MDEV-32086 Server crash for INSERT..SELECT: followup, add comments

TODO: Squash this with the main patch

sql/sql_base.cc

@@ -1184,13 +1184,20 @@ retry:
     DBUG_PRINT("info",
                ("found same copy of table or table which we should skip"));
   }
-  // INSERT SELECT has better dealing with duplicates
+  /*
+     If we've found a duplicate in a derived table, try to work around that.
+
+     For INSERT...SELECT, do not do any workarounds, return the duplicate. The
+     caller will enable buffering to handle this.
+  */
   if (res && res->belong_to_derived &&
       !(check_flag & CHECK_DUP_FOR_INSERT_SELECT))
   {
     /*
-      We come here for queries of type:
-      INSERT INTO t1 (SELECT tmp.a FROM (select * FROM t1) as tmp);
+      We come here for queries like this:
+
+      INSERT INTO t1 VALUES ((SELECT tmp.a FROM (select * FROM t1)));
+      DELETE FROM t1 WHERE ( ... (SELECT ... FROM t1) ) ;
 
       Try to fix by materializing the derived table
     */

sql/sql_insert.cc

@@ -1710,7 +1710,14 @@ int mysql_prepare_insert(THD *thd, TABLE_LIST *table_list,
       DBUG_RETURN(1);
   }
 
-  // SELECT INSERT has good workaround of the problem - buffering results
+  /*
+    Check if we read from the same table we're inserting into.
+    Queries like INSERT INTO t1 VALUES ((SELECT ... FROM t1...)) are not
+    allowed.
+
+    INSERT...SELECT is an exception: it will detect this case and use
+    buffering to handle it correctly.
+  */
   if (!select_insert)
   {
     Item *fake_conds= 0;