Bug #20195: INSERT DELAYED with auto_increment is assigned wrong values

The INSERT DELAYED should not maintain its own private auto-increment
counter, because this is assuming that other threads cannot insert
into the table while the INSERT DELAYED thread is inserting, which is
a wrong assumption.

So the start of processing of a batch of INSERT rows in the 
INSERT DELAYED thread must be treated as a start of a new statement
and cached next_insert_id must be cleared.

mysql-test/r/delayed.result

@@ -39,3 +39,33 @@ select * from t1;
 a
 1
 drop table t1;
+CREATE TABLE t1 ( a int(10) NOT NULL auto_increment, PRIMARY KEY (a));
+insert delayed into t1 values(null);
+insert into t1 values(null);
+insert into t1 values(null);
+insert delayed into t1 values(null);
+insert delayed into t1 values(null);
+insert delayed into t1 values(null);
+insert into t1 values(null);
+insert into t1 values(null);
+insert into t1 values(null);
+delete from t1 where a=6;
+insert delayed into t1 values(null);
+insert delayed into t1 values(null);
+insert delayed into t1 values(null);
+insert delayed into t1 values(null);
+select * from t1 order by a;
+a
+1
+2
+3
+4
+5
+7
+8
+9
+10
+11
+12
+13
+DROP TABLE t1;

mysql-test/t/delayed.test

@@ -50,3 +50,52 @@ insert into t1 values (1);
 insert delayed into t1 values (1);
 select * from t1;
 drop table t1;
+
+#
+# Bug #20195: INSERT DELAYED with auto_increment is assigned wrong values
+#
+CREATE TABLE t1 ( a int(10) NOT NULL auto_increment, PRIMARY KEY (a));
+
+# Make one delayed insert to start the separate thread
+insert delayed into t1 values(null);
+
+# Do some normal inserts
+insert into t1 values(null);
+insert into t1 values(null);
+
+# Discarded, since the delayed-counter is 2, which is already used
+insert delayed into t1 values(null);
+
+# Discarded, since the delayed-counter is 3, which is already used
+insert delayed into t1 values(null);
+
+# Works, since the delayed-counter is 4, which is unused
+insert delayed into t1 values(null);
+
+# Do some more inserts
+insert into t1 values(null);
+insert into t1 values(null);
+insert into t1 values(null);
+
+# Delete one of the above to make a hole
+delete from t1 where a=6;
+
+# Discarded, since the delayed-counter is 5, which is already used
+insert delayed into t1 values(null);
+
+# Works, since the delayed-counter is 6, which is unused (the row we deleted)
+insert delayed into t1 values(null);
+
+# Discarded, since the delayed-counter is 7, which is already used
+insert delayed into t1 values(null);
+
+# Works, since the delayed-counter is 8, which is unused
+insert delayed into t1 values(null);
+
+# Check what we have now
+# must wait so that the delayed thread finishes
+# Note: this must be increased if the test fails
+--sleep 1
+select * from t1 order by a;
+
+DROP TABLE t1;

sql/sql_insert.cc

@@ -1938,6 +1938,14 @@ bool delayed_insert::handle_inserts(void)
   if (!using_bin_log)
     table->file->extra(HA_EXTRA_WRITE_CACHE);
   pthread_mutex_lock(&mutex);
+
+  /* Reset auto-increment cacheing */
+  if (thd.clear_next_insert_id)
+  {
+    thd.next_insert_id= 0;
+    thd.clear_next_insert_id= 0;
+  }
+
   while ((row=rows.get()))
   {
     stacked_inserts--;