Allow sending a membership event to unban a user (#10807)

* Allow membership event to unban user

Signed-off-by: Aaron Raimist <aaron@raim.ist>

4 files changed, 99 insertions, 2 deletions

diff --git a/changelog.d/10807.bugfix b/changelog.d/10807.bugfix
new file mode 100644
index 0000000000..be03f5c738
--- /dev/null
+++ b/changelog.d/10807.bugfix
@@ -0,0 +1 @@
+Allow sending a membership event to unban a user. Contributed by @aaronraimist.
\ No newline at end of file
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index a3e13c2270..7bb3f0bc47 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -668,7 +668,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                     " (membership=%s)" % old_membership,
                     errcode=Codes.BAD_STATE,
                 )
-            if old_membership == "ban" and action != "unban":
+            if old_membership == "ban" and action not in ["ban", "unban", "leave"]:
                 raise SynapseError(
                     403,
                     "Cannot %s user who was banned" % (action,),
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 50100a5ae4..5a01765f4d 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -26,7 +26,7 @@ from twisted.internet import defer
 
 import synapse.rest.admin
 from synapse.api.constants import EventContentFields, EventTypes, Membership
-from synapse.api.errors import HttpResponseException
+from synapse.api.errors import Codes, HttpResponseException
 from synapse.handlers.pagination import PurgeStatus
 from synapse.rest import admin
 from synapse.rest.client import account, directory, login, profile, room, sync
@@ -377,6 +377,91 @@ class RoomPermissionsTestCase(RoomBase):
             expect_code=403,
         )
 
+    # tests the "from banned" line from the table in https://spec.matrix.org/unstable/client-server-api/#mroommember
+    def test_member_event_from_ban(self):
+        room = self.created_rmid
+        self.helper.invite(room=room, src=self.rmcreator_id, targ=self.user_id)
+        self.helper.join(room=room, user=self.user_id)
+
+        other = "@burgundy:red"
+
+        # User cannot ban other since they do not have required power level
+        self.helper.change_membership(
+            room=room,
+            src=self.user_id,
+            targ=other,
+            membership=Membership.BAN,
+            expect_code=403,  # expect failure
+            expect_errcode=Codes.FORBIDDEN,
+        )
+
+        # Admin bans other
+        self.helper.change_membership(
+            room=room,
+            src=self.rmcreator_id,
+            targ=other,
+            membership=Membership.BAN,
+            expect_code=200,
+        )
+
+        # from ban to invite: Must never happen.
+        self.helper.change_membership(
+            room=room,
+            src=self.rmcreator_id,
+            targ=other,
+            membership=Membership.INVITE,
+            expect_code=403,  # expect failure
+            expect_errcode=Codes.BAD_STATE,
+        )
+
+        # from ban to join: Must never happen.
+        self.helper.change_membership(
+            room=room,
+            src=other,
+            targ=other,
+            membership=Membership.JOIN,
+            expect_code=403,  # expect failure
+            expect_errcode=Codes.BAD_STATE,
+        )
+
+        # from ban to ban: No change.
+        self.helper.change_membership(
+            room=room,
+            src=self.rmcreator_id,
+            targ=other,
+            membership=Membership.BAN,
+            expect_code=200,
+        )
+
+        # from ban to knock: Must never happen.
+        self.helper.change_membership(
+            room=room,
+            src=self.rmcreator_id,
+            targ=other,
+            membership=Membership.KNOCK,
+            expect_code=403,  # expect failure
+            expect_errcode=Codes.BAD_STATE,
+        )
+
+        # User cannot unban other since they do not have required power level
+        self.helper.change_membership(
+            room=room,
+            src=self.user_id,
+            targ=other,
+            membership=Membership.LEAVE,
+            expect_code=403,  # expect failure
+            expect_errcode=Codes.FORBIDDEN,
+        )
+
+        # from ban to leave: User was unbanned.
+        self.helper.change_membership(
+            room=room,
+            src=self.rmcreator_id,
+            targ=other,
+            membership=Membership.LEAVE,
+            expect_code=200,
+        )
+
 
 class RoomsMemberListTestCase(RoomBase):
     """Tests /rooms/$room_id/members/list REST events."""
diff --git a/tests/rest/client/utils.py b/tests/rest/client/utils.py
index 954ad1a1fd..c56e45fc10 100644
--- a/tests/rest/client/utils.py
+++ b/tests/rest/client/utils.py
@@ -138,6 +138,7 @@ class RestHelper:
         extra_data: Optional[dict] = None,
         tok: Optional[str] = None,
         expect_code: int = 200,
+        expect_errcode: str = None,
     ) -> None:
         """
         Send a membership state event into a room.
@@ -150,6 +151,7 @@ class RestHelper:
             extra_data: Extra information to include in the content of the event
             tok: The user access token to use
             expect_code: The expected HTTP response code
+            expect_errcode: The expected Matrix error code
         """
         temp_id = self.auth_user_id
         self.auth_user_id = src
@@ -177,6 +179,15 @@ class RestHelper:
             channel.result["body"],
         )
 
+        if expect_errcode:
+            assert (
+                str(channel.json_body["errcode"]) == expect_errcode
+            ), "Expected: %r, got: %r, resp: %r" % (
+                expect_errcode,
+                channel.json_body["errcode"],
+                channel.result["body"],
+            )
+
         self.auth_user_id = temp_id
 
     def send(