Fix that running on a BUILD file expands to all of its targets (pants…

…build#16546)

## Problem

@thejcannon is moving formatting of BUILD files from `update-build-files` to `fmt`. 

But there's a weird edge: `./pants fmt BUILD` often formats much more than `BUILD` because it runs on all the defined targets. Josh and I both believe this is not intuitive - a user would expect only `BUILD` to be changed.

Before:

```
❯ ./pants --cli-build-files-expand-to-targets list BUILD
//BUILD_ROOT:files
//cargo:scripts
//conftest.py:test_utils
//pants:scripts
//pants.toml:files
```

After:

```
❯ ./pants --no-cli-build-files-expand-to-targets list BUILD
15:12:08.84 [WARN] No targets were matched in goal `list`.
```

While this is technically a user API change, we believe the old behavior was buggy and not intentional. It also was not documented. So we fix this as a bug fix, without using a deprecation.

### `--changed-since`

It's important that `--changed-since` does expand BUILD files to their targets, as changing a BUILD file might result in any of the targets changing too. This preserves that behavior, regardless of `--cli-build-files-expand-to-targets `.

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

@@ -705,6 +705,7 @@ class OwnersRequest:
     sources: tuple[str, ...]
     owners_not_found_behavior: OwnersNotFoundBehavior = OwnersNotFoundBehavior.ignore
     filter_by_global_options: bool = False
+    match_if_owning_build_file_included_in_sources: bool = False
 
 
 class Owners(Collection[Address]):
@@ -799,7 +800,10 @@ def create_live_and_deleted_gets(
                 matching_files.update(
                     matches_filespec(secondary_owner_field.filespec, paths=sources_set)
                 )
-            if not matching_files and bfa.rel_path not in sources_set:
+            if not matching_files and not (
+                owners_request.match_if_owning_build_file_included_in_sources
+                and bfa.rel_path in sources_set
+            ):
                 continue
 
             unmatched_sources -= matching_files

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

@@ -708,9 +708,21 @@ def owners_rule_runner() -> RuleRunner:
 
 
 def assert_owners(
-    rule_runner: RuleRunner, requested: Iterable[str], *, expected: Set[Address]
+    rule_runner: RuleRunner,
+    requested: Iterable[str],
+    *,
+    expected: Set[Address],
+    match_if_owning_build_file_included_in_sources: bool = False,
 ) -> None:
-    result = rule_runner.request(Owners, [OwnersRequest(tuple(requested))])
+    result = rule_runner.request(
+        Owners,
+        [
+            OwnersRequest(
+                tuple(requested),
+                match_if_owning_build_file_included_in_sources=match_if_owning_build_file_included_in_sources,
+            )
+        ],
+    )
     assert set(result) == expected
 
 
@@ -823,6 +835,13 @@ def test_owners_build_file(owners_rule_runner: RuleRunner) -> None:
     assert_owners(
         owners_rule_runner,
         ["demo/BUILD"],
+        match_if_owning_build_file_included_in_sources=False,
+        expected=set(),
+    )
+    assert_owners(
+        owners_rule_runner,
+        ["demo/BUILD"],
+        match_if_owning_build_file_included_in_sources=True,
         expected={
             Address("demo", target_name="f1"),
             Address("demo", target_name="f2_first"),

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

@@ -207,7 +207,12 @@ async def addresses_from_raw_specs_with_only_file_owners(
     all_files = tuple(itertools.chain.from_iterable(paths.files for paths in paths_per_include))
     owners = await Get(
         Owners,
-        OwnersRequest(all_files, filter_by_global_options=specs.filter_by_global_options),
+        OwnersRequest(
+            all_files,
+            filter_by_global_options=specs.filter_by_global_options,
+            # Specifying a BUILD file should not expand to all the targets it defines.
+            match_if_owning_build_file_included_in_sources=False,
+        ),
     )
     return Addresses(sorted(owners))
 

src/python/pants/vcs/changed.py

@@ -54,6 +54,8 @@ async def find_changed_owners(
             # 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,
+            # Changing a BUILD file might impact the targets it defines.
+            match_if_owning_build_file_included_in_sources=True,
         ),
     )
     if no_dependees: