Support read-only duplicates of Mojo shared buffers.

mojo/public/c/system/buffer.h

 // Copyright 2014 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.
 
 // This file contains types/constants and functions specific to buffers (and in
 // particular shared buffers).
 // TODO(vtl): Reorganize this file (etc.) to separate general buffer functions
 // from (shared) buffer creation.
 //
 // Note: This header should be compilable as C.
@@ skipping 38 matching lines @@
                    "MojoCreateSharedBufferOptions has wrong size");
 
 // |MojoDuplicateBufferHandleOptions|: Used to specify parameters in duplicating
 // access to a shared buffer to |MojoDuplicateBufferHandle()|.
 //   |uint32_t struct_size|: Set to the size of the
 //       |MojoDuplicateBufferHandleOptions| struct. (Used to allow for future
 //       extensions.)
 //   |MojoDuplicateBufferHandleOptionsFlags flags|: Reserved for future use.
 //       |MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_NONE|: No flags; default
 //       mode.
+//       |MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_READ_ONLY|: The duplicate
+//       shared buffer can only be mapped read-only. A read-only duplicate can
+//       only be created before the buffer is passed over a message pipe.
 //
 // TODO(vtl): Add flags to remove writability (and executability)? Also, COW?
 
 typedef uint32_t MojoDuplicateBufferHandleOptionsFlags;
 
 #ifdef __cplusplus
 const MojoDuplicateBufferHandleOptionsFlags
     MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_NONE = 0;
+const MojoDuplicateBufferHandleOptionsFlags
+    MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_READ_ONLY = 1 << 0;
 #else
 #define MOJO_DUPLICATE_BUFFER_HANDLE_OPTIONS_FLAG_NONE \
   ((MojoDuplicateBufferHandleOptionsFlags)0)
 #endif
 
 struct MojoDuplicateBufferHandleOptions {
   uint32_t struct_size;
   MojoDuplicateBufferHandleOptionsFlags flags;
 };
 MOJO_STATIC_ASSERT(sizeof(MojoDuplicateBufferHandleOptions) == 8,
@@ skipping 102 matching lines @@
 //   |MOJO_RESULT_OK| on success.
 //   |MOJO_RESULT_INVALID_ARGUMENT| if |buffer| is invalid (e.g., is not the
 //       result of |MojoMapBuffer()| or has already been unmapped).
 MOJO_SYSTEM_EXPORT MojoResult MojoUnmapBuffer(void* buffer);  // In.
 
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_BUFFER_H_