blueprints.dms: make sure no double dms happen

by first checking existance, then inserting if
none was found.

litecord/blueprints/dms.py

@@ -38,12 +38,28 @@ async def try_dm_state(user_id: int, dm_id: int):
     """, user_id, dm_id)
 
 
+async def jsonify_dm(dm_id: int, user_id: int):
+    dm_chan = await app.storage.get_dm(dm_id, user_id)
+    return jsonify(dm_chan)
+
+
 async def create_dm(user_id, recipient_id):
     """Create a new dm with a user,
     or get the existing DM id if it already exists."""
-    dm_id = get_snowflake()
 
-    try:
+    dm_id = await app.db.fetchval("""
+    SELECT id
+    FROM dm_channels
+    WHERE (party1_id = $1 OR party2_id = $1) AND
+          (party1_id = $2 OR party2_id = $2)
+    """, user_id, recipient_id)
+
+    if dm_id:
+        return await jsonify_dm(dm_id, user_id)
+
+    # if no dm was found, create a new one
+
+    dm_id = get_snowflake()
     await app.db.execute("""
     INSERT INTO channels (id, channel_type)
     VALUES ($1, $2)
@@ -62,17 +78,7 @@ async def create_dm(user_id, recipient_id):
     # until the user sends a message.
     await try_dm_state(user_id, dm_id)
 
-    except UniqueViolationError:
-        # the dm already exists
-        dm_id = await app.db.fetchval("""
-        SELECT id
-        FROM dm_channels
-        WHERE (party1_id = $1 OR party2_id = $1) AND
-              (party2_id = $2 OR party2_id = $2)
-        """, user_id, recipient_id)
-
-    dm = await app.storage.get_dm(dm_id, user_id)
-    return jsonify(dm)
+    return await jsonify_dm(dm_id, user_id)
 
 
 @bp.route('/@me/channels', methods=['POST'])