API Crashes / External Rendering Question

You are correct that this issue is similar to support request 221 - in fact, I think it is the same. I am the author of the API and I investigated it after that support request. Within available time back then, I was not able to find the root cause. I found that:

• the issue is timing related; when debugging (stepping through at, therefore, a slow speed) the issue does not occur
• multiple fast-following request are more likely to trigger this issue than a single request

The external rendering feature was never developed with your objective in mind. It for this reason has no knowledge of preceding or following slides; its purpose is to render 'this' and send the result back to OpenSong.
Context is available via the API e.g. via the various endpoints: '/presentation/slide/list' gets the full set, '/presentation/status' will get you the current presentation status, to tell whether a presentation is active or not. It won't tell you whether a certain slide is the 'last'. One of the reasons is 'because OpenSong itself does not know'. A slide, or verse, might be the last in the set, but that does not mean it is the last item that is shown before ending the presentation, since the order depends on the actions by the person in control of the presentation helper window.

A good way to get instant notifications of whatever is going on, is to use the websocket interface. When you subscribe to presentation changes on the endpoint '/ws/subscribe/presentation', Opensong will send a notification, so you don't have to poll. This is demonstrated in e.g. http://vwout.github.io/os-secondscreen/

Lastly, you could use the snapshot export feature of OpenSong. When activated, OpenSong will export an image of the currently showing slide, with an xml containing the contents of the currently active slide. That however won't tell you when the presentation ended.

The best integration can probably be achieved by writing an OBS plugin (or a script) using the websocket interface.

Appreciate the quick response, especially around the holidays. I whipped up a quick script and can confirm that WebSocket subscription is the way to go for my use case. I basically set up a REST API call (one session per presentation, torn down once the presentation ends) which calls the /presentation/slide/{slide_id} endpoint.

I should mention that I didn't mean to say that ' I have no way to know when the presentation is "finished"' but rather I need to know when to "clear" information from OBS. The API (when grabbing the slide by the identifier) helpfully gives an empty body for empty slides; thus I was inferring we could clear text files loaded by OBS when we see this slide. However the external render only rendered slides when it needed to (e.g. not when it's empty); Makes sense in hindsight.

Anyways, I did get a very small amount of cases where OpenSong still crashed (only during exiting of presentation mode) with my new implementation. I spent about an hour trying to get it to reproduce myself (using old code, sending requests faster, etc) with WinDbg spawning OpenSong and debugging it and it's child processes but faced similar non-reproducable problems as you stated earlier. Assuming one of the following things is going on as I never actually got a crash exception (application just hung / became "Not Responsive")

1. Thread safety / deadlocking
2. Async code waiting for blocking code to run but not being cleaned up during constructor/destructor related functions

Also had DebugView open on the side, but nothing particularly interesting showed up, which would be expected if you aren't using OutputDebugString-like calls; figured I'd give it a try nonetheless