Fix for bug#19403/12212 "Crash that happens during removing of database name

from cache" and #21216 "Simultaneous DROP TABLE and SHOW OPEN TABLES causes
server to crash".

Crash happened when one ran DROP DATABASE or SHOW OPEN TABLES statements
while concurrently doing DROP TABLE (or RENAME TABLE, CREATE TABLE LIKE
or any other command that takes name-lock) in other connection.

This problem was caused by the fact that table placeholders which were
added to table cache in order to obtain name-lock on table had
TABLE_SHARE::db and table_name set to 0. Therefore they broke assumption
that these members are non-0 for all tables in table cache on which some
of our code relies.

The fix sets these members for such placeholders to appropriate value making
this assumption true again. As attempt to avoid such problems in future
we introduce auxiliary TABLE_SHARE::set_table_cache_key() methods which
should be used when one wants to set TABLE_SHARE::table_cache_key and which
ensure that TABLE_SHARE::table_name/db are set properly.

Test cases for these bugs were added to 5.0 test-suite (with 5.0-specific
fix for bug #21216).


sql/lock.cc:
  Our code assumes that TABLE_SHARE::table_name/db for objects in table cache
  is set properly (and is non-NULL). For example look in list_open_tables()
  and remove_db_from_cache(). This was not true for table placeholders that
  were added to table cache for name-locking. Changed lock_table_name() to
  preserve this assumption (now it uses TABLE_SHARE::set_table_cache_key()
  to set all three table_cache_key/db/table_name members at once).
  Also now we use my_multi_malloc() to allocate memory for TABLE and
  TABLE_SHARE objects instead of using my_malloc() + summing sizes as
  it automatically provides proper alignment for memory allocated.
sql/sql_base.cc:
  Now we use TABLE_SHARE::set_table_cache_key() auxiliary methods to set
  TABLE_SHARE::table_cache_key/db/table_name members at once.
  We also use multi_alloc_root() instead of alloc_root() for allocating
  memory for several objects as it is less error prone.
  Finally, we also got rid of unused code in reopen_name_locked_table().
sql/sql_select.cc:
  Got rid of redundant code. TABLE_SHARE::db/table_cache_key are both set to
  empty string by the call to init_tmp_table_share() routine.
sql/table.cc:
  Now alloc_table_share() uses auxiliary TABLE_SHARE::set_table_cache_key()
  method to properly set TABLE_SHARE::table_cache_key/db/table_name members.
  Also now we use multi_alloc_root() instead of alloc_root() for allocating
  memory for several objects as it is more clear/less error-prone.
sql/table.h:
  Added comment about importance of apropriate setting of
  TABLE_SHARE::table_name/db/table_cache_key for tables in table cache.
  Introduced two auxiliary TABLE_SHARE::set_table_cache_key() methods which
  allow to set these three members at once.

sql/lock.cc

@@ -874,6 +874,8 @@ int lock_and_wait_for_table_name(THD *thd, TABLE_LIST *table_list)
 int lock_table_name(THD *thd, TABLE_LIST *table_list, bool check_in_use)
 {
   TABLE *table;
+  TABLE_SHARE *share;
+  char *key_buff;
   char  key[MAX_DBKEY_LENGTH];
   char *db= table_list->db;
   uint  key_length;
@@ -903,17 +905,18 @@ int lock_table_name(THD *thd, TABLE_LIST *table_list, bool check_in_use)
   }
   /*
     Create a table entry with the right key and with an old refresh version
-    Note that we must use my_malloc() here as this is freed by the table
-    cache
+    Note that we must use my_multi_malloc() here as this is freed by the
+    table cache
   */
-  if (!(table= (TABLE*) my_malloc(sizeof(*table)+ sizeof(TABLE_SHARE)+
-                                  key_length, MYF(MY_WME | MY_ZEROFILL))))
+  if (!my_multi_malloc(MYF(MY_WME | MY_ZEROFILL),
+                       &table, sizeof(*table),
+                       &share, sizeof(*share),
+                       &key_buff, key_length,
+                       NULL))
     DBUG_RETURN(-1);
-  table->s= (TABLE_SHARE*) (table+1);
-  memcpy((table->s->table_cache_key.str= (char*) (table->s+1)), key,
-         key_length);
-  table->s->table_cache_key.length= key_length;
-  table->s->tmp_table= INTERNAL_TMP_TABLE;  // for intern_close_table
+  table->s= share;
+  share->set_table_cache_key(key_buff, key, key_length);
+  share->tmp_table= INTERNAL_TMP_TABLE;  // for intern_close_table
   table->in_use= thd;
   table->locked_by_name=1;
   table_list->table=table;

sql/sql_base.cc

@@ -634,6 +634,7 @@ TABLE_SHARE *get_cached_table_share(const char *db, const char *table_name)
 static void close_handle_and_leave_table_as_lock(TABLE *table)
 {
   TABLE_SHARE *share, *old_share= table->s;
+  char *key_buff;
   MEM_ROOT *mem_root= &table->mem_root;
   DBUG_ENTER("close_handle_and_leave_table_as_lock");
 
