Major part of #73
Unfortunately, it wasn't possible to split it to multiple smaller
commits, since the changes touched the entire application substantially.
Here is a short list of major changes:
1. Create a separate library (headscale-api), which is used as a
convenient abstraction layer providing Pythonic interface with
Pydantic. Headscale API is fully asynchronous library, benefitting
from improved concurrency for backend requests thus increasing page
load speed, e.g., on "Machines" page.
2. Create a common common, validated with flask-pydantic API passthrough
layer from GUI to the backend.
3. Move authentication to a separate (auth.py), consolidating the
functionality in a single place (with better place for expansion in
the future).
4. Move configuration management to a separate module (config.py). Use
Pydantic's BaseSettings for reading values from environment, with
extensive validation and error reporting.
5. Reduce the number of health checks.
- Now, most are performed during server initialization. If any test
fails, the server is started in tainted mode, with only the error
page exposed (thus reducing the surface of attack in invalid
state).
- Key checks are implicit in the requests to the backend and
guarded by `@headscale.key_check_guard` decorator.
- Key renewal is moved to server-side scheduler.
6. Introduce type hints to the level satisfactory for mypy static
analysis. Also, enable some other linters in CI and add optional
pre-commit hooks.
7. Properly handle some error states. Instead of returning success and
handling different responses, if something fails, there is HTTP error
code and standard response for it.
8. General formatting, small rewrites for clarity and more idiomatic
Python constructs.
Signed-off-by: Marek Pikuła <marek.pikula@embevity.com>
