Extend package discovery to support passive mechanisms (#685)

The current implementation of package discovery follows one of two
paths:
1. None of the extensions were specifically enabled via a command line
   argument, in which case all of the extensions are engaged.
2. Some subset of the extensions were given arguments, in which case the
   others are ignored and their discover() methods are not called.

This change adds support for discovery extensions which do not expose
command line arguments and cannot therefore be specifically enabled, and
ensures that those extensions are always engaged.

This is a non-breaking enhancement of the PackageDiscoveryExtensionPoint
interface.

Author: cottsay

colcon_core/package_discovery/__init__.py

@@ -25,14 +25,14 @@ class PackageDiscoveryExtensionPoint:
     """
 
     """The version of the discovery extension interface."""
-    EXTENSION_POINT_VERSION = '1.0'
+    EXTENSION_POINT_VERSION = '1.1'
 
     """The default priority of discovery extensions."""
     PRIORITY = 100
 
     def has_default(self):
         """
-        Check if the extension has a default parameter is none are provided.
+        Check if the extension has a default parameter if none are provided.
 
         The method is intended to be overridden in a subclass.
 
@@ -62,7 +62,9 @@ def has_parameters(self, *, args):
         This method must be overridden in a subclass.
 
         :param args: The parsed command line arguments
-        :returns: True if `discover()` should be called, False otherwise
+        :returns: True if `discover()` should be called, False if no
+          parameters were given, or None if this extension has no
+          parameters to be specified.
         :rtype: bool
         """
         raise NotImplementedError()
@@ -219,6 +221,7 @@ def expand_dir_wildcards(paths):
 def _get_extensions_with_parameters(
     args, discovery_extensions
 ):
+    explicitly_specified = False
     with_parameters = OrderedDict()
     for extension in discovery_extensions.values():
         logger.log(
@@ -235,8 +238,11 @@ def _get_extensions_with_parameters(
             # skip failing extension, continue with next one
         else:
             if has_parameter:
-                with_parameters[extension.PACKAGE_DISCOVERY_NAME] = extension
-    return with_parameters
+                explicitly_specified = True
+            elif has_parameter is not None:
+                continue
+            with_parameters[extension.PACKAGE_DISCOVERY_NAME] = extension
+    return with_parameters if explicitly_specified else OrderedDict()
 
 
 def _discover_packages(

test/test_package_discovery.py

@@ -110,6 +110,7 @@ def test_discover_packages():
             return_value={PackageDescriptor('/extension1/pkg1')})
         extensions['extension2'].discover = Mock(
             return_value={PackageDescriptor('/extension2/pkg1')})
+        extensions['extension2'].has_parameters = Mock(return_value=None)
 
         descs = discover_packages(None, None, discovery_extensions=extensions)
         assert len(descs) == 2
@@ -126,11 +127,13 @@ def test_discover_packages():
                 PackageDescriptor('/extension3/pkg2')})
 
         descs = discover_packages(None, None, discovery_extensions=extensions)
-        assert len(descs) == 2
+        assert len(descs) == 3
         expected_path = '/extension3/pkg1'.replace('/', os.sep)
         assert expected_path in (str(d.path) for d in descs)
         expected_path = '/extension3/pkg2'.replace('/', os.sep)
         assert expected_path in (str(d.path) for d in descs)
+        expected_path = '/extension2/pkg1'.replace('/', os.sep)
+        assert expected_path in (str(d.path) for d in descs)
 
 
 def test_expand_dir_wildcards():