Replace base::SharedMemory read-only methods with GetReadOnlyHandle.

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 34cdc1ed16273befbaa8258f01777b95af33efaf..b03d1d36f4ad3f03c7e2bbe0419aa28b87d04dfa 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -80,8 +80,8 @@ class BASE_EXPORT SharedMemory {
   // shared memory file.
   //
   // WARNING: This does not reduce the OS-level permissions on the handle; it
-  // only affects how the SharedMemory will be mmapped.  Use
-  // ShareReadOnlyToProcess to drop permissions.  TODO(jln,jyasskin): DCHECK
+  // only affects how the SharedMemory will be mmapped. Use
+  // GetReadOnlyHandle to drop permissions. TODO(jln,jyasskin): DCHECK
   // that |read_only| matches the permissions of the handle.
   SharedMemory(const SharedMemoryHandle& handle, bool read_only);
 
@@ -206,31 +206,12 @@ class BASE_EXPORT SharedMemory {
   // It is safe to call Close repeatedly.
   void Close();
 
-  // Shares the shared memory to another process.  Attempts to create a
-  // platform-specific new_handle which can be used in a remote process to read
-  // the shared memory file.  new_handle is an output parameter to receive the
-  // handle for use in the remote process.
-  //
-  // |*this| must have been initialized using one of the Create*() or Open()
-  // methods with share_read_only=true. If it was constructed from a
-  // SharedMemoryHandle, this call will CHECK-fail.
-  //
-  // Returns true on success, false otherwise.
-  bool ShareReadOnlyToProcess(ProcessHandle process,
-                              SharedMemoryHandle* new_handle) {
-    return ShareToProcessCommon(process, new_handle, false, SHARE_READONLY);
-  }
-
-  // Logically equivalent to:
-  //   bool ok = ShareReadOnlyToProcess(process, new_handle);
-  //   Close();
-  //   return ok;
-  // Note that the memory is unmapped by calling this method, regardless of the
-  // return value.
-  bool GiveReadOnlyToProcess(ProcessHandle process,
-                             SharedMemoryHandle* new_handle) {
-    return ShareToProcessCommon(process, new_handle, true, SHARE_READONLY);
-  }
+  // Returns a read-only handle to this shared memory region. The caller takes
+  // ownership of the handle. CHECK-fails if the region wasn't Created or

[Nico, 2017/05/02 15:48:39]

I didn't see this promised CHECK on all platforms.

[erikchen, 2017/05/02 17:52:18]

It's only relevant for POSIX handles. I've updated the comment.

+  // Opened with share_read_only=true. When the handle is passed to the IPC
+  // subsystem, that takes ownership of the handle. As such, it's not valid to
+  // pass the sample handle to the IPC subsystem twice.

[Nico, 2017/05/02 15:48:39]

Say that this will return an invalid handle on failure.

[erikchen, 2017/05/02 17:52:18]

Done.

+  SharedMemoryHandle GetReadOnlyHandle();
 
   // Shares the shared memory to another process.  Attempts
   // to create a platform-specific new_handle which can be
@@ -240,7 +221,7 @@ class BASE_EXPORT SharedMemory {
   // Returns true on success, false otherwise.
   bool ShareToProcess(ProcessHandle process,
                       SharedMemoryHandle* new_handle) {
-    return ShareToProcessCommon(process, new_handle, false, SHARE_CURRENT_MODE);
+    return ShareToProcessCommon(process, new_handle, false);
   }
 
   // Logically equivalent to:
@@ -251,7 +232,7 @@ class BASE_EXPORT SharedMemory {
   // return value.
   bool GiveToProcess(ProcessHandle process,
                      SharedMemoryHandle* new_handle) {
-    return ShareToProcessCommon(process, new_handle, true, SHARE_CURRENT_MODE);
+    return ShareToProcessCommon(process, new_handle, true);
   }
 
 #if defined(OS_POSIX) && (!defined(OS_MACOSX) || defined(OS_IOS)) && \
@@ -275,19 +256,13 @@ class BASE_EXPORT SharedMemory {
   bool FilePathForMemoryName(const std::string& mem_name, FilePath* path);
 #endif
 
-  enum ShareMode {
-    SHARE_READONLY,
-    SHARE_CURRENT_MODE,
-  };
-
 #if defined(OS_MACOSX)
-  bool Share(SharedMemoryHandle* new_handle, ShareMode share_mode);
+  bool Share(SharedMemoryHandle* new_handle);
 #endif
 
   bool ShareToProcessCommon(ProcessHandle process,
                             SharedMemoryHandle* new_handle,
-                            bool close_self,
-                            ShareMode);
+                            bool close_self);
 
 #if defined(OS_WIN)
   // If true indicates this came from an external source so needs extra checks
@@ -295,20 +270,21 @@ class BASE_EXPORT SharedMemory {
   bool external_section_;
   std::wstring       name_;
   win::ScopedHandle  mapped_file_;
-#elif defined(OS_MACOSX) && !defined(OS_IOS)
+#else
   // The OS primitive that backs the shared memory region.
   SharedMemoryHandle shm_;
 
+  // If valid, points to the same memory region as shm_, but with readonly
+  // permissions.
+  SharedMemoryHandle readonly_shm_;
+#endif
+
+#if defined(OS_MACOSX) && !defined(OS_IOS)
   // The mechanism by which the memory is mapped. Only valid if |memory_| is not
   // |nullptr|.
   SharedMemoryHandle::Type mapped_memory_mechanism_;
-
-  int readonly_mapped_file_;
-#elif defined(OS_POSIX)
-  // The OS primitive that backs the shared memory region.
-  SharedMemoryHandle shm_;
-  int                readonly_mapped_file_;
 #endif
+
   size_t             mapped_size_;
   void*              memory_;
   bool               read_only_;