Re-work extension point test mocking (#622)

The existing mocks are fragile and don't reliably work in all cases.
This approach uses an actual Distribution instance so that we don't need
to modify it to reflect upstream changes to the interface.

test/test_extension_point.py

@@ -6,6 +6,12 @@
 from unittest.mock import DEFAULT
 from unittest.mock import patch
 
+try:
+    from importlib.metadata import Distribution
+except ImportError:
+    # TODO: Drop this with Python 3.7 support
+    from importlib_metadata import Distribution
+
 from colcon_core.extension_point import EntryPoint
 from colcon_core.extension_point import EXTENSION_POINT_GROUP_NAME
 from colcon_core.extension_point import get_all_extension_points
@@ -17,57 +23,55 @@
 from .environment_context import EnvironmentContext
 
 
-Group1 = EntryPoint('group1', 'g1', EXTENSION_POINT_GROUP_NAME)
-Group2 = EntryPoint('group2', 'g2', EXTENSION_POINT_GROUP_NAME)
-ExtA = EntryPoint('extA', 'eA', Group1.name)
-ExtB = EntryPoint('extB', 'eB', Group1.name)
-
-
-class Dist():
-
-    version = '0.0.0'
+class _FakeDistribution(Distribution):
 
     def __init__(self, entry_points):
-        self.metadata = {'Name': f'dist-{id(self)}'}
-        self._entry_points = entry_points
+        entry_points_spec = []
+        for group_name, group_members in entry_points.items():
+            entry_points_spec.append(f'[{group_name}]')
+            for member_name, member_value in group_members:
+                entry_points_spec.append(f'{member_name} = {member_value}')
+            entry_points_spec.append('')
+
+        self._files = {
+            'PKG-INFO': f'Name: dist-{id(self)}\nVersion: 0.0.0\n',
+            'entry_points.txt': '\n'.join(entry_points_spec) + '\n',
+        }
 
-    @property
-    def entry_points(self):
-        return list(self._entry_points)
+    def read_text(self, filename):
+        return self._files.get(filename)
 
-    @property
-    def name(self):
-        return self.metadata['Name']
+    def locate_file(self, path):
+        return path
 
 
-def iter_entry_points(*, group=None):
-    if group == EXTENSION_POINT_GROUP_NAME:
-        return [Group1, Group2]
-    elif group == Group1.name:
-        return [ExtA, ExtB]
-    assert not group
-    return {
-        EXTENSION_POINT_GROUP_NAME: [Group1, Group2],
-        Group1.name: [ExtA, ExtB],
-    }
+def _distributions():
+    yield _FakeDistribution({
+        EXTENSION_POINT_GROUP_NAME: [('group1', 'g1')],
+        'group1': [('extA', 'eA'), ('extB', 'eB')],
+    })
+    yield _FakeDistribution({
+        EXTENSION_POINT_GROUP_NAME: [('group2', 'g2')],
+        'group2': [('extC', 'eC')],
+    })
+    yield _FakeDistribution({
+        'groupX': [('extD', 'eD')],
+    })
 
 
-def distributions():
-    return [
-        Dist([Group1, ExtA, ExtB]),
-        Dist([Group2, EntryPoint('extC', 'eC', Group2.name)]),
-        Dist([EntryPoint('extD', 'eD', 'groupX')]),
-    ]
+def _entry_points():
+    for dist in _distributions():
+        yield from dist.entry_points
 
 
 def test_all_extension_points():
     with patch(
         'colcon_core.extension_point.entry_points',
-        side_effect=iter_entry_points
+        side_effect=_entry_points
     ):
         with patch(
             'colcon_core.extension_point.distributions',
-            side_effect=distributions
+            side_effect=_distributions
         ):
             # successfully load a known entry point
             extension_points = get_all_extension_points()
@@ -84,11 +88,11 @@ def test_extension_point_blocklist():
     # successful loading of extension point without a blocklist
     with patch(
         'colcon_core.extension_point.entry_points',
-        side_effect=iter_entry_points
+        side_effect=_entry_points
     ):
         with patch(
             'colcon_core.extension_point.distributions',
-            side_effect=distributions
+            side_effect=_distributions
         ):
             extension_points = get_extension_points('group1')
     assert 'extA' in extension_points.keys()