Use json.dump in FileExfiltrationWriter (#15095)

To directly write to the open file, instead of writing to an
in-memory string first.

changelog.d/15095.misc

@@ -0,0 +1 @@
+Refactor writing json data in `FileExfiltrationWriter`.

synapse/app/admin_cmd.py

@@ -149,7 +149,7 @@ def write_events(self, room_id: str, events: List[EventBase]) -> None:
 
         with open(events_file, "a") as f:
             for event in events:
-                print(json.dumps(event.get_pdu_json()), file=f)
+                json.dump(event.get_pdu_json(), fp=f)
 
     def write_state(
         self, room_id: str, event_id: str, state: StateMap[EventBase]
@@ -162,7 +162,7 @@ def write_state(
 
         with open(event_file, "a") as f:
             for event in state.values():
-                print(json.dumps(event.get_pdu_json()), file=f)
+                json.dump(event.get_pdu_json(), fp=f)
 
     def write_invite(
         self, room_id: str, event: EventBase, state: StateMap[EventBase]
@@ -178,7 +178,7 @@ def write_invite(
 
         with open(invite_state, "a") as f:
             for event in state.values():
-                print(json.dumps(event), file=f)
+                json.dump(event, fp=f)
 
     def write_knock(
         self, room_id: str, event: EventBase, state: StateMap[EventBase]
@@ -194,15 +194,15 @@ def write_knock(
 
         with open(knock_state, "a") as f:
             for event in state.values():
-                print(json.dumps(event), file=f)
+                json.dump(event, fp=f)
 
     def write_profile(self, profile: JsonDict) -> None:
         user_directory = os.path.join(self.base_directory, "user_data")
         os.makedirs(user_directory, exist_ok=True)
         profile_file = os.path.join(user_directory, "profile")
 
         with open(profile_file, "a") as f:
-            print(json.dumps(profile), file=f)
+            json.dump(profile, fp=f)
 
     def write_devices(self, devices: List[JsonDict]) -> None:
         user_directory = os.path.join(self.base_directory, "user_data")
@@ -211,7 +211,7 @@ def write_devices(self, devices: List[JsonDict]) -> None:
 
         for device in devices:
             with open(device_file, "a") as f:
-                print(json.dumps(device), file=f)
+                json.dump(device, fp=f)
 
     def write_connections(self, connections: List[JsonDict]) -> None:
         user_directory = os.path.join(self.base_directory, "user_data")
@@ -220,7 +220,7 @@ def write_connections(self, connections: List[JsonDict]) -> None:
 
         for connection in connections:
             with open(connection_file, "a") as f:
-                print(json.dumps(connection), file=f)
+                json.dump(connection, fp=f)
 
     def write_account_data(
         self, file_name: str, account_data: Mapping[str, JsonDict]
@@ -233,7 +233,7 @@ def write_account_data(
         account_data_file = os.path.join(account_data_directory, file_name)
 
         with open(account_data_file, "a") as f:
-            print(json.dumps(account_data), file=f)
+            json.dump(account_data, fp=f)
 
     def finished(self) -> str:
         return self.base_directory