conf: add exit_on_dl_error option

[tmatth]

This makes dlopen or dlsym errors optionally fatal, which is especially useful for CI.

[tmatth]

For reference I added this option as I've been burned a few times by plugins, transports, etc. not loading at runtime due to undefined symbols. The latest example was trying to use the websocket transport with libwebsockets built with LWS_WITHOUT_CLIENT=ON, which fails at runtime with:

Couldn't load event handler plugin 'libjanus_wsevh.so': /opt/app/lib/janus/events/libjanus_wsevh.so: undefined symbol: lws_client_connect_via_info

As an aside, does it make sense to set that option in CI here?
https://github.com/meetecho/janus-gateway/blame/1d3f68d094df200d7246a3a349ec50bed1e79985/.github/workflows/janus-ci.yml#L129

[lminiero]

This does sound useful, thanks! Added a couple of notes inline. I'll leave it to @atoppi to comment on whether it might be useful in CI, but in that case my guess is we'd need to add a command line argument to that: if this can only be enabled via config file, not sure we can automate that easily.

[lminiero]

As an aside, does it make sense to set that option in CI here?

@tmatth I just realized you were talking of something entirely different, i.e., the libwebsockets option in the CI build. Again, letting @atoppi address that, though 😆

For the rest, this looks good to me, so I'll merge 👍

[atoppi]

@tmatth sorry for being late to the party
Indeed this is an interesting feature and I can see its appliance.
I agree with Lorenzo about the CLI argument, that will be easier for any automation tool in future.

As of

As an aside, does it make sense to set that option in CI here?
https://github.com/meetecho/janus-gateway/blame/1d3f68d094df200d7246a3a349ec50bed1e79985/.github/workflows/janus-ci.yml#L129

The rationale here is to speed-up the building of lws.
We never actually starts janus in the CI workflow (maybe in future we could add this?) since we only check that compilation completes successfully, hence a symbol failure after dlopen can never be triggered.