Fix entry point discovery on Python < 3.10

It seems that the behavior of the importlib.metadata.entry_points
function changed in Python 3.10 to automatically de-duplicate
distributions, but prior to that the "shadowed" distributions were also
enumerated. This change specifically ignores "shadowed" distributions so
that they aren't identified as extension point overwrites.

colcon_core/extension_point.py

@@ -37,6 +37,15 @@
 EXTENSION_POINT_GROUP_NAME = 'colcon_core.extension_point'
 
 
+def _get_unique_distributions():
+    seen = set()
+    for dist in distributions():
+        dist_name = dist.metadata['Name']
+        if dist_name not in seen:
+            seen.add(dist_name)
+            yield dist
+
+
 def get_all_extension_points():
     """
     Get all extension points related to `colcon` and any of its extensions.
@@ -51,12 +60,7 @@ def get_all_extension_points():
     colcon_extension_points.setdefault(EXTENSION_POINT_GROUP_NAME, None)
 
     entry_points = defaultdict(dict)
-    seen = set()
-    for dist in distributions():
-        dist_name = dist.metadata['Name']
-        if dist_name in seen:
-            continue
-        seen.add(dist_name)
+    for dist in _get_unique_distributions():
         for entry_point in dist.entry_points:
             # skip groups which are not registered as extension points
             if entry_point.group not in colcon_extension_points:
@@ -70,7 +74,7 @@ def get_all_extension_points():
                     f"from '{dist._path}' "
                     f"overwriting '{previous}'")
             entry_points[entry_point.group][entry_point.name] = \
-                (entry_point.value, dist_name, dist.version)
+                (entry_point.value, dist.metadata['Name'], dist.version)
     return entry_points
 
 
@@ -87,7 +91,11 @@ def get_extension_points(group):
         # Python 3.10 and newer
         query = entry_points(group=group)
     except TypeError:
-        query = entry_points().get(group, ())
+        query = (
+            entry_point
+            for dist in _get_unique_distributions()
+            for entry_point in dist.entry_points
+            if entry_point.group == group)
     for entry_point in query:
         if entry_point.name in extension_points:
             previous_entry_point = extension_points[entry_point.name]

test/test_extension_point.py

@@ -54,8 +54,8 @@ def iter_entry_points(*, group=None):
 
 def distributions():
     return [
-        Dist(iter_entry_points(group='group1')),
-        Dist([EntryPoint('extC', 'eC', Group2.name)]),
+        Dist([Group1, ExtA, ExtB]),
+        Dist([Group2, EntryPoint('extC', 'eC', Group2.name)]),
         Dist([EntryPoint('extD', 'eD', 'groupX')]),
     ]
 
@@ -71,7 +71,11 @@ def test_all_extension_points():
         ):
             # successfully load a known entry point
             extension_points = get_all_extension_points()
-            assert set(extension_points.keys()) == {'group1', 'group2'}
+            assert set(extension_points.keys()) == {
+                EXTENSION_POINT_GROUP_NAME,
+                'group1',
+                'group2',
+            }
             assert set(extension_points['group1'].keys()) == {'extA', 'extB'}
             assert extension_points['group1']['extA'][0] == 'eA'