Fix Android multi-process tests on M.

base/posix/global_descriptors.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 BASE_POSIX_GLOBAL_DESCRIPTORS_H_
 #define BASE_POSIX_GLOBAL_DESCRIPTORS_H_
 
 #include "build/build_config.h"
 
 #include <vector>
@@ skipping 37 matching lines @@
     // Optional region, defaults to kWholeFile.
     base::MemoryMappedFile::Region region;
   };
   typedef std::vector<Descriptor> Mapping;
 
   // Often we want a canonical descriptor for a given Key. In this case, we add
   // the following constant to the key value:
 #if !defined(OS_ANDROID)
   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
 #else
-  static const int kBaseDescriptor = 4;  // 3 used by __android_log_write().
+  // 3 used by __android_log_write().
+  // 4 used by... something important on Android M.

[Mark Mentovai, 2016/05/04 12:07:12]

Wish we had more confidence in what this was. What if it’s a leaked FD to
something that we really don’t want access to?

[Anand Mistry (off Chromium), 2016/05/04 21:48:42]

The problem I came across is that closing or dup'ing over this fd will hang.
Even in the forked helper process which has no need to access anything beside
the stdout/stderr and the parent pipe. All I know is that this is a socket or
pipe (it's not a file). I didn't do any further digging as to what this is.

Fortunately, I don't believe this is used outside of tests. The normal Android
child process starter (in content/) doesn't have any control over fd mappings
due to all the Java that runs along side the C++ in the same process. So the
test child helper should be the only thing on Android which uses this.

+  static const int kBaseDescriptor = 5;
 #endif
 
   // Return the singleton instance of GlobalDescriptors.
   static GlobalDescriptors* GetInstance();
 
   // Get a descriptor given a key. It is a fatal error if the key is not known.
   int Get(Key key) const;
 
   // Get a descriptor given a key. Returns -1 on error.
   int MaybeGet(Key key) const;
@@ skipping 14 matching lines @@
   friend struct DefaultSingletonTraits<GlobalDescriptors>;
   GlobalDescriptors();
   ~GlobalDescriptors();
 
   Mapping descriptors_;
 };
 
 }  // namespace base
 
 #endif  // BASE_POSIX_GLOBAL_DESCRIPTORS_H_