MDEV-20516: Assertion `!lex->proc_list.first && !lex->result &&...

MDEV-20516: Assertion `!lex->proc_list.first && !lex->result && !lex->param_list.elements' failed in mysql_create_view

Execution of the CREATE VIEW statement sent via binary protocol
where the flags of the COM_STMT_EXECUTE request a cursor to be opened
before running the statement results in an assert failure.

This assert fails since the data member thd->lex->result has not null
value pointing to an instance of the class Select_materialize.
The data member thd->lex->result is assigned a pointer to the class
Select_materialize in the function mysql_open_cursor() that invoked
in case the packet COM_STMT_EXECUTE requests a cursor to be opened.

After thd->lex->result is assigned a pointer to an instance of the
class Select_materialize the function mysql_create_view() is called
(indirectly via the function mysql_execute_statement()) and the assert
fails.

The assert
  DBUG_ASSERT(!lex->proc_list.first && !lex->result &&
              !lex->param_list.elements);

was added by the commit 591c06d4.
Unfortunately , the condition
  !lex->result
was specified incorrect. It was supposed that the thd->lex->result
is set only by parser on handling the clauses SELECT ... INTO
but indeed it is also set inside mysql_open_cursor() and
that fact was missed by the assert's condition.

So, the fix for this issue is to just remove the condition
  !lex->result
from the failing assert.

sql/sql_view.cc

@@ -408,8 +408,18 @@ bool mysql_create_view(THD *thd, TABLE_LIST *views,
   bool res= FALSE;
   DBUG_ENTER("mysql_create_view");
 
-  /* This is ensured in the parser. */
-  DBUG_ASSERT(!lex->proc_list.first && !lex->result &&
+  /*
+    This is ensured in the parser.
+    NOTE: Originally, the assert below contained the extra condition
+      && !lex->result
+    but in this form the assert is failed in case CREATE VIEW run under
+    cursor (the case when the byte 'flags' in the COM_STMT_EXECUTE packet has
+    the flag CURSOR_TYPE_READ_ONLY set). For the cursor use case
+    thd->lex->result is assigned a pointer to the class Select_materialize
+    inside the function mysql_open_cursor() just before handling of a statement
+    will be started and the function mysql_create_view() called.
+  */
+  DBUG_ASSERT(!lex->proc_list.first &&
               !lex->param_list.elements);
 
   /*

tests/mysql_client_test.c

@@ -20013,6 +20013,39 @@ static void test_mdev24827()
   myquery(rc);
 }
 
+static void test_mdev_20516()
+{
+  MYSQL_STMT *stmt;
+  int        rc;
+  unsigned long cursor= CURSOR_TYPE_READ_ONLY;
+
+  myheader("test_mdev_20516");
+
+  rc= mysql_query(mysql, "DROP TABLE IF EXISTS t1");
+  myquery(rc);
+
+  rc= mysql_query(mysql, "CREATE TABLE t1(a INT)");
+  myquery(rc);
+
+  const char* query=
+    "CREATE VIEW v1 AS SELECT * FROM t1";
+
+  stmt= mysql_stmt_init(mysql);
+  check_stmt(stmt);
+
+  rc= mysql_stmt_prepare(stmt, query, strlen(query));
+  check_execute(stmt, rc);
+
+  rc= mysql_stmt_attr_set(stmt, STMT_ATTR_CURSOR_TYPE, &cursor);
+  check_execute(stmt, rc);
+
+  rc= mysql_stmt_execute(stmt);
+  check_execute(stmt, rc);
+
+  rc= mysql_query(mysql, "DROP TABLE t1");
+  myquery(rc);
+}
+
 #ifndef EMBEDDED_LIBRARY
 #define MDEV19838_MAX_PARAM_COUNT 32
 #define MDEV19838_FIELDS_COUNT 17
@@ -20163,6 +20196,7 @@ static void test_mdev19838()
 #endif // EMBEDDED_LIBRARY
 
 static struct my_tests_st my_tests[]= {
+  { "test_mdev_20516", test_mdev_20516 },
   { "test_mdev24827", test_mdev24827 },
   { "test_mdev_26145", test_mdev_26145 },
   { "disable_query_logs", disable_query_logs },