[jln (very slow on Chromium), 2015/01/06 00:43:36]

If we go this route, we should make the warning even stronger. Code between
fork() and exec() needs to be essentially async-signal safe. (And
base::LaunchProcess() is already taking some worrisome liberties). It's also not
obvious how to check that callbacks themselves will stay safe. 

Since, as per our discussion, you only need this to build a synchronization
primitive (so that the child waits for the parent to signal a pipe), perhaps we
should build that in this API instead of giving such a generic API.

I'm not convinced either way, so let's see what mdempsky@ prefers and also get
input from a base/ owner before changing everything.

+#endif
+
 #if defined(OS_CHROMEOS)
   // If non-negative, the specified file descriptor will be set as the launched
   // process' controlling terminal.
   int ctrl_terminal_fd;
 #endif  // defined(OS_CHROMEOS)
 
 #if defined(OS_MACOSX)
   // If this name is non-empty, the new child, after fork() but before exec(),
   // will look up this server name in the bootstrap namespace. The resulting
   // service port will be replaced as the bootstrap port in the child. Because
@@ skipping 128 matching lines @@
 void ReplaceBootstrapPort(const std::string& replacement_bootstrap_name);
 #endif  // defined(OS_MACOSX)
 
 // Creates a LaunchOptions object suitable for launching processes in a test
 // binary. This should not be called in production/released code.
 BASE_EXPORT LaunchOptions LaunchOptionsForTest();
 
 }  // namespace base
 
 #endif  // BASE_PROCESS_LAUNCH_H_