Avoid computing expensive default values when the value is overridden anyways

The most expensive call at the moment is repo.get_changed_files, which
does down the drain if:
- another default_fn overrides the value
- an explicit override is given when creating the `Parameters`

With this change, the default function can return a function as a value,
which is not evaluated unless necessary.

Fixes #616

Author: glandium

src/taskgraph/parameters.py

@@ -110,7 +110,7 @@ def _get_defaults(repo_root=None):
         "do_not_optimize": [],
         "enable_always_target": True,
         "existing_tasks": {},
-        "files_changed": repo.get_changed_files("AM"),
+        "files_changed": lambda: repo.get_changed_files("AM"),
         "filters": ["target_tasks_method"],
         "head_ref": repo.branch or repo.head_rev,
         "head_repository": repo_url,
@@ -210,7 +210,7 @@ def _fill_defaults(repo_root=None, **kwargs):
 
         for name, default in defaults.items():
             if name not in kwargs:
-                kwargs[name] = default
+                kwargs[name] = default() if callable(default) else default
         return kwargs
 
     def check(self):

test/test_parameters.py

@@ -264,6 +264,11 @@ def test_extend_parameters_schema(monkeypatch):
             }
         ),
     )
+    monkeypatch.setattr(
+        parameters,
+        "defaults_functions",
+        list(parameters.defaults_functions),
+    )
 
     with pytest.raises(ParameterMismatch):
         Parameters(strict=False).check()
@@ -437,7 +442,7 @@ def test_extend_parameters_schema(monkeypatch):
         ),
     ),
 )
-def test_get_defaults(
+def test_defaults(
     monkeypatch, repo_root, is_repo, raises, expected_repo_root, expected
 ):
     def mock_get_repository(repo_root):
@@ -478,4 +483,4 @@ def mock_parse(url):
     monkeypatch.setattr(parameters, "datetime", datetime_mock)
     monkeypatch.setattr(parameters, "get_version", lambda *_, **__: "1.0.0")
 
-    assert parameters._get_defaults(repo_root) == expected
+    assert parameters.Parameters(strict=False, repo_root=repo_root) == expected