Use RecvMsgWithPid to find real PID for zygote children

content/zygote/zygote_linux.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_ZYGOTE_ZYGOTE_H_
 #define CONTENT_ZYGOTE_ZYGOTE_H_
 
 #include <string>
 
 #include "base/containers/small_map.h"
@@ skipping 59 matching lines @@
                             int* exit_code);
 
   void HandleGetTerminationStatus(int fd,
                                   const Pickle& pickle,
                                   PickleIterator iter);
 
   // This is equivalent to fork(), except that, when using the SUID sandbox, it
   // returns the real PID of the child process as it appears outside the
   // sandbox, rather than returning the PID inside the sandbox. Optionally, it
   // fills in uma_name et al with a report the helper wants to make via
   // UMA_HISTOGRAM_ENUMERATION.

[jln (very slow on Chromium), 2014/05/05 19:01:42]

It's a little unfortunately that this method is so poorly documented, but let's
add some documentation for |pid_oracle| at least.

[mdempsky, 2014/05/05 21:38:18]

Done.  Tried to improve the uma_* docs a little bit too.

   int ForkWithRealPid(const std::string& process_type,
                       const base::GlobalDescriptors::Mapping& fd_mapping,
                       const std::string& channel_id,
+                      base::ScopedFD pid_oracle,
                       std::string* uma_name,
                       int* uma_sample,
                       int* uma_boundary_value);
 
   // Unpacks process type and arguments from |pickle| and forks a new process.
   // Returns -1 on error, otherwise returns twice, returning 0 to the child
   // process and the child process ID to the parent process, like fork().
   base::ProcessId ReadArgsAndFork(const Pickle& pickle,
                                   PickleIterator iter,
                                   ScopedVector<base::ScopedFD> fds,
@@ skipping 26 matching lines @@
   // These might be set by helper_->InitialUMA. They supply a UMA enumeration
   // sample we should report on the first fork.
   std::string initial_uma_name_;
   int initial_uma_sample_;
   int initial_uma_boundary_value_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_ZYGOTE_ZYGOTE_H_