@@ -642,20 +643,14 @@ static void close_handle_and_leave_table_as_lock(TABLE *table)
     This has to be done to ensure that the table share is removed from
     the table defintion cache as soon as the last instance is removed
   */
-  if ((share= (TABLE_SHARE*) alloc_root(mem_root, sizeof(*share))))
+  if (multi_alloc_root(mem_root,
+                       &share, sizeof(*share),
+                       &key_buff, old_share->table_cache_key.length,
+                       NULL))
   {
     bzero((char*) share, sizeof(*share));
-    share->db.str= memdup_root(mem_root, old_share->db.str,
-                               old_share->db.length+1);
-    share->db.length= old_share->db.length;
-    share->table_name.str= memdup_root(mem_root,
-                                       old_share->table_name.str,
-                                       old_share->table_name.length+1);
-    share->table_name.length= old_share->table_name.length;
-    share->table_cache_key.str= memdup_root(mem_root,
-                                            old_share->table_cache_key.str,
-                                            old_share->table_cache_key.length);
-    share->table_cache_key.length= old_share->table_cache_key.length;
+    share->set_table_cache_key(key_buff, old_share->table_cache_key.str,
+                               old_share->table_cache_key.length);
     share->tmp_table= INTERNAL_TMP_TABLE;       // for intern_close_table()
   }
 
@@ -1603,28 +1598,18 @@ bool rename_temporary_table(THD* thd, TABLE *table, const char *db,
 			    const char *table_name)
 {
   char *key;
+  uint key_length;
   TABLE_SHARE *share= table->s;
   TABLE_LIST table_list;
-  uint db_length, table_length;
   DBUG_ENTER("rename_temporary_table");
 
-  if (!(key=(char*) alloc_root(&share->mem_root,
-			       (uint) (db_length= strlen(db))+
-			       (uint) (table_length= strlen(table_name))+6+4)))
+  if (!(key=(char*) alloc_root(&share->mem_root, MAX_DBKEY_LENGTH)))
     DBUG_RETURN(1);				/* purecov: inspected */
 
   table_list.db= (char*) db;
   table_list.table_name= (char*) table_name;
-  share->db.str= share->table_cache_key.str= key;
-  share->db.length= db_length;
-  share->table_cache_key.length= create_table_def_key(thd, key,
-                                                      &table_list, 1);
-  /*
-    Here we use the fact that table_name is stored as the second component
-    in the 'key' (after db_name), where components are separated with \0
-  */
-  share->table_name.str=    key+db_length+1;
-  share->table_name.length= table_length;
+  key_length= create_table_def_key(thd, key, &table_list, 1);
+  share->set_table_cache_key(key, key_length);
   DBUG_RETURN(0);
 }
 
@@ -1749,10 +1734,7 @@ bool reopen_name_locked_table(THD* thd, TABLE_LIST* table_list)
 {
   TABLE *table= table_list->table;
   TABLE_SHARE *share;
-  char *db= table_list->db;
   char *table_name= table_list->table_name;
-  char key[MAX_DBKEY_LENGTH];
-  uint key_length;
   TABLE orig_table;
   DBUG_ENTER("reopen_name_locked_table");
 
@@ -1762,7 +1744,6 @@ bool reopen_name_locked_table(THD* thd, TABLE_LIST* table_list)
     DBUG_RETURN(TRUE);
 
   orig_table= *table;
-  key_length=(uint) (strmov(strmov(key,db)+1,table_name)-key)+1;
 
   if (open_unireg_entry(thd, table, table_list, table_name,
                         table->s->table_cache_key.str,

sql/sql_select.cc

@@ -8645,8 +8645,6 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
   share->primary_key= MAX_KEY;               // Indicate no primary key
   share->keys_for_keyread.init();
   share->keys_in_use.init();
-  /* For easier error reporting */
-  share->table_cache_key= share->db;
 
