Fix firefox sync-server

[Nadrieril]

Motivation for this change

The nixos syncserver module was broken, this PR fixes it by rolling back a dependency. serversyncstorage requires cornice v0.17 and does not work with any later version.
Based on #52654.
I have tested it in production and it seems to work well.
Fixes #38830.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[Nadrieril]

I had to switch to using gunicorn otherwise it somehow failed on my server. This is the recommended setup from the documentation and should not break anything.

[dotlambda]

Even though this does not look right, let's keep it that way. The bepasty module uses a similar approach.

[dotlambda]

Building syncserver yields

Ran 0 tests in 0.000s

A proper checkPhase or doCheck = false + comment are needed.

[Nadrieril]

You're nitpicking a lot, I feel like you're wasting both our times. I appreciate the effort though. I'll see if I have time to look at this tomorrow. I'll add myself to the maintainers as well.

[dotlambda]

@GrahamcOfBorg build syncserver

[dotlambda]

@GrahamcOfBorg build python2.pkgs.serversyncstorage

[dotlambda]

Thanks a lot for bearing with me!

[dotlambda]

Btw, I'd have liked to mark serversyncstorage as broken, but that's not possible due to #48663.

[andir]

Since this is also broken on 18.09 do you think it is feasible to backport or will it have too many conflicts / dependencies on changes made to master?

[Nadrieril]

@dotlambda Thanks for your patience !
@andir AFAIK, those packages don't need very recent versions of their dependencies, so I'd guess it should not be too hard to backport. If there is a problem with a dependency, you can always add more overrides to pkgs/servers/syncserver/default.nix.

[risicle]

This is digging up quite an old PR, but could we not simply set

  disabled = !(isPy27 && cornice.version == "0.17");

to stop this appearing to be a "broken" package?