testing, messenger: make dummy_context picklable

Note that unittest.MagicMock is unfortunately not pickable, so if one
tries to cross over to a ChildJob then we're in trouble.

Sacrifice some convenience and a bit of safety by implementing our own
_DummyTask instead of using MagicMock.

Don't try to pickle the 'simple_task' context manager, if it's been
overridden when creating a test context.

2 files changed, 35 insertions, 3 deletions

diff --git a/src/buildstream/_messenger.py b/src/buildstream/_messenger.py
index f5f570319..20c327728 100644
--- a/src/buildstream/_messenger.py
+++ b/src/buildstream/_messenger.py
@@ -405,6 +405,13 @@ class Messenger():
         #
         del state['_render_status_cb']
 
+        # The "simple_task" context manager is not needed outside the main
+        # process. During testing we override it to something that cannot
+        # pickle, so just drop it when pickling to a child job. Note that it
+        # will only appear in 'state' if it has been overridden.
+        #
+        state.pop('simple_task', None)
+
         # The State object is not needed outside the main process
         del state['_state']
 
diff --git a/tests/testutils/context.py b/tests/testutils/context.py
index 849895e92..821adef0a 100644
--- a/tests/testutils/context.py
+++ b/tests/testutils/context.py
@@ -15,13 +15,11 @@
 #  License along with this library. If not, see <http://www.gnu.org/licenses/>.
 
 import os
-from unittest.mock import MagicMock
 
 from types import MethodType
 from contextlib import contextmanager
 
 from buildstream._context import Context
-from buildstream._state import _Task
 
 
 # Handle messages from the pipeline
@@ -29,9 +27,36 @@ def _dummy_message_handler(message, is_silenced):
     pass
 
 
+class _DummyTask:
+    # Note that unittest.mock.MagicMock doesn't pickle, so we must make our
+    # _DummyTask manually here.
+    def __init__(self, state, action_name, full_name, elapsed_offset):
+        self._state = state
+        self.action_name = action_name
+        self.full_name = full_name
+        self.elapsed_offset = elapsed_offset
+        self.current_progress = None
+        self.maximum_progress = None
+
+    def set_render_cb(self, callback):
+        pass
+
+    def set_current_progress(self, progress):
+        pass
+
+    def set_maximum_progress(self, progress):
+        pass
+
+    def add_current_progress(self):
+        pass
+
+    def add_maximum_progress(self):
+        pass
+
+
 @contextmanager
 def _get_dummy_task(self, activity_name, *, element_name=None, full_name=None, silent_nested=False):
-    yield MagicMock(spec=_Task("state", activity_name, full_name, 0))
+    yield _DummyTask("state", activity_name, full_name, 0)
 
 
 # dummy_context()