Move JsonDict import to type-checking path only

[anoadragon453]

I ran into the usual case where my synapse_rust module was outdated and needed to rebuild them before running the Complement tests.

However, instead of getting the typical error of Rust module outdated. Please rebuild using poetry install`, I instead got the following:

COMPLEMENT_DIR="../complement" ./scripts-dev/complement.sh -run
  "TestMSC4429ProfileUpdates"
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/work/code/synapse/synapse/util/__init__.py", line 41, in <module>
      from synapse.types import JsonDict
    File "/home/work/code/synapse/synapse/types/__init__.py", line 70, in <module>
      from synapse.synapse_rust.types import Requester
  ModuleNotFoundError: No module named 'synapse.synapse_rust.types'; 'synapse.synapse_rust' is not a package.

which is much more confusing for a irregular contributor to diagnose than the one that tells you to run poetry install.

The cause was an edge case: I did not yet have the synapse.synapse_rust.types module, which was added only recently in #19828. So you could argue this is a one-time hurdle.

However, the need to import synapse.synapse_rust.types solely comes from needing to import JsonDict, which is only used for typechecking!

Hence moving the import under if typing.TYPE_CHECKING. With this, I would have received the more noob-friendly error message instead of one about a synapse_rust.types module not existing.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[erikjohnston]

Hmm, this won't help for future cases though. The root cause is that we started importing stuff from Rust before we checked if the Rust was up to date. This is because check_rust_lib_up_to_date lives in synapse.util.rust, but we import Rust stuff in synapse.util.

I think we should move check_rust_lib_up_to_date into a top-level module and make sure that it is imported/called before anything else is imported (other than maybe the Python version check). I'm tempted to have something like synapse.startup_checks that checks the appropriate things on import with no/minimal imports from other synapse modules.

Thoughts?

[erikjohnston]

(FTR you can stop an import line being resorted by adding a # isort: skip comment after it)

[anoadragon453]

That does sound more correct, and future-proof. I'll give it a shot!