Bug#11527 - Setting myisam_repair_threads to >1 leads to corruption

A wrong cast led to numeric overflow for data files
greater than 4GB. The parallel repair assumed end of
file after reading the amount of data that the file
was bigger than 4GB. It truncated the data file and
noted the number of records it found so far in the
index file header as the number of rows in the table.
Removing the cast fixed the problem.
I added some cosmetic changes too.

The normal repair worked because it uses a different
function to read from the data file.

mysys/mf_iocache.c

@@ -397,11 +397,31 @@ my_bool reinit_io_cache(IO_CACHE *info, enum cache_type type,
 
 
 /*
-  Read buffered. Returns 1 if can't read requested characters
-  This function is only called from the my_b_read() macro
-  when there isn't enough characters in the buffer to
-  satisfy the request.
-  Returns 0 we succeeded in reading all data
+  Read buffered.
+
+  SYNOPSIS
+    _my_b_read()
+      info                      IO_CACHE pointer
+      Buffer                    Buffer to retrieve count bytes from file
+      Count                     Number of bytes to read into Buffer
+
+  NOTE
+    This function is only called from the my_b_read() macro when there
+    isn't enough characters in the buffer to satisfy the request.
+
+  WARNING
+
+    When changing this function, be careful with handling file offsets
+    (end-of_file, pos_in_file). Do not cast them to possibly smaller
+    types than my_off_t unless you can be sure that their value fits.
+    Same applies to differences of file offsets.
+
+    When changing this function, check _my_b_read_r(). It might need the
+    same change.
+
+  RETURN
+    0      we succeeded in reading all data
+    1      Error: can't read requested characters
 */
 
 int _my_b_read(register IO_CACHE *info, byte *Buffer, uint Count)
@@ -429,7 +449,7 @@ int _my_b_read(register IO_CACHE *info, byte *Buffer, uint Count)
   if (Count >= (uint) (IO_SIZE+(IO_SIZE-diff_length)))
   {					/* Fill first intern buffer */
     uint read_length;
-    if (info->end_of_file == pos_in_file)
+    if (info->end_of_file <= pos_in_file)
     {					/* End of file */
       info->error=(int) left_length;
       DBUG_RETURN(1);
@@ -544,43 +564,70 @@ static void unlock_io_cache(IO_CACHE *info)
 
 /*
   Read from IO_CACHE when it is shared between several threads.
-  It works as follows: when a thread tries to read from a file
-  (that is, after using all the data from the (shared) buffer),
-  it just hangs on lock_io_cache(), wating for other threads.
-  When the very last thread attempts a read, lock_io_cache()
-  returns 1, the thread does actual IO and unlock_io_cache(),
-  which signals all the waiting threads that data is in the buffer.
+
+  SYNOPSIS
+    _my_b_read_r()
+      info                      IO_CACHE pointer
+      Buffer                    Buffer to retrieve count bytes from file
+      Count                     Number of bytes to read into Buffer
+
+  NOTE
+    This function is only called from the my_b_read() macro when there
+    isn't enough characters in the buffer to satisfy the request.
+
+  IMPLEMENTATION
+
+    It works as follows: when a thread tries to read from a file (that
+    is, after using all the data from the (shared) buffer), it just
+    hangs on lock_io_cache(), wating for other threads. When the very
+    last thread attempts a read, lock_io_cache() returns 1, the thread
+    does actual IO and unlock_io_cache(), which signals all the waiting
+    threads that data is in the buffer.
+
+  WARNING
+
+    When changing this function, be careful with handling file offsets
+    (end-of_file, pos_in_file). Do not cast them to possibly smaller
+    types than my_off_t unless you can be sure that their value fits.
+    Same applies to differences of file offsets. (Bug #11527)
+
+    When changing this function, check _my_b_read(). It might need the
+    same change.
+
+  RETURN
+    0      we succeeded in reading all data
+    1      Error: can't read requested characters
 */
 
 int _my_b_read_r(register IO_CACHE *info, byte *Buffer, uint Count)
 {
   my_off_t pos_in_file;
-  uint length,diff_length,read_len;
+  uint length, diff_length, left_length;
   DBUG_ENTER("_my_b_read_r");
 
-  if ((read_len=(uint) (info->read_end-info->read_pos)))
+  if ((left_length= (uint) (info->read_end - info->read_pos)))
   {
-    DBUG_ASSERT(Count >= read_len);	/* User is not using my_b_read() */
-    memcpy(Buffer,info->read_pos, (size_t) (read_len));
-    Buffer+=read_len;
-    Count-=read_len;
+    DBUG_ASSERT(Count >= left_length);	/* User is not using my_b_read() */
+    memcpy(Buffer, info->read_pos, (size_t) (left_length));
+    Buffer+= left_length;
+    Count-= left_length;
   }
   while (Count)
   {
     int cnt, len;
 
-    pos_in_file= info->pos_in_file + (uint)(info->read_end - info->buffer);
+    pos_in_file= info->pos_in_file + (info->read_end - info->buffer);
     diff_length= (uint) (pos_in_file & (IO_SIZE-1));
     length=IO_ROUND_UP(Count+diff_length)-diff_length;
     length=(length <= info->read_length) ?
                    length + IO_ROUND_DN(info->read_length - length) :
                    length - IO_ROUND_UP(length - info->read_length) ;
     if (info->type != READ_FIFO &&
-	(length > (uint) (info->end_of_file - pos_in_file)))
+	(length > (info->end_of_file - pos_in_file)))
       length= (uint) (info->end_of_file - pos_in_file);
     if (length == 0)
     {
-      info->error=(int) read_len;
+      info->error= (int) left_length;
       DBUG_RETURN(1);
     }
     if (lock_io_cache(info, pos_in_file))
@@ -605,15 +652,15 @@ int _my_b_read_r(register IO_CACHE *info, byte *Buffer, uint Count)
     info->seek_not_done=0;
     if (len <= 0)
     {
-      info->error=(int) read_len;
+      info->error= (int) left_length;
       DBUG_RETURN(1);
     }
-    cnt=((uint) len > Count) ? (int) Count : len;
-    memcpy(Buffer,info->read_pos, (size_t)cnt);
-    Count -=cnt;
-    Buffer+=cnt;
-    read_len+=cnt;
-    info->read_pos+=cnt;
+    cnt= ((uint) len > Count) ? (int) Count : len;
+    memcpy(Buffer, info->read_pos, (size_t) cnt);
+    Count -= cnt;
+    Buffer+= cnt;
+    left_length+= cnt;
+    info->read_pos+= cnt;
   }
   DBUG_RETURN(0);
 }