memroot improvement: fix savepoint support

savepoint support was added a year ago as get_last_memroot_block()
and free_all_new_blocks(), but it didn't work and was disabled.

* fix it to work
* instead of freeing the memory, only mark blocks free - this feature
  is supposed to be used inside a loop (otherwise there is no need
  to free anything, end of statement will do it anyway). And freeing
  blocks inside a loop is a bad idea, as they'll be all malloc-ed
  on the next iteration again. So, don't.
* fix a bug in mark_blocks_free() - it doesn't change the number of blocks

include/my_alloc.h

@@ -60,6 +60,14 @@ typedef struct st_mem_root
   PSI_memory_key psi_key;
 } MEM_ROOT;
 
+typedef struct st_mem_root_savepoint
+{
+  MEM_ROOT *root;
+  USED_MEM *free;
+  USED_MEM *used;
+  unsigned short first_block_usage;
+} MEM_ROOT_SAVEPOINT;
+
 #ifdef  __cplusplus
 }
 #endif

include/my_sys.h

@@ -921,8 +921,8 @@ extern void move_root(MEM_ROOT *to, MEM_ROOT *from);
 extern void set_prealloc_root(MEM_ROOT *root, char *ptr);
 extern void reset_root_defaults(MEM_ROOT *mem_root, size_t block_size,
                                 size_t prealloc_size);
-extern USED_MEM *get_last_memroot_block(MEM_ROOT* root);
-extern void free_all_new_blocks(MEM_ROOT *root, USED_MEM *last_block);
+extern void root_make_savepoint(MEM_ROOT *root, MEM_ROOT_SAVEPOINT *sv);
+extern void root_free_to_savepoint(const MEM_ROOT_SAVEPOINT *sv);
 extern void protect_root(MEM_ROOT *root, int prot);
 extern char *strdup_root(MEM_ROOT *root,const char *str);
 static inline char *safe_strdup_root(MEM_ROOT *root, const char *str)

mysys/my_alloc.c

@@ -266,8 +266,8 @@ void *alloc_root(MEM_ROOT *mem_root, size_t length)
 {
   size_t get_size, block_size;
   uchar* point;
-  reg1 USED_MEM *next= 0;
-  reg2 USED_MEM **prev;
+  USED_MEM *next= 0;
+  USED_MEM **prev;
   size_t original_length __attribute__((unused)) = length;
   DBUG_ENTER("alloc_root");
   DBUG_PRINT("enter",("root: %p", mem_root));
@@ -425,10 +425,10 @@ void *multi_alloc_root(MEM_ROOT *root, ...)
 #if !(defined(HAVE_valgrind) && defined(EXTRA_DEBUG))
 /** Mark all data in blocks free for reusage */
 
-static inline void mark_blocks_free(MEM_ROOT* root)
+static void mark_blocks_free(MEM_ROOT* root)
 {
-  reg1 USED_MEM *next;
-  reg2 USED_MEM **last;
+  USED_MEM *next;
+  USED_MEM **last;
 
   /* iterate through (partially) free blocks, mark them free */
   last= &root->free;
@@ -451,7 +451,6 @@ static inline void mark_blocks_free(MEM_ROOT* root)
   /* Now everything is set; Indicate that nothing is used anymore */
   root->used= 0;
   root->first_block_usage= 0;
-  root->block_num= 4;
 }
 #endif
 
@@ -477,7 +476,7 @@ static inline void mark_blocks_free(MEM_ROOT* root)
 
 void free_root(MEM_ROOT *root, myf MyFlags)
 {
-  reg1 USED_MEM *next,*old;
+  USED_MEM *next,*old;
   DBUG_ENTER("free_root");
   DBUG_PRINT("enter",("root: %p  flags: %lu", root, MyFlags));
 
@@ -567,45 +566,61 @@ void move_root(MEM_ROOT *to, MEM_ROOT *from)
   from->used= 0;
 }
 
-
-
 /*
-  Remember last MEM_ROOT block.
+  Prepare MEM_ROOT to a later truncation. Everything allocated after
+  that point can be freed while keeping earlier allocations intact.
 
