base: Introduce SharedMemoryTracker for POSIX (but not macOS)

base/memory/shared_memory.h

Index: base/memory/shared_memory.h
diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index 3c68e90a031ff476dd7585b4d6653cc92e7b7643..95d00c0ad769b2ec89ae543d8f07058dafa0a5d0 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -255,6 +255,13 @@ class BASE_EXPORT SharedMemory {
     return ShareToProcessCommon(process, new_handle, true, SHARE_CURRENT_MODE);
   }
 
+#if defined(OS_POSIX) && !(defined(OS_MACOSX) && !defined(OS_IOS)) && \

[danakj, 2017/01/27 16:56:42]

POSIX && (!MACOSX || IOS) && !NACL avoids double negation.

[hajimehoshi, 2017/02/07 12:33:44]

Done.

+    !defined(OS_NACL)
+  using Id = std::pair<dev_t, ino_t>;

[danakj, 2017/01/27 16:56:42]

I think the type alias just makes use of first/second unclear in the tracker.
I'd prefer to use the pair type literally.

[hajimehoshi, 2017/02/07 12:33:44]

This can be different type among Windows, macOS and other POSIX, so I want to
keep this type alias.

+
+  bool GetUniqueId(Id* id) const;

[danakj, 2017/01/27 16:56:42]

Can you make a comment that this goes to the file system ie can block and is
slow.

[hajimehoshi, 2017/02/07 12:33:44]

Done.

+#endif
+
  private:
 #if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_ANDROID) && \
     !(defined(OS_MACOSX) && !defined(OS_IOS))
@@ -301,6 +308,7 @@ class BASE_EXPORT SharedMemory {
 
   DISALLOW_COPY_AND_ASSIGN(SharedMemory);
 };
+
 }  // namespace base
 
 #endif  // BASE_MEMORY_SHARED_MEMORY_H_