dc_receive_imf: do not create adhoc groups when group ID is known

Adhoc groups for group messages that don't have Chat-Group-ID are already
created above in another create_or_lookup_adhoc_group.

At this point it could be that there is a valid Chat-Group-ID header,
but no group was created because removed_id was non-zero i.e. received
message removes some group member.

If group was not explicitly left, current code creates an adhoc group
instead of trashing late or reordered messages that remove group
members. Such groups have adhoc group IDs locally, but proper group
message-IDs.  Attempts to reply to such groups or leave them creates
"split groups" for all other members of original group. The solution is
not to create adhoc groups in this case.

src/dc_receive_imf.rs

@@ -1199,23 +1199,19 @@ async fn create_or_lookup_group(
 
     // again, check chat_id
     if chat_id.is_special() {
-        return if group_explicitly_left {
-            Ok((ChatId::new(DC_CHAT_ID_TRASH), chat_id_blocked))
+        if mime_parser.decrypting_failed {
+            // It is possible that the message was sent to a valid,
+            // yet unknown group, which was rejected because
+            // Chat-Group-Name, which is in the encrypted part, was
+            // not found. We can't create a properly named group in
+            // this case, so assign error message to 1:1 chat with the
+            // sender instead.
+            return Ok((ChatId::new(0), Blocked::Not));
         } else {
-            create_or_lookup_adhoc_group(
-                context,
-                mime_parser,
-                allow_creation,
-                create_blocked,
-                from_id,
-                to_ids,
-            )
-            .await
-            .map_err(|err| {
-                warn!(context, "failed to create ad-hoc group: {:?}", err);
-                err
-            })
-        };
+            // The message was decrypted successfully, but contains a late "quit" or otherwise
+            // unwanted message.
+            return Ok((ChatId::new(DC_CHAT_ID_TRASH), chat_id_blocked));
+        }
     }
 
     // We have a valid chat_id > DC_CHAT_ID_LAST_SPECIAL.
@@ -1400,12 +1396,10 @@ async fn create_or_lookup_adhoc_group(
         // decrypted.
         //
         // The subject may be encrypted and contain a placeholder such
-        // as "...". Besides that, it is possible that the message was
-        // sent to a valid, yet unknown group, which was rejected
-        // because Chat-Group-Name, which is in the encrypted part,
-        // was not found. Generating a new ID in this case would
-        // result in creation of a twin group with a different group
-        // ID.
+        // as "...". It can also be a COI group, with encrypted
+        // Chat-Group-ID and incompatible Message-ID format.
+        //
+        // Instead, assign the message to 1:1 chat with the sender.
         warn!(
             context,
             "not creating ad-hoc group for message that cannot be decrypted"