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

1 files changed, 27 insertions, 13 deletions

diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index 4f217159e5c..70cab968f43 100644
--- a/sql/sql_show.cc
+++ b/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)
-  {
-    char path[FN_REFLEN+16];
-    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);
-  }
+     (!db_names.elements() /* The database name was too long */||
+      (db_names.at(0) != &INFORMATION_SCHEMA_NAME &&
+       verify_database_directory_exists(lookup_field_vals.db_value))))
+    DBUG_RETURN(0);
 
   for (size_t i=0; i < db_names.elements(); i++)
   {