iotests/linters: Add workaround for mypy bug #9852

This one is insidious: if you write an import as "from {namespace}
import {subpackage}" as mirror-top-perms (now) does, mypy will fail on
every-other invocation *if* the package being imported is a typed,
installed, namespace-scoped package.

Upsettingly, that's exactly what 'qemu.[aqmp|qmp|machine]' et al are in
the context of Python CI tests.

Now, I could just edit mirror-top-perms to avoid this invocation, but
since I tripped on a landmine, I might as well head it off at the pass
and make sure nobody else trips on that same landmine.

It seems to have something to do with the order in which files are
checked as well, meaning the random order in which set(os.listdir())
produces the list of files to test will cause problems intermittently
and not just strictly "every other run".

This will be fixed in mypy >= 0.920, which is not released yet. The
workaround for now is to disable incremental checking, which avoids the
issue.

Note: This workaround is not applied when running iotest 297 directly,
because the bug does not surface there! Given the nature of CI jobs not
starting with any stale cache to begin with, this really only has a
half-second impact on manual runs of the Python test suite when executed
directly by a developer on their local machine. The workaround may be
removed when the Python package requirements can stipulate mypy 0.920 or
higher, which can happen as soon as it is released. (Barring any
unforseen compatibility issues that 0.920 may bring with it.)

See also:
 https://github.com/python/mypy/issues/11010
 https://github.com/python/mypy/issues/9852

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-id: 20211019144918.3159078-14-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

tests/qemu-iotests/linters.py

@@ -93,7 +93,9 @@ def main() -> None:
     if sys.argv[1] == '--pylint':
         run_linter('pylint', files)
     elif sys.argv[1] == '--mypy':
-        run_linter('mypy', files)
+        # mypy bug #9852; disable incremental checking as a workaround.
+        args = ['--no-incremental'] + files
+        run_linter('mypy', args)
     else:
         print(f"Unrecognized argument: '{sys.argv[1]}'", file=sys.stderr)
         show_usage()