Function key_cache_read

mysys/mf_keycache.c:2498–2709

*/

uchar *key_cache_read(KEY_CACHE *keycache,
                      File file, my_off_t filepos, int level,
                      uchar *buff, uint length,
                      uint block_length __attribute__((unused)),
                      int return_buffer __attribute__((unused)))
{
  my_bool locked_and_incremented= FALSE;
  int error=0;
  uchar *start= buff;
  DBUG_ENTER("key_cache_read");
  DBUG_PRINT("enter", ("fd: %u  pos: %lu  length: %u",
               (uint) file, (ulong) filepos, length));

  if (keycache->key_cache_inited)
  {
    /* Key cache is used */
    reg1 BLOCK_LINK *block;
    uint read_length;
    uint offset;
    int page_st;

    if (MYSQL_KEYCACHE_READ_START_ENABLED())
    {
      MYSQL_KEYCACHE_READ_START(my_filename(file), length,
                                (ulong) (keycache->blocks_used *
                                         keycache->key_cache_block_size),
                                (ulong) (keycache->blocks_unused *
                                         keycache->key_cache_block_size));
    }
  
    /*
      When the key cache is once initialized, we use the cache_lock to
      reliably distinguish the cases of normal operation, resizing, and
      disabled cache. We always increment and decrement
      'cnt_for_resize_op' so that a resizer can wait for pending I/O.
    */
    keycache_pthread_mutex_lock(&keycache->cache_lock);
    /*
      Cache resizing has two phases: Flushing and re-initializing. In
      the flush phase read requests are allowed to bypass the cache for
      blocks not in the cache. find_key_block() returns NULL in this
      case.

      After the flush phase new I/O requests must wait until the
      re-initialization is done. The re-initialization can be done only
      if no I/O request is in progress. The reason is that
      key_cache_block_size can change. With enabled cache, I/O is done
      in chunks of key_cache_block_size. Every chunk tries to use a
      cache block first. If the block size changes in the middle, a
      block could be missed and old data could be read.
    */
    while (keycache->in_resize && !keycache->resize_in_flush)
      wait_on_queue(&keycache->resize_queue, &keycache->cache_lock);
    /* Register the I/O for the next resize. */
    inc_counter_for_resize_op(keycache);
    locked_and_incremented= TRUE;
    /* Requested data may not always be aligned to cache blocks. */
    offset= (uint) (filepos % keycache->key_cache_block_size);