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>
@@ skipping 19 matching lines @@
 
 #if defined(OS_WIN)
 typedef std::vector<HANDLE> HandlesToInheritVector;
 #endif
 // TODO(viettrungluu): Only define this on POSIX?
 typedef std::vector<std::pair<int, int> > FileHandleMappingVector;
 
 // Options for launching a subprocess that are passed to LaunchProcess().
 // The default constructor constructs the object with default options.
 struct BASE_EXPORT LaunchOptions {
+#if defined(OS_POSIX)
+  // Delegate to be run in between fork and exec in the subprocess (see
+  // pre_exec_delegate below)
+  class BASE_EXPORT PreExecDelegate {
+   public:
+    PreExecDelegate() {}
+    virtual ~PreExecDelegate() {}
+
+    // Since this is to be run between fork and exec, and fork may have happened
+    // while multiple threads were running, this function needs to be async
+    // safe.
+    virtual void RunAsyncSafe() = 0;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(PreExecDelegate);
+  };
+#endif  // defined(OS_POSIX)
+
   LaunchOptions();
   ~LaunchOptions();
 
   // If true, wait for the process to complete.
   bool wait;
 
 #if defined(OS_WIN)
   bool start_hidden;
 
   // If non-null, inherit exactly the list of handles in this vector (these
@@ skipping 65 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)
+  // If non-null, a delegate to be run immediately prior to executing the new
+  // program in the child process. Warning: If LaunchProcess was called in the
+  // presence of multiple threads, code running in this delegate essentially
+  // needs to be async-signal safe.

[Lei Zhang, 2015/01/08 23:07:07]

Maybe mention "man 7 signal" for a list of async signal safe functions, and
explicitly mention no mallocing. Have "Warning" in caps because there be
dragons.

[rickyz (no longer on Chrome), 2015/01/13 17:22:36]

Done.

+  PreExecDelegate* pre_exec_delegate;
+#endif  // defined(OS_POSIX)
+
 #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_