MDEV-26772 InnoDB DDL fails with DUPLICATE KEY error

ha_innobase::delete_table(): When the table that is being dropped
has a name starting with #sql, temporarily set
innodb_lock_wait_timeout=0 while attempting to lock the
persistent statistics tables. If the statistics tables cannot be locked,
pretend that statistics did not exist and carry on with dropping
the table. The SQL layer is not really prepared for failures of
this operation. This is what fixes the test case.

ha_innobase::rename_table(): When renaming a table from a name
that starts with #sql, try to lock the statistics tables with an
immediate timeout, and ignore the statistics if the locks were
not available. In fact, during any rename from a #sql name,
dict_stats_rename_table() should have no effect, because already
when an earlier rename to a #sql name took place we should have
deleted the statistics for the table using the non-#sql name.
This change is just analogous to the ha_innobase::delete_table().

mysql-test/suite/innodb/r/innodb-alter-debug.result

@@ -107,3 +107,29 @@ ALTER TABLE t RENAME INDEX i2 to x, ALGORITHM=INPLACE;
 ERROR 40001: Deadlock found when trying to get lock; try restarting transaction
 SET DEBUG_DBUG = @saved_debug_dbug;
 DROP TABLE t;
+#
+# MDEV-26772 InnoDB DDL fails with DUPLICATE KEY error
+#
+create table t1(f1 int not null primary key,
+f2 int not null, index idx(f2))engine=innodb;
+insert into t1 values(1, 1);
+connect con1,localhost,root,,,;
+SET DEBUG_SYNC='before_delete_table_stats SIGNAL blocked WAIT_FOR go';
+SET innodb_lock_wait_timeout=0;
+ALTER TABLE t1 FORCE, ALGORITHM=COPY;
+connection default;
+SET DEBUG_SYNC='now WAIT_FOR blocked';
+BEGIN;
+SELECT * FROM mysql.innodb_table_stats FOR UPDATE;
+database_name	table_name	last_update	n_rows	clustered_index_size	sum_of_other_index_sizes
+SET DEBUG_SYNC='now SIGNAL go';
+connection con1;
+connection default;
+COMMIT;
+SET DEBUG_SYNC=RESET;
+connection con1;
+ALTER TABLE t1 RENAME KEY idx TO idx1, ALGORITHM=COPY;
+disconnect con1;
+connection default;
+DROP TABLE t1;
+# End of 10.6 tests

mysql-test/suite/innodb/t/innodb-alter-debug.test

@@ -142,5 +142,42 @@ SET DEBUG_DBUG = @saved_debug_dbug;
 
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-26772 InnoDB DDL fails with DUPLICATE KEY error
+--echo #
+
+create table t1(f1 int not null primary key,
+
+                f2 int not null, index idx(f2))engine=innodb;
+
+insert into t1 values(1, 1);
+
+connect(con1,localhost,root,,,);
+SET DEBUG_SYNC='before_delete_table_stats SIGNAL blocked WAIT_FOR go';
+SET innodb_lock_wait_timeout=0;
+send ALTER TABLE t1 FORCE, ALGORITHM=COPY;
+
+connection default;
+SET DEBUG_SYNC='now WAIT_FOR blocked';
+BEGIN;
+SELECT * FROM mysql.innodb_table_stats FOR UPDATE;
+SET DEBUG_SYNC='now SIGNAL go';
+
+connection con1;
+reap;
+
+connection default;
+COMMIT;
+SET DEBUG_SYNC=RESET;
+
+connection con1;
+ALTER TABLE t1 RENAME KEY idx TO idx1, ALGORITHM=COPY;
+disconnect con1;
+
+connection default;
+DROP TABLE t1;
+
+--echo # End of 10.6 tests
+
 # Wait till all disconnects are completed
 --source include/wait_until_count_sessions.inc

storage/innobase/handler/ha_innodb.cc

@@ -13473,6 +13473,8 @@ int ha_innobase::delete_table(const char *name)
   }
 #endif
 
+  DEBUG_SYNC(thd, "before_delete_table_stats");
+
   if (err == DB_SUCCESS && dict_stats_is_persistent_enabled(table) &&
       !table->is_stats_table())
   {
@@ -13496,11 +13498,29 @@ int ha_innobase::delete_table(const char *name)
       dict_sys.unfreeze();
     }
 
+    auto &timeout= THDVAR(thd, lock_wait_timeout);
+    const auto save_timeout= timeout;
+    if (table->name.is_temporary())
+      timeout= 0;
+
     if (table_stats && index_stats &&
         !strcmp(table_stats->name.m_name, TABLE_STATS_NAME) &&
         !strcmp(index_stats->name.m_name, INDEX_STATS_NAME) &&
         !(err= lock_table_for_trx(table_stats, trx, LOCK_X)))
       err= lock_table_for_trx(index_stats, trx, LOCK_X);
+
+    if (err != DB_SUCCESS && !timeout)
+    {
+      /* We may skip deleting statistics if we cannot lock the tables,
+      when the table carries a temporary name. */
+      err= DB_SUCCESS;
+      dict_table_close(table_stats, false, thd, mdl_table);
+      dict_table_close(index_stats, false, thd, mdl_index);
+      table_stats= nullptr;
+      index_stats= nullptr;
+    }
+
+    timeout= save_timeout;
   }
 
   if (err == DB_SUCCESS)
@@ -13959,8 +13979,9 @@ ha_innobase::rename_table(
 	normalize_table_name(norm_to, to);
 
 	dberr_t error = DB_SUCCESS;
+	const bool from_temp = dict_table_t::is_temporary_name(norm_from);
 
-	if (dict_table_t::is_temporary_name(norm_from)) {
+	if (from_temp) {
 		/* There is no need to lock any FOREIGN KEY child tables. */
 	} else if (dict_table_t *table = dict_table_open_on_name(
 		    norm_from, false, DICT_ERR_IGNORE_FK_NOKEY)) {
@@ -14003,9 +14024,31 @@ ha_innobase::rename_table(
 
 		if (error == DB_SUCCESS && table_stats && index_stats
 		    && !strcmp(table_stats->name.m_name, TABLE_STATS_NAME)
-		    && !strcmp(index_stats->name.m_name, INDEX_STATS_NAME) &&
-		    !(error = lock_table_for_trx(table_stats, trx, LOCK_X))) {
-			error = lock_table_for_trx(index_stats, trx, LOCK_X);
+		    && !strcmp(index_stats->name.m_name, INDEX_STATS_NAME)) {
+			auto &timeout = THDVAR(thd, lock_wait_timeout);
+			const auto save_timeout = timeout;
+			if (from_temp) {
+				timeout = 0;
+			}
+			error = lock_table_for_trx(table_stats, trx, LOCK_X);
+			if (error == DB_SUCCESS) {
+				error = lock_table_for_trx(index_stats, trx,
+							   LOCK_X);
+			}
+			if (error != DB_SUCCESS && from_temp) {
+				error = DB_SUCCESS;
+				/* We may skip renaming statistics if
+				we cannot lock the tables, when the
+				table is being renamed from from a
+				temporary name. */
+				dict_table_close(table_stats, false, thd,
+						 mdl_table);
+				dict_table_close(index_stats, false, thd,
+						 mdl_index);
+				table_stats = nullptr;
+				index_stats = nullptr;
+			}
+			timeout = save_timeout;
 		}
 	}