-  This allows one to free all new allocated blocks.
+  For this to work we cannot allow new allocations in partially filled blocks,
+  so remove all non-empty blocks from the memroot. For simplicity, let's
+  also remove all used blocks.
 */
-
-USED_MEM *get_last_memroot_block(MEM_ROOT* root)
+void root_make_savepoint(MEM_ROOT *root, MEM_ROOT_SAVEPOINT *sv)
 {
-  return root->used ? root->used : root->pre_alloc;
+  USED_MEM **prev= &root->free, *block= *prev;
+  for ( ; block; prev= &block->next, block= *prev)
+    if (block->left < block->size - ALIGN_SIZE(sizeof(USED_MEM)))
+      break;
+  sv->root= root;
+  sv->free= block;
+  sv->used= root->used;
+  sv->first_block_usage= root->first_block_usage;
+  *prev= 0;
+  root->used= 0;
 }
 
 /*
-  Free all newly allocated blocks
-*/
+  Restore MEM_ROOT to the state before the savepoint was made.
 
-void free_all_new_blocks(MEM_ROOT *root, USED_MEM *last_block)
-{
-  USED_MEM *old, *next;
-  if (!root->used)
-    return;                                     /* Nothing allocated */
-  return;
-  /*
-    Free everying allocated up to, but not including, last_block.
-    However do not go past pre_alloc as we do not want to free
-    that one. This should not be a problem as in almost all normal
-    usage pre_alloc is last in the list.
+  Restore old free and used lists.
+  Mark all new (after savepoint) used and partially used blocks free
+  and put them into the free list.
 */
-
-  for (next= root->used ;
-       next && next != last_block && next != root->pre_alloc ; )
+void root_free_to_savepoint(const MEM_ROOT_SAVEPOINT *sv)
 {
-    old= next; next= next->next;
-    root_free(root, old, old->size);
+  MEM_ROOT *root= sv->root;
+  USED_MEM **prev= &root->free, *block= *prev;
+
+  /* iterate through (partially) free blocks, mark them free */
+  for ( ; block; prev= &block->next, block= *prev)
+  {
+    block->left= block->size - ALIGN_SIZE(sizeof(USED_MEM));
+    TRASH_MEM(block);
   }
-  root->used= next;
-  root->block_num= 4;
-  root->first_block_usage= 0;
+
+  /* Combine the free and the used list */
+  *prev= block=root->used;
+
+  /* now go through the used blocks and mark them free */
+  for ( ; block; prev= &block->next, block= *prev)
+  {
+    block->left= block->size - ALIGN_SIZE(sizeof(USED_MEM));
+    TRASH_MEM(block);
+  }
+
+  /* restore free and used lists from savepoint */
+  *prev= sv->free;
+  root->used= sv->used;
+  root->first_block_usage= prev == &root->free ? sv->first_block_usage : 0;
 }
 
 /**
@@ -615,7 +630,7 @@ void free_all_new_blocks(MEM_ROOT *root, USED_MEM *last_block)
 #if defined(HAVE_MMAP) && defined(HAVE_MPROTECT) && defined(MAP_ANONYMOUS)
 void protect_root(MEM_ROOT *root, int prot)
 {
-  reg1 USED_MEM *next,*old;
+  USED_MEM *next,*old;
   DBUG_ENTER("protect_root");
   DBUG_PRINT("enter",("root: %p  prot: %d", root, prot));
 

sql/sql_admin.cc

@@ -982,7 +982,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
       {
         TABLE *tab= table->table;
         Field **field_ptr= tab->field;
-        USED_MEM *memroot_block;
+        MEM_ROOT_SAVEPOINT memroot_sv;
 
         if (!lex->column_list)
         {
@@ -1084,7 +1084,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
         }
         /* Ensure that number of records are updated */
         tab->file->info(HA_STATUS_VARIABLE);
-        memroot_block= get_last_memroot_block(thd->mem_root);
+        root_make_savepoint(thd->mem_root, &memroot_sv);
         if (!(compl_result_code=
               alloc_statistics_for_table(thd, tab,
                                          &tab->has_value_set)) &&
@@ -1092,7 +1092,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
               collect_statistics_for_table(thd, tab)))
           compl_result_code= update_statistics_for_table(thd, tab);
         free_statistics_for_table(tab);
-        free_all_new_blocks(thd->mem_root, memroot_block);
+        root_free_to_savepoint(&memroot_sv);
       }
       else
         compl_result_code= HA_ADMIN_FAILED;