Take advantage of MojoGetBufferInformation

mojo/services/media/common/cpp/mapped_shared_buffer.cc

 // Copyright 2015 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.
 
 #include "mojo/public/cpp/environment/logging.h"
 #include "mojo/services/media/common/cpp/mapped_shared_buffer.h"
 
 namespace mojo {
 namespace media {
 
 MappedSharedBuffer::MappedSharedBuffer() {}
 
 MappedSharedBuffer::~MappedSharedBuffer() {}
 
 void MappedSharedBuffer::InitNew(uint64_t size) {
   MOJO_DCHECK(size > 0);
 
   buffer_.reset(new SharedBuffer(size));
   handle_.reset();
 
-  InitInternal(buffer_->handle, size);
+  InitInternal(buffer_->handle);
 }
 
-void MappedSharedBuffer::InitFromHandle(
-    ScopedSharedBufferHandle handle,
-    uint64_t size) {
+void MappedSharedBuffer::InitFromHandle(ScopedSharedBufferHandle handle) {
   MOJO_DCHECK(handle.is_valid());
-  MOJO_DCHECK(size > 0);
 
   buffer_.reset();
   handle_ = handle.Pass();
 
-  InitInternal(handle_, size);
+  InitInternal(handle_);
 }
 
-void MappedSharedBuffer::InitInternal(
-    ScopedSharedBufferHandle& handle,
-    uint64_t size) {
+void MappedSharedBuffer::InitInternal(const ScopedSharedBufferHandle& handle) {
   MOJO_DCHECK(handle.is_valid());
+
+  // Query the buffer for its size.
+  // TODO(johngro) :  It would be nice if we could do something other than
+  // DCHECK if things don't go exactly our way.
+  MojoBufferInformation info;
+  MojoResult res = MojoGetBufferInformation(handle.get().value(),
+                                            &info,
+                                            sizeof(info));
+  uint64_t size = info.num_bytes;
+  MOJO_DCHECK(res == MOJO_RESULT_OK);
   MOJO_DCHECK(size > 0);
 
   size_ = size;
   buffer_ptr_.reset();
 
   void* ptr;
   auto result = MapBuffer(
       handle.get(),
-      0, // offset
+      0,  // offset
       size,
       &ptr,
       MOJO_MAP_BUFFER_FLAG_NONE);
   MOJO_DCHECK(result == MOJO_RESULT_OK);
   MOJO_DCHECK(ptr);
 
   buffer_ptr_.reset(reinterpret_cast<uint8_t*>(ptr));
 
   OnInit();
 }
@@ skipping 34 matching lines @@
   if (ptr == nullptr) {
     return FifoAllocator::kNullOffset;
   }
   uint64_t offset = reinterpret_cast<uint8_t*>(ptr) - buffer_ptr_.get();
   MOJO_DCHECK(offset < size_);
   return offset;
 }
 
 void MappedSharedBuffer::OnInit() {}
 
-} // namespace media
-} // namespace mojo
+}  // namespace media
+}  // namespace mojo