blueprints.dms: make sure no double dms happen

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

litecord/blueprints/dms.py

@@ -38,41 +38,47 @@ 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 = 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)
+    """, dm_id, ChannelType.DM.value)
 
-    try:
-        await app.db.execute("""
-        INSERT INTO channels (id, channel_type)
-        VALUES ($1, $2)
-        """, dm_id, ChannelType.DM.value)
+    await app.db.execute("""
+    INSERT INTO dm_channels (id, party1_id, party2_id)
+    VALUES ($1, $2, $3)
+    """, dm_id, user_id, recipient_id)
 
-        await app.db.execute("""
-        INSERT INTO dm_channels (id, party1_id, party2_id)
-        VALUES ($1, $2, $3)
-        """, dm_id, user_id, recipient_id)
+    # the dm state is something we use
+    # to give the currently "open dms"
+    # on the client.
 
-        # the dm state is something we use
-        # to give the currently "open dms"
-        # on the client.
+    # we don't open a dm for the peer/recipient
+    # until the user sends a message.
+    await try_dm_state(user_id, dm_id)
 
-        # we don't open a dm for the peer/recipient
-        # 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'])