Every command failure pickles across a worker

src/agent_comms/coordination_errors.py

@@ -12,7 +12,13 @@ class DrainDeferred(Exception):  # noqa: N818 - capability, not an outcome
 
 
 class CommandFailure(Exception):  # noqa: N818 - capability, not an outcome
-    """A failure a command reports to its caller as a failed result instead of crashing."""
+    """A failure a command reports to its caller as a failed result instead of crashing.
+
+    It crosses a worker's pipe as its class, args and state, never through a member's own
+    ``__init__``, whose parameters are not its args, and keeps its original cause."""
+
+    def __reduce__(self):
+        return __newobj__, (type(self), *self.args), self.__dict__ | {"__cause__": self.__cause__}
 
 
 class DrainUnavailable(DrainDeferred):
@@ -22,10 +28,6 @@ class DrainUnavailable(DrainDeferred):
 class CoordinationError(RuntimeError, DrainUnavailable, CommandFailure):
     """Base class for fail-loud coordinator errors."""
 
-    def __reduce__(self):
-        """Keep the original cause when an observation sends this error over its pipe."""
-        return __newobj__, (type(self), *self.args), self.__dict__ | {"__cause__": self.__cause__}
-
 
 class DeferredRead(Exception):  # noqa: N818 - capability, not an outcome
     """A read refused only for now, for currentness or contention; retrying may answer it."""

tests/test_command_failure_pickles.py

@@ -0,0 +1,43 @@
+"""Every command failure crosses a worker's pipe whole, whatever its constructor takes."""
+
+import importlib
+import pickle
+import pkgutil
+import subprocess
+
+import pytest
+
+import agent_comms
+from agent_comms.coordination_errors import CommandFailure
+from agent_comms.repo import GitCommandError
+
+
+def members(family: type) -> set[type]:
+    return {member for child in family.__subclasses__() for member in {child, *members(child)}}
+
+
+def declared() -> list[type[CommandFailure]]:
+    """The family once every module declaring a member is imported."""
+    for module in pkgutil.walk_packages(agent_comms.__path__, f"{agent_comms.__name__}."):
+        if ".skills." not in module.name:
+            importlib.import_module(module.name)
+    return sorted(members(CommandFailure), key=lambda member: member.__qualname__)
+
+
+@pytest.mark.parametrize("member", declared(), ids=lambda member: member.__qualname__)
+def test_each_member_unpickles_with_its_message_state_and_cause(member) -> None:
+    failure = member.__new__(member, "git exited 128")
+    failure.detail = "kept"
+    failure.__cause__ = ValueError("root")
+    copy = pickle.loads(pickle.dumps(failure))
+    assert type(copy) is member
+    assert copy.args == ("git exited 128",)
+    assert copy.detail == "kept"
+    assert copy.__cause__.args == ("root",)
+
+
+def test_a_raised_git_failure_keeps_its_git_message() -> None:
+    run = subprocess.CompletedProcess(("git",), 128, b"", b"fatal: bad object\n")
+    copy = pickle.loads(pickle.dumps(GitCommandError("/repo", run)))
+    assert str(copy) == "Git in /repo: fatal: bad object"
+    assert (copy.returncode, copy.stderr) == (128, "fatal: bad object\n")