"HW busy now" when calling vaSyncSurface sometimes

Sorry for the vague question, but I'm not understanding the error I'm seeing from libva. My setup is as follows: I have two different video streams that I'm trying to decode simultaneously, so I have one `VADisplay`, two `VAContextID`'s, and two different `VASurfaceID`'s that I'm decoding to. Without getting too much in the weeds, I call something similar to this for each stream:

```
vaBeginPicture(hw_context_->display(), va_context_->context_id(), va_surface_id);
vaEndPicture(hw_context_->display(), va_context_->context_id());
```

Then, for each stream, I want to wait for the surface to be finished decoding:

```
vaSyncSurface(hw_context_->display(), va_surface_id);
```

However, some amount of time (maybe 60% of the time?), I get `VA_STATUS_ERROR_HW_BUSY` when calling `vaSyncSurface`. What does this error mean in the context of decoding? From the docs it appears like this error should only happen if you try and kick off two decoding jobs on the same context, but I have been careful to create two different VAAPI contexts.

Curiously, if I call `vaSyncSurface` immediately after starting the decoding job for one stream (thereby making the whole process serial, and defeating the purpose of having two different streams), then I no longer see this error. It seems like there is some contamination across VAAPI contexts? Is the flow I'm trying to do supported?

I looked at the trace file, but it didn't contain anything useful as far as I could tell. This is an HEVC stream if that is important! Thanks for any help!