mi_cache.cc

/* Copyright (c) 2000, 2025, Oracle and/or its affiliates.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
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   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.

   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */

/*
  Functions for read record caching with myisam
  Used for reading dynamic/compressed records from datafile.

  Can fetch data directly from file (outside cache),
  if reading a small chunk straight before the cached part (with possible
  overlap).

  Can be explicitly asked not to use cache (by not setting READING_NEXT in
  flag) - useful for occasional out-of-cache reads, when the next read is
  expected to hit the cache again.

  Allows "partial read" errors in the record header (when READING_HEADER flag
  is set) - unread part is zerofilled.

  Note: out-of-cache reads are enabled for shared IO_CACHE's too,
  as these reads will be cached by OS cache (and mysql_file_pread is always
  atomic)
*/

#include <sys/types.h>

#include <algorithm>

#include "my_dbug.h"
#include "my_inttypes.h"
#include "my_macros.h"
#include "storage/myisam/myisamdef.h"

int _mi_read_cache(IO_CACHE *info, uchar *buff, my_off_t pos, uint length,
                   int flag) {
  uint read_length, in_buff_length;
  my_off_t offset;
  uchar *in_buff_pos;
  DBUG_TRACE;

  if (pos < info->pos_in_file) {
    read_length = length;
    if ((my_off_t)read_length > (my_off_t)(info->pos_in_file - pos))
      read_length = (uint)(info->pos_in_file - pos);
    info->seek_not_done = true;
    if (mysql_file_pread(info->file, buff, read_length, pos, MYF(MY_NABP)))
      return 1;
    if (!(length -= read_length)) return 0;
    pos += read_length;
    buff += read_length;
  }
  if (pos >= info->pos_in_file &&
      (offset = (my_off_t)(pos - info->pos_in_file)) <
          (my_off_t)(info->read_end - info->request_pos)) {
    in_buff_pos = info->request_pos + (uint)offset;
    in_buff_length = std::min<size_t>(length, (info->read_end - in_buff_pos));
    memcpy(buff, info->request_pos + (uint)offset, (size_t)in_buff_length);
    if (!(length -= in_buff_length)) return 0;
    pos += in_buff_length;
    buff += in_buff_length;
  } else
    in_buff_length = 0;
  if (flag & READING_NEXT) {
    if (pos !=
        (info->pos_in_file + (uint)(info->read_end - info->request_pos))) {
      info->pos_in_file = pos; /* Force start here */
      info->read_pos = info->read_end = info->request_pos; /* Everything used */
      info->seek_not_done = true;
    } else
      info->read_pos = info->read_end; /* All block used */
    if (!(*info->read_function)(info, buff, length)) return 0;
    read_length = info->error;
  } else {
    info->seek_not_done = true;
    if ((read_length =
             mysql_file_pread(info->file, buff, length, pos, MYF(0))) == length)
      return 0;
  }
  if (!(flag & READING_HEADER) || (int)read_length == -1 ||
      read_length + in_buff_length < 3) {
    DBUG_PRINT("error",
               ("Error %d reading next-multi-part block (Got %d bytes)",
                my_errno(), (int)read_length));
    if (!my_errno() || my_errno() == -1) set_my_errno(HA_ERR_WRONG_IN_RECORD);
    return 1;
  }
  memset(buff + read_length, 0,
         MI_BLOCK_INFO_HEADER_LENGTH - in_buff_length - read_length);
  return 0;
} /* _mi_read_cache */