+cromwellian since he did unload support.
Oops, I see now that you attached it. Could you upload it to Gerrit?
I looked pretty hard for a reason why it's window.onUnload and not
window.onload and I can't find any. It dates back to 2010 at least and was
copied from hosted.html, whether deliberately I don't know.
One issue is that it's possible the app itself might want an onunload
handler. See
user/src/com/google/gwt/user/client/impl/WindowImpl.java line 32 where it
can install an onunload handler. It does preserve the old one but it still
seems kind of fragile to install one in the devmode.js. I wonder if there's
some way to detect the unload event in C++?
- Brian
On Tue, Sep 24, 2013 at 5:25 PM, Colin Alworth <niloc...@gmail.com> wrote:
> The patch I provided only tweaks the normal source, and leaves the
> generated sources and binaries alone - I think that is already what you are
> after.
>
> Note that this does *not* stop the leak by itself - we need to decide what
> to do about hosted.html/devmode.js. One (ugly) option is to try to have the
> ff plugin add an event handler to the window's close to call back to its
> own disconnect? I think there are other ways we could think about doing
> this as well, but changing hosted.html keeps it consistent across the board.
>
>
> On Tuesday, September 24, 2013 6:50:07 PM UTC-5, Brian Slesinsky wrote:
>
>> Hi,
>>
>> Wow, thanks for tracking this down! Could you send a patch that just
>> modifies the source (not worrying about the autogenerated files)? I will
>> rebuild them when I do the next release.
>>
>> Usually I just leave the existing dll's alone and move forward; it's not
>> worth fixing older plugins. However, I believe Firefox 24 will be an ESR
>> release so I think it's worth rebuilding that version.
>>
>>
>>
>> On Tue, Sep 24, 2013 at 4:31 PM, Colin Alworth <nilo...@gmail.com> wrote:
>>
>>> I spent a little time this weekend learning how to build firefox
>>> plugins, and a little time spilled out into this week, but I think I'm at a
>>> point where I can share what I've done, what I'm seeing, and start asking
>>> for help to finalize this (if it is as meaningful as I hope it is).
>>>
>>> First, the issue itself: It looks like we're actually leaking on both
>>> ends - that is, not just in the JVM, but in the browser itself. When Dev
>>> Mode transfers control back to the browser itself, it runs
>>> BrowserChannelServer.**reactToMessages(). This listens at the open
>>> socket between the browser's plugin and itself, and waits for the next byte
>>> to float over the wire, meaning that the browser wants something. This
>>> waiting specifically happens in Message.readMessageType, where we block on
>>> stream.readByte(). Once a tab/window has been closed, the thread that is
>>> actively watching that connection stays stuck in readByte(), waiting for
>>> that next message to float over the wire.
>>>
>>> My first thought was "why can't we just ask if that socket is closed?" -
>>> well, the socket *isn't* closed, which means that the browser is leaking
>>> the socket itself, along with whatever supporting objects were set up to
>>> manage that socket. Note that this suggests a workaround to the leak - quit
>>> and relaunch firefox, and since the socket will be forcibly closed then,
>>> readByte() will throw a EOF exception, and reactToMessages will trip off a
>>> RemoteDeathError (not ideal, but at least it just logs it and moves on,
>>> finishing the leak off).
>>>
>>> Next I dug into how to make the plugin actually disconnect when the page
>>> was closed. I started this by finding exactly where the socket was opened
>>> (common/HostChannel.{h|cpp}), then what creates HostChannel objects in the
>>> firefox plugin. This turns out to be achieved in this line in
>>> xpcom/ExternalWrapper.cpp (plus instructive comments):
>>> // TODO(jat): leaks HostChannel -- need to save it in a session object
>>> and
>>> // return that so the host page can call a disconnect method on it at
>>> unload
>>> // time or when it gets GC'd.
>>> HostChannel* channel = new HostChannel();
>>>
>>> Okay, so at the time it was written, it was known that this leaks the
>>> channel. This is where I started losing confidence, as it doesnt look like
>>> it could be this easy... The next block actually opens the connection, and
>>> passes it off to a FFSessionHandler instance, which is stored away in a
>>> field:
>>> Debug::log(Debug::Debugging) << "Connecting..." << Debug::flush;
>>>
>>> if (!channel->connectToHost(**hostPart.c_str(),
>>> atoi(portPart.c_str()))) {
>>> *_retval = false;
>>> return NS_OK;
>>> }
>>>
>>> Debug::log(Debug::Debugging) << "...Connected" << Debug::flush;
>>> sessionHandler.reset(new FFSessionHandler(channel/*, ctx*/));
>>>
>>> All this code is part of ExternalWrapper::Connect (note, defined twice
>>> for varying ff versions), and since Connect is never invoked internally, it
>>> was my assumption that I could build a ExternalWrapper::Disconnect that
>>> simply unwound this channel:
>>>
>>> NS_IMETHODIMP ExternalWrapper::Disconnect() {
>>> Debug::log(Debug::Info) << "Disconnecting..." << Debug::flush;
>>> sessionHandler.get()->**disconnect();
>>> Debug::log(Debug::Info) << "...Disconnected" << Debug::flush;
>>> return NS_OK;
>>> }
>>>
>>> That, combined with two other changes seems to expose this new function
>>> to be called from in the browser and actually triggers the QUIT case in
>>> reactToMessages - First, declare this new method in IOOPHM.idl, and second
>>> tweak ExternalWrapper's CanCallMethod check to allow invoking disconnect
>>> from within the browser. Actually building with the attached patch then
>>> modifies several other files (more on that later).
>>>
>>> Next, how to make disconnect() be invoked from within the browser - it
>>> is already declared and invoked from within hosted.html and devmode.js, but
>>> it seems to be in an invalid window callback:
>>> window.onUnload = function() {
>>> try {
>>> // wrap in try/catch since plugins are not required to supply
>>> this
>>> plugin.disconnect();
>>> } catch (e) {
>>> }
>>> };
>>> As far as I can tell, there is no such onUnload, but should be onunload
>>> instead. Indeed, onunload is defined later in each file, but since it just
>>> assigns an empty function, I suspect that one or the other is a typo:
>>> window.onunload = function() {
>>> };
>>>
>>> To continue my exploration, I kept onUnload's name as is, and just
>>> called it from the onunload declaration - this avoids changing the name of
>>> any existing calls (in case there is actually a good reason for labelling
>>> it so) and should prevent any possible issues from clobbering
>>> com.google.gwt.user.client.**impl.WindowImpl#**initWindowCloseHandler
>>> by re-overriding onunload after initWindowCloseHandler has already replaced
>>> it (and kept a reference to the original):
>>> window.onunload = function() {
>>> window.onUnload && window.onUnload();
>>> };
>>>
>>> With that change to hosted.html (or a copy in my project's public/ dir)
>>> and the attached patch, the leak seems to be resolved. Questions I have:
>>> * How does one perform the complete plugin build process? How far back
>>> in time do we rebuild and test?
>>> * How many auto-generated files (mostly .h from the idl, but also the
>>> .dll/.dylib/etc) do we commit when such a change is made?
>>> * Apparently plugins don't need to provide a disconnect() method, but
>>> if any do, are we *sure* they are actually being called as expected? Or is
>>> onUnload a long-lived typo?
>>>
>>> Looking forward to discussing this here, in ##gwt on freenode, or on the
>>> call tomorrow,
>>> Colin
>>>
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