Allow loading modules in Docker image

[erikjohnston]

This is done by mounting a /modules directory and installing packages into that.

One choice that I made was to ensure that package versions in /modules do not override those installed by Synapse. This ensures that people using custom modules cannot accidentally install an incompatible version of a package.

Closes #9944

[reivilibre]

I'm happy with this approach, I think it's the best one we have.

I seem to remember someone in some meeting ages ago had something they didn't like about it, but honestly it seems like the best option we have.

Might be worth noting that this approach won't work (smoothly, at least) for libraries with native dependencies.

[erikjohnston]

Might be worth noting that this approach won't work (smoothly, at least) for libraries with native dependencies.

Oh, hmm, true. I wonder if we should instead add a way of installing deps via the docker image (e.g. adding a new command to start.py like docker run synapse install <package>? Rather than installing on the host machine.

[MatMaul]

Current approach in this PR looks quite sane to me.

Might be worth noting that this approach won't work (smoothly, at least) for libraries with native dependencies.

Oh, hmm, true. I wonder if we should instead add a way of installing deps via the docker image (e.g. adding a new command to start.py like docker run synapse install <package>? Rather than installing on the host machine.

A lot of env uses a fresh container each time, which doesn't bode well with this proposal I believe.

[erikjohnston]

I think we do need an approach for native packages here, we don't necessarily need to add support immediately but we should ensure we're not painting ourselves into a corner. For example, I think the current approach will sometimes work, sometimes not for native packages, which is a huge footgun.

Another thing we need to worry about is python versions, packages built for one version won't necessarily work after an upgrade.

A lot of env uses a fresh container each time, which doesn't bode well with this proposal I believe.

Yeah, we'd want to ensure that the modules are installed into a volume I think.

[erikjohnston]

I'm not comfortable with this approach due to the aformentioned footguns of native packages / dependencies. I think there is a path forwards, but let's move this back to the issue.

[MatMaul]

Yeah, we'd want to ensure that the modules are installed into a volume I think.

Oh yes good point I didn't understand it that way, with both the volume mount and the entrypoint in the python script it seems rather solid to me. TBH with this PR you can already just do docker run -v modules:/modules pip install --target /modules mydep.

The native deps became less of a problem also I think : if it manages to compile within the docker environment then it should just run fine ?