Fix minion crash when grains config option is empty

[BrianHa94]

What does this PR do?

Fixes a minion crash on startup caused by an empty grains: config option. YAML parses grains: with no value as None instead of a dict, which two separate code paths (apply_minion_config and salt.loader.grains(), which independently re-reads the raw config file off disk) assumed was always a dict. That crashed the minion with TypeError: 'NoneType' object is not iterable when building the __grains__ context wrapper.

Both locations now default the option to an empty dict ({}) when it resolves to None, and log a warning pointing out that grains: {} should be used instead of an empty grains: key.

What issues does this PR fix or reference?

Fixes #61321

Previous Behavior

A minion configured with an empty grains: config option (e.g. grains: with no value) would crash on startup with:

TypeError: 'NoneType' object is not iterable

raised from salt.utils.context.NamespacedDictWrapper.__init__ via the loader's __prep_mod_opts.

New Behavior

The minion starts normally. The empty grains option is treated as {}, and a warning is logged:

Config option 'grains' is set to an empty value. An empty 'grains' config is invalid, a dict is required. To set an empty grains config, use 'grains: {}' instead. Defaulting to an empty dict.

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Add unit tests in tests/unit/test_config.py and tests/pytests/unit/loader/test_loader.py, plus an integration test in tests/pytests/integration/cli/test_salt_minion.py.

Commits signed with GPG?

No

[dwoz]

not isinstance(opts["grains"], dict) -- catches empty and wrong-type in one guard. No valid grains is anything but a mapping. Drop the warning; document the shape.

conf/minion above #grains::

+#
+# The value of 'grains' must be a mapping. Use 'grains: {}' for an
+# explicit empty section. Any non-dict value is silently defaulted
+# to an empty dict.
 #grains:

doc/ref/configuration/minion.rst inside the grains entry, after the example:

+The value of ``grains`` must be a mapping. Use ``grains: {}`` for an
+explicit empty section. Any non-dict value is silently defaulted to
+an empty dict.
+
 .. conf_minion:: grains_blacklist

Tests: parametrize over ["", '""', "[]", "foo", "42", "[1, 2]"] (all defaulting to {}) in tests/unit/test_config.py + tests/pytests/unit/loader/test_loader.py; same for tests/pytests/integration/cli/test_salt_minion.py with [None, "", [], "foo", 42, [1, 2]], drop the log-file assertions and the import salt.utils.files.

[dwoz]

not isinstance(..., dict) -- catches empty and wrong-type. Docs say a dict is required; every non-dict is equally invalid. Drop the warning; document in conf/minion + doc/ref/configuration/minion.rst instead.

Suggested change

	if "grains" in opts and opts["grains"] is None:
	log.warning(
	"Config option 'grains' is set to an empty value. An empty "
	"'grains' config is invalid, a dict is required. To set an "
	"empty grains config, use 'grains: {}' instead. Defaulting to "
	"an empty dict."
	)
	opts["grains"] = defaults.get("grains", {})
	if "grains" in opts and not isinstance(opts["grains"], dict):
	opts["grains"] = {}

[dwoz]

Same shape here. Collapses the two-if structure.

Suggested change

	if "grains" in pre_opts and pre_opts["grains"] is None:
	log.warning(
	"Config option 'grains' is set to an empty value. An empty "
	"'grains' config is invalid, a dict is required. To set an "
	"empty grains config, use 'grains: {}' instead. Defaulting "
	"to an empty dict."
	)
	if pre_opts.get("grains") is not None:
	opts["grains"] = pre_opts["grains"]
	else:
	opts["grains"] = {}
	if "grains" in pre_opts and isinstance(pre_opts["grains"], dict):
	opts["grains"] = pre_opts["grains"]
	else:
	opts["grains"] = {}

[dwoz]

Widen wording to match; mention the docs addition.

Suggested change

Fixed minion crashing on startup when the ``grains`` config option was present but empty (e.g. ``grains:`` with no value), which previously caused a ``TypeError: 'NoneType' object is not iterable``.

Fixed minion crashing on startup when the ``grains`` config option was present but not a mapping (e.g. ``grains:`` with no value, an empty string, or a scalar), which previously caused a ``TypeError: 'NoneType' object is not iterable`` and similar. Any non-dict value is now silently defaulted to an empty dict, and the required shape of the ``grains`` option is documented in the minion configuration reference.