nsIProtocolHandler: trouble loading image for html page

Question

I'm building an nsIProtocolHandler implementation in Delphi. (more here) And it's working already. Data the module builds gets streamed over an nsIInputStream. I've got all the nsIRequest, nsIChannel and nsIHttpChannel methods and properties working.

I've started testing and I run into something strange. I have a page "a.html" with this simple HTML:

<img src="a.png">

Both "xxm://test/a.html" and "xxm://test/a.png" work in Firefox, and give above HTML or the PNG image data. The problem is with displaying the HTML page, the image doesn't get loaded. When I debug, I see:

• NewChannel gets called for a.png, (when Firefox is processing an OnDataAvailable notice on a.html),
• NotificationCallbacks is set (I only need to keep a reference, right?)
• RequestHeader "Accept" is set to "image/png,image/*;q=0.8,*/*;q=0.5"
• but then, the channel object is released (most probably due to a zero reference count)

Looking at other requests, I would expect some other properties to get set (such as LoadFlags or OriginalURI) and AsyncOpen to get called, from where I can start getting the request responded to.

Does anybody recognise this? Am I doing something wrong? Perhaps with LoadFlags or the LoadGroup? I'm not sure when to call AddRequest and RemoveRequest on the LoadGroup, and peeping from nsHttpChannel and nsBaseChannel I'm not sure it's better to call RemoveRequest early or late (before or after OnStartRequest or OnStopRequest)?

Update: Checked on the freshly new Firefox 3.5, still the same

Update: To try to further isolate the issue, I try "file://test/a1.html" with <img src="xxm://test/a.png" /> and still only get above sequence of events happening. If I'm supposed to add this secundary request to a load-group to get AsyncOpen called on it, I have no idea where to get a reference to it.

There's more: I find only one instance of the "Accept" string that get's added to the request headers, it queries for nsIHttpChannelInternal right after creating a new channel, but I don't even get this QueryInterface call through... (I posted it here)

Answer 1

Me again.

I am going to quote the same stuff from nsIChannel::asyncOpen():

If asyncOpen returns successfully, the channel is responsible for keeping itself alive until it has called onStopRequest on aListener or called onChannelRedirect.

If you go back to nsViewSourceChannel.cpp, there's one place where loadGroup->AddRequest is called and two places where loadGroup->RemoveRequest is being called.

nsViewSourceChannel::AsyncOpen(nsIStreamListener *aListener, nsISupports *ctxt)
{
    NS_ENSURE_TRUE(mChannel, NS_ERROR_FAILURE);

    mListener = aListener;

    /*
     * We want to add ourselves to the loadgroup before opening
     * mChannel, since we want to make sure we're in the loadgroup
     * when mChannel finishes and fires OnStopRequest()
     */

    nsCOMPtr<nsILoadGroup> loadGroup;
    mChannel->GetLoadGroup(getter_AddRefs(loadGroup));
    if (loadGroup)
        loadGroup->AddRequest(NS_STATIC_CAST(nsIViewSourceChannel*,
                                             this), nsnull);

    nsresult rv = mChannel->AsyncOpen(this, ctxt);

    if (NS_FAILED(rv) && loadGroup)
        loadGroup->RemoveRequest(NS_STATIC_CAST(nsIViewSourceChannel*,
                                                this),
                                 nsnull, rv);

    if (NS_SUCCEEDED(rv)) {
        mOpened = PR_TRUE;
    }

    return rv;
}

and

nsViewSourceChannel::OnStopRequest(nsIRequest *aRequest, nsISupports* aContext,
                               nsresult aStatus)
{
    NS_ENSURE_TRUE(mListener, NS_ERROR_FAILURE);
    if (mChannel)
    {
        nsCOMPtr<nsILoadGroup> loadGroup;
        mChannel->GetLoadGroup(getter_AddRefs(loadGroup));
        if (loadGroup)
        {
            loadGroup->RemoveRequest(NS_STATIC_CAST(nsIViewSourceChannel*,
                                                    this),
                                     nsnull, aStatus);
        }
    }
    return mListener->OnStopRequest(NS_STATIC_CAST(nsIViewSourceChannel*,
                                                   this),
                                    aContext, aStatus);
}

Edit:

As I have no clue about how Mozilla works, so I have to guess from reading some code. From the channel's point of view, once the original file is loaded, its job is done. If you want to load the secondary items linked in file like an image, you have to implement that in the listener. See TestPageLoad.cpp. It implements a crude parser and it retrieves child items upon OnDataAvailable:

NS_IMETHODIMP
MyListener::OnDataAvailable(nsIRequest *req, nsISupports *ctxt,
                            nsIInputStream *stream,
                            PRUint32 offset, PRUint32 count)
{
    //printf(">>> OnDataAvailable [count=%u]\n", count);
    nsresult rv = NS_ERROR_FAILURE;
    PRUint32 bytesRead=0;
    char buf[1024];

    if(ctxt == nsnull) {
      bytesRead=0;
      rv = stream->ReadSegments(streamParse, &offset, count, &bytesRead);
    } else {
      while (count) {
        PRUint32 amount = PR_MIN(count, sizeof(buf));
        rv = stream->Read(buf, amount, &bytesRead);  
        count -= bytesRead;
      }
    }

    if (NS_FAILED(rv)) {
      printf(">>> stream->Read failed with rv=%x\n", rv);
      return rv;
    }

    return NS_OK;
}

The important thing is that it calls streamParse(), which looks at src attribute of img and script element, and calls auxLoad(), which creates new channel with new listener and calls AsyncOpen().

uriList->AppendElement(uri);
rv = NS_NewChannel(getter_AddRefs(chan), uri, nsnull, nsnull, callbacks);
RETURN_IF_FAILED(rv, "NS_NewChannel");

gKeepRunning++;
rv = chan->AsyncOpen(listener, myBool);
RETURN_IF_FAILED(rv, "AsyncOpen");

Since it's passing in another instance of MyListener object in there, that can also load more child items ad infinitum like a Russian doll situation.

Answer 2

I think I found it (myself), take a close look at this page. Why it doesn't highlight that the UUID has been changed over versions, isn't clear to me, but it would explain why things fail when (or just prior to) calling QueryInterface on nsIHttpChannelInternal.

With the new(er) UUID, I'm getting better results. As I mentioned in an update to the question, I've posted this on bugzilla.mozilla.org, I'm curious if and which response I will get there.