Correct ExternaVR shutdown

[MortimerGoro]

Fixes #538

Related Gecko PR: https://phabricator.services.mozilla.com/D6236

[bluemarvin]

I wish the WebVR external wasn't so fragile. It seems like small changes in gfxExternalVR will break this. I guess we just need to add some tests at some point. 🙂

[bluemarvin]

This got me thinking about how to deal with the PullBrowserStateWhileLocked() issue. I wonder if we could use a closure to fix this issue where the code would look something like:

void PullBrowserState(std::function<bool(const BrowserState&)>& aFunc, float aWait = 0.0f) {
  wait.lock();
  copy_data();
  while(!aFunc(m.browserState)) {
    if (!wait.DoWait(aWait)) {
     break;
    }
    copy_data()
  }
}

[bluemarvin]

I applied the GV patch and pulled this and built and I don't think it is working. STR on the Go:

1. Go to https://tonite.dance
2. Enter immersive mode.
3. Press the home button.
4. Exit FxR
5. Relaunch FxR

Expected: FxR launches with https://tonite.dance as the currently loaded page not in immersive mode. But the user is able to enter immersive mode.
Actual: Black screen and the Go is hung requiring a system reboot, or FxR crashes, or FxR fails to enter immersive mode.

[bluemarvin]

From my testing the external struct is not getting removed from Gecko.

[bluemarvin]

I'm seeing this in the log:
09-19 14:34:35.860 2437 3092 W VRB : FXR Shutdown wait timeout

[MortimerGoro]

I changed the approach to use the static struct we talked about in slack.

Even with that sometimes the next relaunch was a black screen. I found that sometimes there is a deadlock in onDestroy when the browser widget is released (on the display.SurfaceDestroyed() call, which stops the compositor using a sync() call). This only happens when the activity is destroyed while presenting immersive.

It seems that there is a race condition where the immersive mode is not paused before onDestroy arrived, probably because the frame loop is stopped before the event sent from onPause is processed. I made the exitImmersive() call sync in onPause to fix the issue.

This problem could happen with the previous killProcess too.

[bluemarvin]

@MortimerGoro I assume this no longer requires the Gecko patch?

[MortimerGoro]

@bluemarvin yes, the GV patch is no longer required. I just set the shutdown flag for correctness.

[bluemarvin]

Just fix-up where ExternVR is creating the shared_ptr<> in Create. Otherwise, looks good.

[bluemarvin]

Since the state is no longer instanced with the class this should be:

return std::make_shared<ExternalVR>();

And #include "vrb/ConcreteClass.h" can be removed.

[bluemarvin]

I'm seeing a dead lock when restarting:
STR:

1. Go to https://tonite.dance
2. Enter immersive mode
3. Press home button on controller
4. Press exit button
5. Launch FxR again
6. Goto 2)

Expected: Should be able to do this indefinitely
Actual: After going in and out several times, FxR will hang on launch and just show a black screen after step 5)

[MortimerGoro]

I removed the shutdown flag, it was causing some intermittent EnterVR problem after a restart. I can't reproduce the deadlock when restarting (trying with the Go).

The only thing I saw is sometimes there is a whole app restart because of #586. It's not easy to reproduce, it seems easier following the STR steps with https://webvr.info/samples/03-vr-presentation.html

[bluemarvin]

Can no longer reproduce hang issue.