segfault in in cs::MjpegServerImpl::ConnThread::SendStream

[ariovistus]

robotpy-cscore==2019.0.3, fedora 28 with their packaged opencv, seems to be 3.4.1

I run the following code:

import time
from cscore import CameraServer, VideoSource, UsbCamera
inst = CameraServer.getInstance()
camera = UsbCamera()
server = inst.startAutomaticCapture(camera=camera, return_server=True)
camera.setConnectionStrategy(VideoSource.ConnectionStrategy.kKeepOpen)

while True:
    time.sleep(10)

as my first attempt to do something with cscore. When I try to connect to http://localhost:1181, the segfault occurs. gdb doesn't give me much:

0x00007fffea0120a2 in cs::MjpegServerImpl::ConnThread::SendStream(wpi::raw_socket_ostream&) ()
   from /home/ellery/.virtualenvs/tabularasa/lib/python3.6/site-packages/cscore/_cscore.cpython-36m-x86_64-linux-gnu.so

workaround is simple:

camera = UsbCamera("name", 0)

and everything works, but help(UsbCamera) suggests the former invocation is valid:

 |  __init__(...)
 |      __init__(*args, **kwargs)
 |      Overloaded function.
 |      
 |      1. __init__(self: cscore._cscore.UsbCamera) -> None
 |      
 |      2. __init__(self: cscore._cscore.UsbCamera, name: wpi::Twine, dev: int) -> None

[auscompgeek]

A bunch of the VideoSink and VideoSource subclasses appear to have default no-arg constructors. @PeterJohnson are these intended to be usable?

[PeterJohnson]

The no-args constructor initializes the C++ wrapper object with an invalid handle (handle=0). No underlying implementation object is created in this case, and calling other functions on that object won't work (they will return an error / set the error flag in the class). That should simply result in nothing happening, not in a crash though.

[PeterJohnson]

It looks like I may be missing a couple of null checks in MjpegServerImpl. Will fix.

[auscompgeek]

I suppose we should also not expose the no-args constructors as well, since they're not very useful from Python land.