There was an import cycle in the Python modules:
- `qmk.build_targets` imported `qmk.cli.generate.compilation_database`;
- importing `qmk.cli.generate.compilation_database` requires
initializing `qmk.cli` first;
- the initialization of `qmk.cli` imported the modules for all CLI
commands;
- `qmk.cli.compile` imported `qmk.build_targets`.
This cycle did not matter in most cases, because `qmk.cli` was imported
first, and in that case importing `qmk.cli.generate.compilation_database`
did not trigger the initialization of `qmk.cli` again. However, there was
one corner case when `qmk.bulld_targets` was getting imported first:
- The `qmk find` command uses the `multiprocessing` module.
- The `multiprocessing` module uses the `spawn` start method on macOS
and Windows.
- When the `spawn` method is used, the child processes initialize
without any Python modules loaded, and the required modules are loaded
on demand by the `pickle` module when receiving the serialized objects
from the main process.
The result was that the `qmk find` command did not work properly on macOS
(and probably Windows too); it reported exceptions like this:
ImportError: cannot import name 'KeyboardKeymapBuildTarget' from partially initialized module 'qmk.build_targets' (most likely due to a circular import)
Moving the offending `qmk.cli.generate.compilation_database` import into
the method which actually uses it fixes the problem.
When multiple `-f FILTER` options were specified, `qmk find` did not
return anything at all instead of printing the list of entries that
matched all of the specified filters.
The problem was that the statement in `_filter_keymap_targets()` that
filled `targets` had a wrong indent and therefore was executed for every
filter instead of only once after applying all filters, and
`valid_keymaps` was actually an iterator and therefore could be used
only once. Moving the statement outside of the loop fixes the problem.
