MDEV-4688: empty @@gtid_slave_pos during slave commit.

In record_gtid(), too many rows were deleted from the slave position
hash - we need to always keep on to the most recent committed row,
so we have a valid slave position at all times.

1 files changed, 33 insertions, 7 deletions

diff --git a/sql/rpl_gtid.cc b/sql/rpl_gtid.cc
index d8e79c1e002..4783fb763c8 100644
--- a/sql/rpl_gtid.cc
+++ b/sql/rpl_gtid.cc
@@ -363,6 +363,14 @@ rpl_slave_state::record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
     goto end;
   }
 
+  if(opt_bin_log &&
+     (err= mysql_bin_log.bump_seq_no_counter_if_needed(gtid->domain_id,
+                                                       gtid->seq_no)))
+  {
+    my_error(ER_OUT_OF_RESOURCES, MYF(0));
+    goto end;
+  }
+
   lock();
   if ((elem= get_element(gtid->domain_id)) == NULL)
   {
@@ -371,7 +379,30 @@ rpl_slave_state::record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
     err= 1;
     goto end;
   }
-  elist= elem->grab_list();
+  if ((elist= elem->grab_list()) != NULL)
+  {
+    /* Delete any old stuff, but keep around the most recent one. */
+    list_element *cur= elist;
+    uint64 best_sub_id= cur->sub_id;
+    list_element **best_ptr_ptr= &elist;
+    while ((next= cur->next))
+    {
+      if (next->sub_id > best_sub_id)
+      {
+        best_sub_id= next->sub_id;
+        best_ptr_ptr= &cur->next;
+      }
+      cur= next;
+    }
+    /*
+      Delete the highest sub_id element from the old list, and put it back as
+      the single-element new list.
+    */
+    cur= *best_ptr_ptr;
+    *best_ptr_ptr= cur->next;
+    cur->next= NULL;
+    elem->list= cur;
+  }
   unlock();
 
   if (!elist)
@@ -393,7 +424,7 @@ rpl_slave_state::record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
     DBUG_EXECUTE_IF("gtid_slave_pos_simulate_failed_delete",
                     { err= ENOENT;
                       table->file->print_error(err, MYF(0));
-                      /* `break' does not work in DBUG_EXECUTE_IF */
+                      /* `break' does not work inside DBUG_EXECUTE_IF */
                       goto dbug_break; });
 
     next= elist->next;
@@ -420,11 +451,6 @@ rpl_slave_state::record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
 IF_DBUG(dbug_break:, )
   table->file->ha_index_end();
 
-  if(!err && opt_bin_log &&
-     (err= mysql_bin_log.bump_seq_no_counter_if_needed(gtid->domain_id,
-                                                       gtid->seq_no)))
-    my_error(ER_OUT_OF_RESOURCES, MYF(0));
-
 end:
 
   if (table_opened)