MDEV-22941 Assertion `idx < array.elements' failed in Dynamic_array<st_mysql_const_lex_string*>::at

The code in fill_schema_schemata() did not take into account that
make_db_list() can leave empty db_names if the requested database
name was too long, so the call for db_names.at(0) crashed on assert.

- Moving the code testing if the database directory exists
  into a separate function verify_database_directory_exists()
- Modifying the test to check if db_names is not empty

mysql-test/r/information_schema.result

@@ -2101,3 +2101,13 @@ column_name
 c1
 c2
 DROP TABLE tt1, tt2;
+#
+# MDEV-13242 Wrong results for queries with row constructors and information_schema
+#
+SELECT SCHEMA_NAME from information_schema.schemata where schema_name='aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
+SCHEMA_NAME
+SELECT SCHEMA_NAME from information_schema.schemata where schema_name=REPEAT('a',193);
+SCHEMA_NAME
+#
+# End of 10.1 tests
+#

mysql-test/t/information_schema.test

@@ -1936,3 +1936,15 @@ SELECT count(*) FROM information_schema.columns WHERE table_schema='test' AND (t
 SELECT count(*) FROM information_schema.columns WHERE table_schema='test' AND (table_name='tt1' AND column_name='c1') OR (table_name='tt2' AND column_name='c2');
 SELECT column_name FROM information_schema.columns WHERE (table_name, column_name) IN (('tt1','c1'),('tt2', 'c2')) ORDER BY column_name;
 DROP TABLE tt1, tt2;
+
+--echo #
+--echo # MDEV-13242 Wrong results for queries with row constructors and information_schema
+--echo #
+
+SELECT SCHEMA_NAME from information_schema.schemata where schema_name='aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
+SELECT SCHEMA_NAME from information_schema.schemata where schema_name=REPEAT('a',193);
+
+
+--echo #
+--echo # End of 10.1 tests
+--echo #

sql/sql_show.cc

@@ -4953,6 +4953,29 @@ bool store_schema_shemata(THD* thd, TABLE *table, LEX_STRING *db_name,
 }
 
 
+/*
+  Check if the specified database exists on disk.
+
+  @param dbname - the database name
+  @retval true  - on error, the database directory does not exists
+  @retval false - on success, the database directory exists
+*/
+static bool verify_database_directory_exists(const LEX_STRING &dbname)
+{
+  DBUG_ENTER("verity_database_exists");
+  char path[FN_REFLEN + 16];
+  uint path_len;
+  MY_STAT stat_info;
+  if (!dbname.str[0])
+    DBUG_RETURN(true); // Empty database name: does not exits.
+  path_len= build_table_filename(path, sizeof(path) - 1, dbname.str, "", "", 0);
+  path[path_len - 1]= 0;
+  if (!mysql_file_stat(key_file_misc, path, &stat_info, MYF(0)))
+    DBUG_RETURN(true); // The database directory was not found: does not exists.
+  DBUG_RETURN(false);  // The database directory was found.
+}
+
+
 int fill_schema_schemata(THD *thd, TABLE_LIST *tables, COND *cond)
 {
   /*
@@ -4981,19 +5004,10 @@ int fill_schema_schemata(THD *thd, TABLE_LIST *tables, COND *cond)
     If we have lookup db value we should check that the database exists
   */
   if(lookup_field_vals.db_value.str && !lookup_field_vals.wild_db_value &&
-     db_names.at(0) != &INFORMATION_SCHEMA_NAME)
+     (!db_names.elements() /* The database name was too long */||
-  {
+      (db_names.at(0) != &INFORMATION_SCHEMA_NAME &&
-    char path[FN_REFLEN+16];
+       verify_database_directory_exists(lookup_field_vals.db_value))))
-    uint path_len;
-    MY_STAT stat_info;
-    if (!lookup_field_vals.db_value.str[0])
     DBUG_RETURN(0);
-    path_len= build_table_filename(path, sizeof(path) - 1,
-                                   lookup_field_vals.db_value.str, "", "", 0);
-    path[path_len-1]= 0;
-    if (!mysql_file_stat(key_file_misc, path, &stat_info, MYF(0)))
-      DBUG_RETURN(0);
-  }
 
   for (size_t i=0; i < db_names.elements(); i++)
   {