channel.pins: deny pins from system messages

suggested by @Cynosphere

litecord/blueprints/channel/pins.py

@@ -25,11 +25,17 @@ from litecord.snowflake import snowflake_datetime
 from litecord.types import timestamp_
 
 from litecord.system_messages import send_sys_message
-from litecord.enums import MessageType
+from litecord.enums import MessageType, SYS_MESSAGES
+from litecord.errors import BadRequest
 
 bp = Blueprint('channel_pins', __name__)
 
 
+class SysMsgInvalidAction(BadRequest):
+    """Invalid action on a system message."""
+    error_code = 50021
+
+
 @bp.route('/<int:channel_id>/pins', methods=['GET'])
 async def get_pins(channel_id):
     """Get the pins for a channel"""
@@ -62,6 +68,16 @@ async def add_pin(channel_id, message_id):
 
     await channel_perm_check(user_id, channel_id, 'manage_messages')
 
+    mtype = await app.db.fetchval("""
+    SELECT message_type
+    FROM messages
+    WHERE id = $1
+    """, message_id)
+
+    if mtype in SYS_MESSAGES:
+        raise SysMsgInvalidAction(
+            'Cannot execute action on a system message')
+
     await app.db.execute("""
     INSERT INTO channel_pins (channel_id, message_id)
     VALUES ($1, $2)

litecord/enums.py

@@ -101,6 +101,17 @@ class MessageType(EasyEnum):
     GUILD_MEMBER_JOIN = 7
 
 
+SYS_MESSAGES = (
+    MessageType.RECIPIENT_ADD,
+    MessageType.RECIPIENT_REMOVE,
+    MessageType.CALL,
+    MessageType.CHANNEL_NAME_CHANGE,
+    MessageType.CHANNEL_ICON_CHANGE,
+    MessageType.CHANNEL_PINNED_MESSAGE,
+    MessageType.GUILD_MEMBER_JOIN
+)
+
+
 class MessageActivityType(EasyEnum):
     JOIN = 1
     SPECTATE = 2

litecord/system_messages.py

@@ -42,7 +42,7 @@ async def _handle_pin_msg(app, channel_id, pinned_id, author_id):
 
 
 async def send_sys_message(app, channel_id: int, m_type: MessageType,
-                           *args, **kwargs):
+                           *args, **kwargs) -> int:
     """Send a system message."""
     handler = {
         MessageType.CHANNEL_PINNED_MESSAGE: _handle_pin_msg,
@@ -55,3 +55,5 @@ async def send_sys_message(app, channel_id: int, m_type: MessageType,
     await app.dispatcher.dispatch(
         'channel', channel_id, 'MESSAGE_CREATE', message
     )
+
+    return message_id