Bug 309386 Document what a null stream listener from loadImageWithChannel means

r=pavlov sr=bz

modules/libpr0n/public/imgILoader.idl

@@ -96,6 +96,10 @@ interface imgILoader : nsISupports
    * @param uri the URI to load
    * @param aObserver the observer
    * @param cx some random data
+   * @param aListener [out]
+   *        A listener that should receive the data. Can be null, in which
+   *        case imagelib has found a cached image and is not interested in
+   *        the data. The caller needs not cancel the channel in this case.
    *
    * libpr0n does NOT keep a strong ref to the observer; this prevents
    * reference cycles.  This means that callers of loadImageWithChannel should

modules/libpr0n/src/imgLoader.cpp

@@ -622,9 +622,6 @@ NS_IMETHODIMP imgLoader::LoadImageWithChannel(nsIChannel *channel, imgIDecoderOb
   if (request) {
     // we have this in our cache already.. cancel the current (document) load
 
-    /* XXX If |*listener| is null when we return here, the caller should 
-       probably cancel the channel instead of us doing it here.
-    */
     channel->Cancel(NS_IMAGELIB_ERROR_LOAD_ABORTED); // this should fire an OnStopRequest
 
     *listener = nsnull; // give them back a null nsIStreamListener