Latest posts of series Things I learnt in November 2022

• str.endswith() can take a tuple of possible endings instead of a single string

About JACK and Debian

• There are 3 JACK implementations: jackd1, jackd2, pipewire-jack.
• jackd1 is mostly superseded in favour of jackd2, and as far as I understand, can be ignored
• pipewire-jack integrates well with pipewire and the rest of the Linux audio world
• jackd2 is the native JACK server. When started it handles the sound card directly, and will steal it from pipewire. Non-JACK audio applications will likely cease to see the sound card until JACK is stopped and wireplumber is restarted. Pipewire should be able to keep working as a JACK client but I haven't gone down that route yet
• pipewire-jack mostly works. At some point I experienced glitches in complex JACK apps like giada or ardour that went away after switching to jackd2. I have not investigated further into the glitches
• So: try things with pw-jack. If you see odd glitches, try without pw-jack to use the native jackd2. Keep in mind, if you do so, that you will lose standard pipewire until you stop jackd2 and restart wireplumber.

Python: typing.overload

typing.overload makes it easier to type functions with behaviour that depends on input types. Functions marked with @overload are ignored by Python and only used by the type checker:

@overload
def process(response: None) -> None:
    ...
@overload
def process(response: int) -> tuple[int, str]:
    ...
@overload
def process(response: bytes) -> str:
    ...
def process(response):
    # <actual implementation>

Python's multiprocessing and deadlocks

Python's multiprocessing is prone to deadlocks in a number of conditions. In my case, the running program was a standard single-process, non-threaded script, but it used complex native libraries which might have been the triggers for the deadlocks.

The suggested workaround is using set_start_method("spawn"), but when we tried it we hit serious performance penalties.

Lesson learnt: multiprocessing is good for prototypes, and may end up being too hacky for production.

In my case, I was already generating small python scripts corresponding to worker tasks, which were useful for reproducing and debugging Magics issues, so I switched to running those as the actual workers. In the future, this may come in handy for dispatching work to HPC nodes, too.

Here's a parallel execution scheduler based on asyncio that I wrote to run them, which may always come in handy on other projects.

Debian:

• You can Build-Depend on debhelper-compat (=version) and get rid of debhelper as a build-dependency, and of debian/compat (details)
• You can Build-Depend on dh-sequence-foo and get rid of the corresponding dh-foo build-dependency, and of the need to add --with foo in debian/rules (details)
• You can (and should) get rid of dh-buildinfo, which is now handled automatically
• In salsa.debian.org there is a default CI pipeline for Debian packages that works beautifully without needing to add any .gitlab-ci.yml to a repository
• Add Testsuite: autopkgtest-pkg-python to debian/control, and you get a free autopkgtest that verifies that your packaged Python module can be imported. The default CI pipeline in salsa will automatically run the tests. (specification, details)

Python:

• From Python 3.8, you can use = in format strings to make it easier to debug variables and expressions (details):

>>> name="test"
>>> print(f"{name=}")
name='test'
>>> print(f"{3*8=}")
3*8=24

Leaflet:

• [abc].tile.openstreetmap.org links need to be replaced with tile.openstreetmap.org (details)