   /* Calculate which type of fields we will store in the temporary table */
 

sql/table.cc

@@ -93,6 +93,7 @@ TABLE_SHARE *alloc_table_share(TABLE_LIST *table_list, char *key,
 {
   MEM_ROOT mem_root;
   TABLE_SHARE *share;
+  char *key_buff, *path_buff;
   char path[FN_REFLEN];
   uint path_length;
   DBUG_ENTER("alloc_table_share");
@@ -103,22 +104,17 @@ TABLE_SHARE *alloc_table_share(TABLE_LIST *table_list, char *key,
                                     table_list->db,
                                     table_list->table_name, "", 0);
   init_sql_alloc(&mem_root, TABLE_ALLOC_BLOCK_SIZE, 0);
-  if ((share= (TABLE_SHARE*) alloc_root(&mem_root,
-					sizeof(*share) + key_length +
-                                        path_length +1)))
+  if (multi_alloc_root(&mem_root,
+                       &share, sizeof(*share),
+                       &key_buff, key_length,
+                       &path_buff, path_length + 1,
+                       NULL))
   {
     bzero((char*) share, sizeof(*share));
-    share->table_cache_key.str=    (char*) (share+1);
-    share->table_cache_key.length= key_length;
-    memcpy(share->table_cache_key.str, key, key_length);
 
-    /* Use the fact the key is db/0/table_name/0 */
-    share->db.str=            share->table_cache_key.str;
-    share->db.length=         strlen(share->db.str);
-    share->table_name.str=    share->db.str + share->db.length + 1;
-    share->table_name.length= strlen(share->table_name.str);
+    share->set_table_cache_key(key_buff, key, key_length);
 
-    share->path.str= share->table_cache_key.str+ key_length;
+    share->path.str= path_buff;
     share->path.length= path_length;
     strmov(share->path.str, path);
     share->normalized_path.str=    share->path.str;

sql/table.h

@@ -138,7 +138,16 @@ typedef struct st_table_share
   CHARSET_INFO *table_charset;		/* Default charset of string fields */
 
   MY_BITMAP all_set;
-  /* A pair "database_name\0table_name\0", widely used as simply a db name */
+  /*
+    Key which is used for looking-up table in table cache and in the list
+    of thread's temporary tables. Has the form of:
+      "database_name\0table_name\0" + optional part for temporary tables.
+
+    Note that all three 'table_cache_key', 'db' and 'table_name' members
+    must be set (and be non-zero) for tables in table cache. They also
+    should correspond to each other.
+    To ensure this one can use set_table_cache() methods.
+  */
   LEX_STRING table_cache_key;
   LEX_STRING db;                        /* Pointer to db */
   LEX_STRING table_name;                /* Table name (for open) */
@@ -223,6 +232,60 @@ typedef struct st_table_share
   uint part_state_len;
   handlerton *default_part_db_type;
 #endif
+
+
+  /*
+    Set share's table cache key and update its db and table name appropriately.
+
+    SYNOPSIS
+      set_table_cache_key()
+        key_buff    Buffer with already built table cache key to be
+                    referenced from share.
+        key_length  Key length.
+
+    NOTES
+      Since 'key_buff' buffer will be referenced from share it should has same
+      life-time as share itself.
+      This method automatically ensures that TABLE_SHARE::table_name/db have
+      appropriate values by using table cache key as their source.
+  */
+
+  void set_table_cache_key(char *key_buff, uint key_length)
+  {
+    table_cache_key.str= key_buff;
+    table_cache_key.length= key_length;
+    /*
+      Let us use the fact that the key is "db/0/table_name/0" + optional
+      part for temporary tables.
+    */
+    db.str=            table_cache_key.str;
+    db.length=         strlen(db.str);
+    table_name.str=    db.str + db.length + 1;
+    table_name.length= strlen(table_name.str);
+  }
+
+
+  /*
+    Set share's table cache key and update its db and table name appropriately.
+
+    SYNOPSIS
+      set_table_cache_key()
+        key_buff    Buffer to be used as storage for table cache key
+                    (should be at least key_length bytes).
+        key         Value for table cache key.
+        key_length  Key length.
+
+    NOTE
+      Since 'key_buff' buffer will be used as storage for table cache key
+      it should has same life-time as share itself.
+  */
+
+  void set_table_cache_key(char *key_buff, const char *key, uint key_length)
+  {
+    memcpy(key_buff, key, key_length);
+    set_table_cache_key(key_buff, key_length);
+  }
+
 } TABLE_SHARE;