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1) Decision to divert data to other thread should only be made within OnStartRequest().
There seemed to be general agreement that the decision to offload OnDataAvailable() to a different thread should only be possible within OnStartRequest(). By this point, we will have the final channel for the request (i.e. any redirects, auth requests, etc, will have been resolved). We will also (usually!) have the Content-type of the request, which we will need to determine whether the data should be handled by an off-thread consumer (like the HTML parser) in the first place.
2) How to notify other-thread listener when request is complete?
Two obvious ways come to mind. 1) We could also divert OnStopRequest() to the listener's thread. Or, 2) we could send an additional OnDataAvailable() with data length == 0 to signal the end of the Request. #1 is more intuitive for the programmer; #2 is more conservative, in that the main thread will still receive OnStopRequest. Since compliant nsChannels will need to be modified anyway, no one could think of a good reason not to change them to no longer expect OnStopRequest(), and so #1 is the current winner, pending discovery of any problems with not delivering OnStopReq() to the main thread.
3) Code would need to query both the Channel nsIChannel and any StreamListeners chained (via nsITraceableChannel) between the socket nsIChannel and the target listener, to make sure they are all ALL safe to be switched to a different thread for OnDataAvailable. (We could modify any relevant existing internal listeners to become safe, but since some listeners could come from extensions, we cannot guarantee all listeners will always be safe). Code must be prepared to handle the case where OnDataAvailable cannot be redirected to the desired thread.
4) The API would probably consist of two new XPCOM interfaces, one for thread-retargetable Channels, one for Listeners (nsIThreadSinkableChannel and nsIThreadSinkableStreamListener?). The Channel interface would have a method (RedirectDeliveryToThread?) that would take a thread object argument, and redirect OnDataAvailable (and OnStopRequest) to that thread. The method would first check the chain of Listeners to ensure that they are all ThreadSink listeners, else the method would fail and the redirect would not happen.
