Fix tag filtering with --changed-dependees

src/python/pants/engine/internals/mapper.py

@@ -190,3 +190,6 @@ def matches(self, target: Target) -> bool:
         """Check that the target matches the provided `--tag` and `--exclude-target-regexp`
         options."""
         return self._tag_filter(target) and not self._is_excluded_by_pattern(target.address)
+
+    def __bool__(self) -> bool:
+        return bool(self.tags or self.exclude_target_regexps)

src/python/pants/vcs/changed.py

@@ -10,10 +10,12 @@
 from pants.backend.project_info import dependees
 from pants.backend.project_info.dependees import Dependees, DependeesRequest
 from pants.base.build_environment import get_buildroot
-from pants.engine.addresses import Address
+from pants.engine.addresses import Address, Addresses
 from pants.engine.collection import Collection
 from pants.engine.internals.graph import Owners, OwnersRequest
+from pants.engine.internals.mapper import SpecsFilter
 from pants.engine.rules import Get, collect_rules, rule
+from pants.engine.target import UnexpandedTargets
 from pants.option.option_types import EnumOption, StrOption
 from pants.option.option_value_container import OptionValueContainer
 from pants.option.subsystem import Subsystem
@@ -40,9 +42,21 @@ class ChangedAddresses(Collection[Address]):
 
 
 @rule
-async def find_changed_owners(request: ChangedRequest) -> ChangedAddresses:
-    owners = await Get(Owners, OwnersRequest(request.sources, filter_by_global_options=True))
-    if request.dependees == DependeesOption.NONE:
+async def find_changed_owners(
+    request: ChangedRequest, specs_filter: SpecsFilter
+) -> ChangedAddresses:
+    no_dependees = request.dependees == DependeesOption.NONE
+    owners = await Get(
+        Owners,
+        OwnersRequest(
+            request.sources,
+            # If `--changed-dependees` is used, we cannot eagerly filter out root targets. We
+            # need to first find their dependees, and only then should we filter. See
+            # https://github.com/pantsbuild/pants/issues/15544
+            filter_by_global_options=no_dependees,
+        ),
+    )
+    if no_dependees:
         return ChangedAddresses(owners)
 
     # See https://github.com/pantsbuild/pants/issues/15313. We filter out target generators because
@@ -63,7 +77,19 @@ async def find_changed_owners(request: ChangedRequest) -> ChangedAddresses:
             include_roots=False,
         ),
     )
-    return ChangedAddresses(FrozenOrderedSet(owners) | (dependees - owner_target_generators))
+    result = FrozenOrderedSet(owners) | (dependees - owner_target_generators)
+    if specs_filter:
+        # Finally, we must now filter out the result to only include what matches our tags, as the
+        # last step of https://github.com/pantsbuild/pants/issues/15544.
+        #
+        # Note that we use `UnexpandedTargets` rather than `Targets` or `FilteredTargets` so that
+        # we preserve target generators.
+        result_as_tgts = await Get(UnexpandedTargets, Addresses(result))
+        result = FrozenOrderedSet(
+            tgt.address for tgt in result_as_tgts if specs_filter.matches(tgt)
+        )
+
+    return ChangedAddresses(result)
 
 
 @dataclass(frozen=True)

src/python/pants/vcs/changed_integration_test.py

@@ -157,6 +157,11 @@ def test_delete_generated_target(repo: str) -> None:
     for dependees in DependeesOption:
         assert_list_stdout(repo, ["//:lib"], dependees=dependees)
 
+    # Make sure that our fix for https://github.com/pantsbuild/pants/issues/15544 does not break
+    # this test when using `--tag`.
+    for dependees in (DependeesOption.NONE, DependeesOption.TRANSITIVE):
+        assert_list_stdout(repo, ["//:lib"], dependees=dependees, extra_args=["--tag=a"])
+
     # If we also edit a sibling generated target, we should still (for now at least) include the
     # target generator.
     append_to_file(repo, "app.sh", "# foo")
@@ -208,3 +213,15 @@ def test_tag_filtering(repo: str) -> None:
         dependees=DependeesOption.TRANSITIVE,
         extra_args=["--tag=-b"],
     )
+
+    # Regression test for https://github.com/pantsbuild/pants/issues/15544. Don't filter
+    # `--changed-since` roots until the very end if using `--changed-dependees`.
+    #
+    # We change `dep.sh`, which has the tag `b`. When we filter for only tag `a`, we should still
+    # find the dependees of `dep.sh`, like `app.sh`, and only then apply the filter.
+    reset_edits()
+    append_to_file(repo, "dep.sh", "# foo")
+    assert_list_stdout(repo, [], dependees=DependeesOption.NONE, extra_args=["--tag=a"])
+    assert_list_stdout(
+        repo, ["//app.sh:lib"], dependees=DependeesOption.TRANSITIVE, extra_args=["--tag=a"]
+    )