Mojo: Make parallel sync/async embedder channel creation/destruction APIs.

mojo/embedder/embedder.h

Index: mojo/embedder/embedder.h
diff --git a/mojo/embedder/embedder.h b/mojo/embedder/embedder.h
index 57da5a0964d91cf41cd407ae1a543f34a70bb613..6b5a1c526595407c3e761efaaa8a0b82d5d71eac 100644
--- a/mojo/embedder/embedder.h
+++ b/mojo/embedder/embedder.h
@@ -18,44 +18,84 @@ namespace embedder {
 // Must be called first to initialize the (global, singleton) system.
 MOJO_SYSTEM_IMPL_EXPORT void Init();
 
-// Creates a new "channel", returning a handle to the bootstrap message pipe on
-// that channel. |platform_handle| should be an OS-dependent handle to one side
-// of a suitable bidirectional OS "pipe" (e.g., a file descriptor to a socket on
-// POSIX, a handle to a named pipe on Windows); this "pipe" should be connected
-// and ready for operation (e.g., to be written to or read from).
-// |io_thread_task_runner| should be a |TaskRunner| for the thread on which the
-// "channel" will run (read data and demultiplex).
+// A "channel" is a connection on top of an OS "pipe", on top of which Mojo
+// message pipes (etc.) can be multiplexed. It must "live" on some I/O thread.
 //
-// On completion, it will run |callback| with a pointer to a |ChannelInfo|
-// (which is meant to be opaque to the embedder). If
-// |callback_thread_task_runner| is non-null, it the callback will be posted to
-// that task runner. Otherwise, it will be run on the I/O thread directly.
+// There are two "channel" creation/destruction APIs: the synchronous
+// |CreateChannelOnIOThread()|/|DestroyChannelOnIOThread()|, which must be
+// called from the I/O thread, and the asynchronous
+// |CreateChannel()|/|DestroyChannel()|, which may be called from any thread.
 //
-// Returns an invalid |MOJO_HANDLE_INVALID| on error. Note that this will happen
-// only if, e.g., the handle table is full (operation of the channel begins
-// asynchronously and if, e.g., the other end of the "pipe" is closed, this will
-// report an error to the returned handle in the usual way).
+// Both creation functions have a |platform_handle| argument, which should be an
+// OS-dependent handle to one side of a suitable bidirectional OS "pipe" (e.g.,
+// a file descriptor to a socket on POSIX, a handle to a named pipe on Windows);
+// this "pipe" should be connected and ready for operation (e.g., to be written
+// to or read from).
 //
-// Notes: The handle returned is ready for use immediately, with messages
-// written to it queued. E.g., it would be perfectly valid for a message to be
-// immediately written to the returned handle and the handle closed, all before
-// the channel has begun operation on the IO thread. In this case, the channel
-// is expected to connect as usual, send the queued message, and report that the
-// handle was closed to the other side. (This message may well contain another
-// handle, so there may well still be message pipes "on" this channel.)
+// Both (synchronously) return a handle to the bootstrap message pipe on the
+// channel that was (or is to be) created, or |MOJO_HANDLE_INVALID| on error
+// (but note that this will happen only if, e.g., the handle table is full).
+// This message pipe may be used immediately, but since channel operation
+// actually begins asynchronously, other errors may still occur (e.g., if the
+// other end of the "pipe" is closed) and be reported in the usual way to the
+// returned handle.
+//
+// (E.g., a message written immediately to the returned handle will be queued
+// and the handle immediately closed, before the channel begins operation. In
+// this case, the channel should connect as usual, send the queued message, and
+// report that the handle was closed to the other side. The message sent may
+// have other handles, so there may still be message pipes "on" this channel.)
+//
+// Both also produce a |ChannelInfo*| (a pointer to an opaque object) -- the
+// first synchronously and second asynchronously.
+//
+// The destruction functions are similarly synchronous and asynchronous,
+// respectively, and take the |ChannelInfo*| produced by the creation function.
+// (Note: One may call |DestroyChannelOnIOThread()| with the result of
+// |CreateChannel()|, but not |DestroyChannel()| with the result of
+// |CreateChannelOnIOThread()|.)
 //
 // TODO(vtl): Figure out channel teardown.
 struct ChannelInfo;
+
+// Creates a channel; must only be called from the I/O thread. |platform_handle|
+// should be a handle to a connected OS "pipe". Eventually (even on failure),
+// the "out" value |*channel_info| should be passed to
+// |DestroyChannelOnIOThread()| to tear down the channel. Returns a handle to
+// the bootstrap message pipe.
+MOJO_SYSTEM_IMPL_EXPORT ScopedMessagePipeHandle
+    CreateChannelOnIOThread(ScopedPlatformHandle platform_handle,
+                            ChannelInfo** channel_info);
+
 typedef base::Callback<void(ChannelInfo*)> DidCreateChannelCallback;
+// Creates a channel asynchronously; may be called from any thread.
+// |platform_handle| should be a handle to a connected OS "pipe".
+// |io_thread_task_runner| should be the |TaskRunner| for the I/O thread.
+// |callback| should be the callback to call with the |ChannelInfo*|, which
+// should eventually be passed to |DestroyChannel()| (or
+// |DestroyChannelOnIOThread()|) to tear down the channel; the callback will be
+// called using |callback_thread_task_runner| if that is non-null, or otherwise
+// it will be called using |io_thread_task_runner|. Returns a handle to the
+// bootstrap message pipe.
 MOJO_SYSTEM_IMPL_EXPORT ScopedMessagePipeHandle
     CreateChannel(ScopedPlatformHandle platform_handle,
                   scoped_refptr<base::TaskRunner> io_thread_task_runner,
                   DidCreateChannelCallback callback,
                   scoped_refptr<base::TaskRunner> callback_thread_task_runner);
 
+// Destroys a channel that was created using either |CreateChannelOnIOThread()|
+// or |CreateChannel()|; must only be called from the I/O thread. |channel_info|
+// should be the "out" value from |CreateChannelOnIOThread()| or the value
+// provided to the callback to |CreateChannel()|.
 MOJO_SYSTEM_IMPL_EXPORT void DestroyChannelOnIOThread(
     ChannelInfo* channel_info);
 
+// Destroys a channel (asynchronously) that was created using |CreateChannel()|
+// (note: NOT |CreateChannelOnIOThread()|); may be called from any thread.
+// |channel_info| should be the value provided to the callback to
+// |CreateChannel()|.
+MOJO_SYSTEM_IMPL_EXPORT void DestroyChannel(ChannelInfo* channel_info);
+
 // Creates a |MojoHandle| that wraps the given |PlatformHandle| (taking
 // ownership of it). This |MojoHandle| can then, e.g., be passed through message
 // pipes. Note: This takes ownership (and thus closes) |platform_handle| even on