Add subsystem for appending tags to targets from a single source (#7315)

Problem:
It can be cumbersome to apply the same tags to many targets as it requires editing every associated BUILD file. Having a single source of tags would make it more straightforward to configure tags by which to filter targets (#6902), and can be applied to other use cases such as grouping targets with a particular suffix (ex. _integration in pants) under a single tag.

Solution:
Add a subsystem within Target responsible for fetching, parsing, and matching tags configured in a JSON file to targets. The subsystem matches against the target's address (though could be expanded to a regex to address additional use cases).

Closes #7302

src/python/pants/build_graph/target.py

@@ -28,7 +28,7 @@
 from pants.source.payload_fields import SourcesField
 from pants.source.wrapped_globs import EagerFilesetWithSpec, FilesetWithSpec
 from pants.subsystem.subsystem import Subsystem
-from pants.util.memo import memoized_property
+from pants.util.memo import memoized_method, memoized_property
 
 
 logger = logging.getLogger(__name__)
@@ -139,9 +139,32 @@ def check_unknown(self, target, kwargs, payload):
           args=''.join('\n  {} = {}'.format(key, value) for key, value in unknown_args.items())
         ))
 
+  class TagAssignments(Subsystem):
+    """Tags to add to targets in addition to any defined in their BUILD files."""
+
+    options_scope = 'target-tag-assignments'
+
+    @classmethod
+    def register_options(cls, register):
+      register('--tag-targets-mappings', type=dict, default=None, fromfile=True, fingerprint=True,
+               help='Dict with tag assignments for targets. Ex: { "tag1": ["path/to/target:foo"] }')
+
+    @classmethod
+    def tags_for(cls, target_address):
+      return cls.global_instance()._invert_tag_targets_mappings().get(target_address, [])
+
+    @memoized_method
+    def _invert_tag_targets_mappings(self):
+      result = {}
+      for tag, targets in (self.get_options().tag_targets_mappings or {}).items():
+        for target in targets:
+          target_tags = result.setdefault(Address.parse(target).spec, []) 
+          target_tags.append(tag)
+      return result
+
   @classmethod
   def subsystems(cls):
-    return super(Target, cls).subsystems() + (cls.Arguments,)
+    return super(Target, cls).subsystems() + (cls.Arguments, cls.TagAssignments)
 
   @classmethod
   def get_addressable_type(target_cls):
@@ -312,7 +335,7 @@ def __init__(self, name, address, build_graph, type_alias=None, payload=None, ta
     self.address = address
     self._build_graph = build_graph
     self._type_alias = type_alias
-    self._tags = set(tags or [])
+    self._tags = set(tags or []).union(self.TagAssignments.tags_for(address.spec))
     self.description = description
 
     self._cached_fingerprint_map = {}

tests/python/pants_test/build_graph/BUILD

@@ -41,6 +41,7 @@ python_tests(
   sources = ['test_build_file_aliases.py'],
   dependencies = [
     'src/python/pants/build_graph',
+    'tests/python/pants_test/subsystem:subsystem_utils',
   ]
 )
 

tests/python/pants_test/build_graph/test_build_file_aliases.py

@@ -11,6 +11,7 @@
 from pants.build_graph.build_file_aliases import BuildFileAliases, TargetMacro
 from pants.build_graph.mutable_build_graph import MutableBuildGraph
 from pants.build_graph.target import Target
+from pants_test.subsystem.subsystem_util import init_subsystem
 
 
 class BuildFileAliasesTest(unittest.TestCase):
@@ -19,6 +20,7 @@ class BlueTarget(Target):
     pass
 
   def setUp(self):
+    init_subsystem(Target.TagAssignments)
     self.target_macro_factory = TargetMacro.Factory.wrap(
       lambda ctx: ctx.create_object(self.BlueTarget,
                                     type_alias='jill',

tests/python/pants_test/build_graph/test_target.py

@@ -110,6 +110,28 @@ def test_unknown_kwargs(self):
     target = self.make_target('foo:bar', Target, foobar='barfoo')
     self.assertFalse(hasattr(target, 'foobar'))
 
+  def test_tags_applied_from_configured_dict(self):
+    options = {Target.TagAssignments.options_scope: {
+      'tag_targets_mappings': {
+        'special_tag': ['foo:bar', 'path/to/target:foo', 'path/to/target'],
+        'special_tag2': ['path/to/target:target', '//base:foo'],
+        'nonextant_target_tag': ['i/dont/exist'],
+      }
+    }}
+
+    init_subsystem(Target.TagAssignments, options)
+    target1 = self.make_target('foo:bar', Target, tags=['tag1', 'tag2'])
+    target2 = self.make_target('path/to/target:foo', Target, tags=['tag1'])
+    target3 = self.make_target('path/to/target', Target, tags=['tag2'])
+    target4 = self.make_target('//base:foo', Target, tags=['tag3'])
+    target5 = self.make_target('baz:qux', Target, tags=['tag3'])
+
+    self.assertEqual({'tag1', 'tag2', 'special_tag'}, target1.tags)
+    self.assertEqual({'tag1', 'special_tag'}, target2.tags)
+    self.assertEqual({'tag2', 'special_tag', 'special_tag2'}, target3.tags)
+    self.assertEqual({'tag3', 'special_tag2'}, target4.tags)
+    self.assertEqual({'tag3'}, target5.tags)
+
   def test_target_id_long(self):
     long_path = 'dummy'
     for i in range(1,30):

tests/python/pants_test/test_base.py

@@ -323,6 +323,7 @@ def setUp(self):
 
     self._build_configuration = self.build_config()
     self._inited_target = False
+    subsystem_util.init_subsystem(Target.TagAssignments)
 
   def buildroot_files(self, relpath=None):
     """Returns the set of all files under the test build root.