kunit: tool: reconfigure when the used kunitconfig changes

Problem: currently, if you remove something from your kunitconfig,
kunit.py will not regenerate the .config file.
The same thing happens if you did --kunitconfig_add=CONFIG_KASAN=y [1]
and then ran again without it. Your new run will still have KASAN.

The reason is that kunit.py won't regenerate the .config file if it's a
superset of the kunitconfig. This speeds it up a bit for iterating.

This patch adds an additional check that forces kunit.py to regenerate
the .config file if the current kunitconfig doesn't match the previous
one.

What this means:
* deleting entries from .kunitconfig works as one would expect
* dropping  a --kunitconfig_add also triggers a rebuild
* you can still edit .config directly to turn on new options

We implement this by creating a `last_used_kunitconfig` file in the
build directory (so .kunit, by default) after we generate the .config.
When comparing the kconfigs, we compare python sets, so duplicates and
permutations don't trip us up.

The majority of this patch is adding unit tests for the existing logic
and for the new case where `last_used_kunitconfig` differs.

[1] https://lore.kernel.org/linux-kselftest/20211106013058.2621799-2-dlatypov@google.com/

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/start.rst

@@ -50,10 +50,10 @@ It'll warn you if you haven't included the dependencies of the options you're
 using.
 
 .. note::
-   Note that removing something from the ``.kunitconfig`` will not trigger a
-   rebuild of the ``.config`` file: the configuration is only updated if the
-   ``.kunitconfig`` is not a subset of ``.config``. This means that you can use
-   other tools (such as make menuconfig) to adjust other config options.
+   If you change the ``.kunitconfig``, kunit.py will trigger a rebuild of the
+   ``.config`` file. But you can edit the ``.config`` file directly or with
+   tools like ``make menuconfig O=.kunit``. As long as its a superset of
+   ``.kunitconfig``, kunit.py won't overwrite your changes.
 
 
 Running the tests (KUnit Wrapper)

tools/testing/kunit/kunit_kernel.py

@@ -21,6 +21,7 @@
 
 KCONFIG_PATH = '.config'
 KUNITCONFIG_PATH = '.kunitconfig'
+OLD_KUNITCONFIG_PATH = 'last_used_kunitconfig'
 DEFAULT_KUNITCONFIG_PATH = 'tools/testing/kunit/configs/default.config'
 BROKEN_ALLCONFIG_PATH = 'tools/testing/kunit/configs/broken_on_uml.config'
 OUTFILE_PATH = 'test.log'
@@ -179,6 +180,9 @@ def get_kconfig_path(build_dir) -> str:
 def get_kunitconfig_path(build_dir) -> str:
 	return get_file_path(build_dir, KUNITCONFIG_PATH)
 
+def get_old_kunitconfig_path(build_dir) -> str:
+	return get_file_path(build_dir, OLD_KUNITCONFIG_PATH)
+
 def get_outfile_path(build_dir) -> str:
 	return get_file_path(build_dir, OUTFILE_PATH)
 
@@ -289,24 +293,38 @@ def build_config(self, build_dir, make_options) -> bool:
 		except ConfigError as e:
 			logging.error(e)
 			return False
-		return self.validate_config(build_dir)
+		if not self.validate_config(build_dir):
+			return False
+
+		old_path = get_old_kunitconfig_path(build_dir)
+		if os.path.exists(old_path):
+			os.remove(old_path)  # write_to_file appends to the file
+		self._kconfig.write_to_file(old_path)
+		return True
+
+	def _kunitconfig_changed(self, build_dir: str) -> bool:
+		old_path = get_old_kunitconfig_path(build_dir)
+		if not os.path.exists(old_path):
+			return True
+
+		old_kconfig = kunit_config.parse_file(old_path)
+		return old_kconfig.entries() != self._kconfig.entries()
 
 	def build_reconfig(self, build_dir, make_options) -> bool:
 		"""Creates a new .config if it is not a subset of the .kunitconfig."""
 		kconfig_path = get_kconfig_path(build_dir)
-		if os.path.exists(kconfig_path):
-			existing_kconfig = kunit_config.parse_file(kconfig_path)
-			self._ops.make_arch_qemuconfig(self._kconfig)
-			if not self._kconfig.is_subset_of(existing_kconfig):
-				print('Regenerating .config ...')
-				os.remove(kconfig_path)
-				return self.build_config(build_dir, make_options)
-			else:
-				return True
-		else:
+		if not os.path.exists(kconfig_path):
 			print('Generating .config ...')
 			return self.build_config(build_dir, make_options)
 
+		existing_kconfig = kunit_config.parse_file(kconfig_path)
+		self._ops.make_arch_qemuconfig(self._kconfig)
+		if self._kconfig.is_subset_of(existing_kconfig) and not self._kunitconfig_changed(build_dir):
+			return True
+		print('Regenerating .config ...')
+		os.remove(kconfig_path)
+		return self.build_config(build_dir, make_options)
+
 	def build_kernel(self, alltests, jobs, build_dir, make_options) -> bool:
 		try:
 			if alltests:

tools/testing/kunit/kunit_tool_test.py

@@ -413,6 +413,51 @@ def fake_start(unused_args, unused_build_dir):
 			with open(kunit_kernel.get_outfile_path(build_dir), 'rt') as outfile:
 				self.assertEqual(outfile.read(), 'hi\nbye\n', msg='Missing some output')
 
+	def test_build_reconfig_no_config(self):
+		with tempfile.TemporaryDirectory('') as build_dir:
+			with open(kunit_kernel.get_kunitconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y')
+
+			tree = kunit_kernel.LinuxSourceTree(build_dir)
+			mock_build_config = mock.patch.object(tree, 'build_config').start()
+
+			# Should generate the .config
+			self.assertTrue(tree.build_reconfig(build_dir, make_options=[]))
+			mock_build_config.assert_called_once_with(build_dir, [])
+
+	def test_build_reconfig_existing_config(self):
+		with tempfile.TemporaryDirectory('') as build_dir:
+			# Existing .config is a superset, should not touch it
+			with open(kunit_kernel.get_kunitconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y')
+			with open(kunit_kernel.get_old_kunitconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y')
+			with open(kunit_kernel.get_kconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y\nCONFIG_KUNIT_TEST=y')
+
+			tree = kunit_kernel.LinuxSourceTree(build_dir)
+			mock_build_config = mock.patch.object(tree, 'build_config').start()
+
+			self.assertTrue(tree.build_reconfig(build_dir, make_options=[]))
+			self.assertEqual(mock_build_config.call_count, 0)
+
+	def test_build_reconfig_remove_option(self):
+		with tempfile.TemporaryDirectory('') as build_dir:
+			# We removed CONFIG_KUNIT_TEST=y from our .kunitconfig...
+			with open(kunit_kernel.get_kunitconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y')
+			with open(kunit_kernel.get_old_kunitconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y\nCONFIG_KUNIT_TEST=y')
+			with open(kunit_kernel.get_kconfig_path(build_dir), 'w') as f:
+				f.write('CONFIG_KUNIT=y\nCONFIG_KUNIT_TEST=y')
+
+			tree = kunit_kernel.LinuxSourceTree(build_dir)
+			mock_build_config = mock.patch.object(tree, 'build_config').start()
+
+			# ... so we should trigger a call to build_config()
+			self.assertTrue(tree.build_reconfig(build_dir, make_options=[]))
+			mock_build_config.assert_called_once_with(build_dir, [])
+
 	# TODO: add more test cases.