Support generic invocation of package_discovery functions

Other function groups like augmentation, identification, and discovery,
can be invoked with an explicit list of extensions instances. This
supports scenarios where we want to use extensions other than the ones
registered for colcon_core.

colcon_core/package_selection/__init__.py

@@ -71,7 +71,9 @@ def select_packages(self, *, args, decorators):
         raise NotImplementedError()
 
 
-def add_arguments(parser):
+def add_arguments(
+    parser, *, discovery_extensions=None, selection_extensions=None,
+):
     """
     Add the command line arguments for the package selection extensions.
 
@@ -80,9 +82,9 @@ def add_arguments(parser):
 
     :param parser: The argument parser
     """
-    add_package_discovery_arguments(parser)
+    add_package_discovery_arguments(parser, extensions=discovery_extensions)
 
-    _add_package_selection_arguments(parser)
+    _add_package_selection_arguments(parser, extensions=selection_extensions)
 
 
 def get_package_selection_extensions(*, group_name=None):
@@ -101,15 +103,16 @@ def get_package_selection_extensions(*, group_name=None):
     return order_extensions_by_priority(extensions)
 
 
-def _add_package_selection_arguments(parser):
+def _add_package_selection_arguments(parser, *, extensions=None):
     """
     Add the command line arguments for the package selection extensions.
 
     :param parser: The argument parser
     """
-    package_selection_extensions = get_package_selection_extensions()
+    if extensions is None:
+        extensions = get_package_selection_extensions()
     group = parser.add_argument_group(title='Package selection arguments')
-    for extension in package_selection_extensions.values():
+    for extension in extensions.values():
         try:
             retval = extension.add_arguments(parser=group)
             assert retval is None, 'add_arguments() should return None'
@@ -125,7 +128,9 @@ def _add_package_selection_arguments(parser):
 def get_packages(
     args, *,
     additional_argument_names=None,
-    direct_categories=None, recursive_categories=None
+    direct_categories=None, recursive_categories=None,
+    discovery_extensions=None, identification_extensions=None,
+    augmentation_extensions=None, selection_extensions=None,
 ):
     """
     Get the selected package decorators in topological order.
@@ -146,12 +151,18 @@ def get_packages(
       package names
     """
     descriptors = get_package_descriptors(
-        args, additional_argument_names=additional_argument_names)
+        args, additional_argument_names=additional_argument_names,
+        discovery_extensions=discovery_extensions,
+        identification_extensions=identification_extensions,
+        augmentation_extensions=augmentation_extensions,
+        selection_extensions=selection_extensions)
     decorators = topological_order_packages(
         descriptors,
         direct_categories=direct_categories,
         recursive_categories=recursive_categories)
-    select_package_decorators(args, decorators)
+    select_package_decorators(
+        args, decorators,
+        selection_extensions=selection_extensions)
 
     # check for duplicate package names
     pkgs = [m.descriptor for m in decorators if m.selected]
@@ -169,7 +180,11 @@ def get_packages(
     return decorators
 
 
-def get_package_descriptors(args, *, additional_argument_names=None):
+def get_package_descriptors(
+    args, *, additional_argument_names=None, discovery_extensions=None,
+    identification_extensions=None, augmentation_extensions=None,
+    selection_extensions=None,
+):
     """
     Get the package descriptors.
 
@@ -185,20 +200,28 @@ def get_package_descriptors(args, *, additional_argument_names=None):
       :py:class:`colcon_core.package_descriptor.PackageDescriptor`
     :rtype: set
     """
-    extensions = get_package_identification_extensions()
-    descriptors = discover_packages(args, extensions)
+    if identification_extensions is None:
+        identification_extensions = get_package_identification_extensions()
+    descriptors = discover_packages(
+        args, identification_extensions,
+        discovery_extensions=discovery_extensions)
 
     pkg_names = {d.name for d in descriptors}
-    _check_package_selection_parameters(args, pkg_names)
+    _check_package_selection_parameters(
+        args, pkg_names, selection_extensions=selection_extensions)
 
     augment_packages(
-        descriptors, additional_argument_names=additional_argument_names)
+        descriptors, additional_argument_names=additional_argument_names,
+        augmentation_extensions=augmentation_extensions)
     return descriptors
 
 
-def _check_package_selection_parameters(args, pkg_names):
-    package_selection_extensions = get_package_selection_extensions()
-    for extension in package_selection_extensions.values():
+def _check_package_selection_parameters(
+    args, pkg_names, *, selection_extensions=None,
+):
+    if selection_extensions is None:
+        selection_extensions = get_package_selection_extensions()
+    for extension in selection_extensions.values():
         try:
             retval = extension.check_parameters(args=args, pkg_names=pkg_names)
             assert retval is None, 'check_parameters() should return None'
@@ -211,7 +234,9 @@ def _check_package_selection_parameters(args, pkg_names):
             # skip failing extension, continue with next one
 
 
-def select_package_decorators(args, decorators):
+def select_package_decorators(
+    args, decorators, *, selection_extensions=None,
+):
     """
     Select the package decorators based on the command line arguments.
 
@@ -222,8 +247,9 @@ def select_package_decorators(args, decorators):
     """
     # filtering must happen after the topological ordering since otherwise
     # packages in the middle of the dependency graph might be missing
-    package_selection_extensions = get_package_selection_extensions()
-    for extension in package_selection_extensions.values():
+    if selection_extensions is None:
+        selection_extensions = get_package_selection_extensions()
+    for extension in selection_extensions.values():
         try:
             retval = extension.select_packages(
                 args=args, decorators=decorators)