owners: handle CLs which only have partial OWNERS

If every file in a change has OWNERS, then the code would find
them fine. Similarly, if no files has OWNERS, then this code would
return an empty set just fine.

But if some files had OWNERS while others didn't, it would crash
when it tried to find an OWNER for file 'foo' while all the possible
OWNERS only provided coverage for file 'bar'.

This code purges the list of possible OWNERS as they become useless
for providing additional coverage, and returns whatever set we have
accumulated so far when the set of possible OWNERS becomes empty.

R=iannucci@chromium.org

Bug: 715062
Change-Id: I408601bd89379381db1cc7df56beed97ab3c27e6
Reviewed-on: https://chromium-review.googlesource.com/506239
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

owners.py

@@ -418,19 +418,28 @@ class Database(object):
     dirs_remaining = set(self._enclosing_dir_with_owners(f) for f in files)
     all_possible_owners = self.all_possible_owners(dirs_remaining, author)
     suggested_owners = set()
-    if len(all_possible_owners) == 0:
-      return suggested_owners
-    while dirs_remaining:
+    while dirs_remaining and all_possible_owners:
       owner = self.lowest_cost_owner(all_possible_owners, dirs_remaining)
       suggested_owners.add(owner)
       dirs_to_remove = set(el[0] for el in all_possible_owners[owner])
       dirs_remaining -= dirs_to_remove
+      # Now that we've used `owner` and covered all their dirs, remove them
+      # from consideration.
+      del all_possible_owners[owner]
+      for o, dirs in all_possible_owners.items():
+        new_dirs = [(d, dist) for (d, dist) in dirs if d not in dirs_to_remove]
+        if not new_dirs:
+          del all_possible_owners[o]
+        else:
+          all_possible_owners[o] = new_dirs
     return suggested_owners
 
   def all_possible_owners(self, dirs, author):
-    """Returns a list of (potential owner, distance-from-dir) tuples; a
-    distance of 1 is the lowest/closest possible distance (which makes the
-    subsequent math easier)."""
+    """Returns a dict of {potential owner: (dir, distance)} mappings.
+
+    A distance of 1 is the lowest/closest possible distance (which makes the
+    subsequent math easier).
+    """
     all_possible_owners = {}
     for current_dir in dirs:
       dirname = current_dir

tests/owners_unittest.py

@@ -467,6 +467,13 @@ class ReviewersForTest(_BaseTestCase):
     self.assert_reviewers_for(['chrome/gpu/gpu_channel.h'],
                               [[ben], [brett]], author=ken)
 
+
+  def test_reviewers_for__ignores_unowned_files(self):
+    # Clear the root OWNERS file.
+    self.files['/OWNERS'] = ''
+    self.assert_reviewers_for(['base/vlog.h', 'chrome/browser/deafults/h'],
+                              [[brett]])
+
   def test_reviewers_file_includes__absolute(self):
     self.assert_reviewers_for(['content/qux/foo.cc'],
                               [[peter], [brett], [john], [darin]])