Ensure that PP_Instance values are unique within a plugin process in addition...

ppapi/proxy/ppapi_messages_internal.h

 // Copyright (c) 2011 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 "gpu/command_buffer/common/command_buffer.h"
 #include "gpu/ipc/gpu_command_buffer_traits.h"
 #include "ipc/ipc_message_macros.h"
 #include "ppapi/c/dev/pp_file_info_dev.h"
 #include "ppapi/c/ppb_var.h"
 
 #define IPC_MESSAGE_START PpapiMsgStart
 
 // These are from the plugin to the renderer
 // Loads the given plugin.
 IPC_MESSAGE_CONTROL1(PpapiMsg_LoadPlugin, FilePath /* path */)
 
 // Creates a channel to talk to a renderer. The plugin will respond with
 // PpapiHostMsg_ChannelCreated.
 IPC_MESSAGE_CONTROL2(PpapiMsg_CreateChannel,
                      base::ProcessHandle /* host_process_handle */,
                      int /* renderer_id */);
 
+// Each plugin may be referenced by multiple renderers. We need the instance
+// IDs to be unique within a plugin, despite coming from different renderers,
+// and unique within a renderer, despite going to different plugins. This means
+// that neither the renderer nor the plugin can generate instance IDs without
+// consulting the other.
+//
+// We resolve this by having the renderer generate a unique instance ID inside
+// its process. It then asks the plugin to reserve that ID by sending this sync
+// message. If the plugin has not yet seen this ID, it will remember it as used
+// (to prevent a race condition if another renderer tries to then use the same
+// instance), and set usable as true.
+//
+// If the plugin has already seen the instance ID, it will set usable as false
+// and the renderer must retry a new instance ID.
+IPC_SYNC_MESSAGE_CONTROL1_1(PpapiMsg_ReserveInstanceId,
+                            PP_Instance /* instance */,
+                            bool /* usable */)
+
 // Sent in both directions to see if the other side supports the given
 // interface.
 IPC_SYNC_MESSAGE_CONTROL1_1(PpapiMsg_SupportsInterface,
                             std::string /* interface_name */,
                             bool /* result */)
 
 IPC_MESSAGE_CONTROL2(PpapiMsg_ExecuteCallback,
                      uint32 /* serialized_callback */,
                      int32 /* param */)
 
@@ skipping 650 matching lines @@
 IPC_SYNC_MESSAGE_ROUTED2_2(PpapiHostMsg_PPBVar_IsInstanceOfDeprecated,
                            pp::proxy::SerializedVar /* var */,
                            int64 /* object_class */,
                            int64 /* object-data */,
                            PP_Bool /* result */)
 IPC_SYNC_MESSAGE_ROUTED3_1(PpapiHostMsg_PPBVar_CreateObjectDeprecated,
                            PP_Instance /* instance */,
                            int64 /* object_class */,
                            int64 /* object_data */,
                            pp::proxy::SerializedVar /* result */)