BUG#25712 - insert delayed and check table run together report crashed

            tables

In case system doesn't have native pread/pwrite calls (e.g. Windows)
and there is CHECK TABLE runs concurrently with another statement that
reads from a table, the table may be reported as crashed.

This is fixed by locking file descriptor when my_seek is executed on
MyISAM index file and emulated pread/pwrite may be executed concurrently.

Affects MyISAM tables on platforms that do not have native
pread/pwrite calls (e.g. Windows).

No deterministic test case for this bug.

myisam/mi_check.c

@@ -336,7 +336,7 @@ int chk_size(MI_CHECK *param, register MI_INFO *info)
   flush_key_blocks(info->s->key_cache,
 		   info->s->kfile, FLUSH_FORCE_WRITE);
 
-  size=my_seek(info->s->kfile,0L,MY_SEEK_END,MYF(0));
+  size= my_seek(info->s->kfile, 0L, MY_SEEK_END, MYF(MY_THREADSAFE));
   if ((skr=(my_off_t) info->state->key_file_length) != size)
   {
     if (skr > size)
@@ -595,7 +595,8 @@ static int chk_index_down(MI_CHECK *param, MI_INFO *info, MI_KEYDEF *keyinfo,
   {
     /* purecov: begin tested */
     /* Give it a chance to fit in the real file size. */
-    my_off_t max_length= my_seek(info->s->kfile, 0L, MY_SEEK_END, MYF(0));
+    my_off_t max_length= my_seek(info->s->kfile, 0L, MY_SEEK_END,
+                                 MYF(MY_THREADSAFE));
     mi_check_print_error(param, "Invalid key block position: %s  "
                          "key block size: %u  file_length: %s",
                          llstr(page, llbuff), keyinfo->block_length,
@@ -4039,10 +4040,10 @@ int test_if_almost_full(MI_INFO *info)
 {
   if (info->s->options & HA_OPTION_COMPRESS_RECORD)
     return 0;
-  return (my_seek(info->s->kfile,0L,MY_SEEK_END,MYF(0))/10*9 >
-	  (my_off_t) (info->s->base.max_key_file_length) ||
-	  my_seek(info->dfile,0L,MY_SEEK_END,MYF(0))/10*9 >
-	  (my_off_t) info->s->base.max_data_file_length);
+  return my_seek(info->s->kfile, 0L, MY_SEEK_END, MYF(MY_THREADSAFE)) / 10 * 9 >
+         (my_off_t) info->s->base.max_key_file_length ||
+         my_seek(info->dfile, 0L, MY_SEEK_END, MYF(0)) / 10 * 9 >
+         (my_off_t) info->s->base.max_data_file_length;
 }
 
 	/* Recreate table with bigger more alloced record-data */

mysys/my_seek.c

@@ -24,7 +24,9 @@
   my_off_t pos     The expected position (absolute or relative)
   int whence       A direction parameter and one of 
                    {SEEK_SET, SEEK_CUR, SEEK_END}
-  myf MyFlags      Not used.
+  myf MyFlags      MY_THREADSAFE must be set in case my_seek may be mixed
+                   with my_pread/my_pwrite calls and fd is shared among
+                   threads.
 
   DESCRIPTION
     The my_seek  function  is a wrapper around the system call lseek and
@@ -51,7 +53,16 @@ my_off_t my_seek(File fd, my_off_t pos, int whence,
 		   whence, MyFlags));
   DBUG_ASSERT(pos != MY_FILEPOS_ERROR);		/* safety check */
 
-  newpos=lseek(fd, pos, whence);
+#if defined(THREAD) && !defined(HAVE_PREAD)
+  if (MyFlags & MY_THREADSAFE)
+  {
+    pthread_mutex_lock(&my_file_info[fd].mutex);
+    newpos= lseek(fd, pos, whence);
+    pthread_mutex_lock(&my_file_info[fd].mutex);
+  }
+  else
+#endif
+    newpos= lseek(fd, pos, whence);
   if (newpos == (os_off_t) -1)
   {
     my_errno=errno;