owners: add owners_rooted_at_file()

This method allows retrieving the owners listed in a file, as well as those
transitively listed in files included by it. This change also adds a new test to
cover this method, which is just a wrapper for the existing
_read_just_the_owners() internal method.

Bug: None
Change-Id: Iee956f115d3846acf0ee0806451807b0aa96d2f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1773904
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>

owners.py

@@ -610,3 +610,12 @@ class Database(object):
         lambda owner: total_costs_by_owner[owner] == lowest_cost,
         total_costs_by_owner)
     return random.Random().choice(lowest_cost_owners)
+
+  def owners_rooted_at_file(self, filename):
+    """Returns a set of all owners transitively listed in filename.
+
+    This function returns a set of all the owners either listed in filename, or
+    in a file transitively included by filename. Lines that are not plain owners
+    (i.e. per-file owners) are ignored.
+    """
+    return self._read_just_the_owners(filename)

tests/owners_unittest.py

@@ -388,6 +388,14 @@ class OwnersDatabaseTest(_BaseTestCase):
         john: {'': 'comment preceeded by empty line'},
         peter: {'': 'comment in the middle'}})
 
+  def test_owners_rooted_at_file(self):
+    self.files['/foo/OWNERS'] = owners_file(darin, file='//bar/OWNERS')
+    self.files['/bar/OWNERS'] = owners_file(john,
+        lines=['per-file nope.cc=' + ben])
+    db = self.db()
+    self.assertEqual(db.owners_rooted_at_file('foo/OWNERS'),
+                     set([john, darin]))
+
 
 class ReviewersForTest(_BaseTestCase):
   def assert_reviewers_for(self, files, potential_suggested_reviewers,