Bug#38848 myisam_use_mmap causes widespread myisam corruption on windows

            
Currently the memory map is being created
with a size that is greater than the size 
of the underlying datafile. This can cause
varying behaviour,

e.g. 

In windows the size of the datafile
is increased, while on linux it remains
the same.

This fix removes the increment margin to
the size that is used while creating the
memory map.

storage/myisam/mi_dynrec.c:
  remove MEMMAP_EXTRA_MARGIN that is used as
  the increment margin to the underlying 
  datafile size while creating the mmap.
storage/myisam/mi_packrec.c:
  The size of the underlying datafile is
  increased by MEMMAP_EXTRA_MARGIN when
  using a packed record format. Hence in 
  this case the size of the memory map should 
  be incremented by the same factor.

storage/myisam/mi_dynrec.c

@@ -66,7 +66,7 @@ static int _mi_cmp_buffer(File file, const uchar *buff, my_off_t filepos,
 my_bool mi_dynmap_file(MI_INFO *info, my_off_t size)
 {
   DBUG_ENTER("mi_dynmap_file");
-  if (size > (my_off_t) (~((size_t) 0)) - MEMMAP_EXTRA_MARGIN)
+  if (size > (my_off_t) (~((size_t) 0)))
   {
     DBUG_PRINT("warning", ("File is too large for mmap"));
     DBUG_RETURN(1);
@@ -80,7 +80,7 @@ my_bool mi_dynmap_file(MI_INFO *info, my_off_t size)
       upon a write if no physical memory is available.
   */
   info->s->file_map= (uchar*)
-                  my_mmap(0, (size_t)(size + MEMMAP_EXTRA_MARGIN),
+                  my_mmap(0, (size_t) size,
                           info->s->mode==O_RDONLY ? PROT_READ :
                           PROT_READ | PROT_WRITE,
                           MAP_SHARED | MAP_NORESERVE,
@@ -113,7 +113,7 @@ void mi_remap_file(MI_INFO *info, my_off_t size)
   if (info->s->file_map)
   {
     VOID(my_munmap((char*) info->s->file_map,
-                   (size_t) info->s->mmaped_length + MEMMAP_EXTRA_MARGIN));
+                   (size_t) info->s->mmaped_length));
     mi_dynmap_file(info, size);
   }
 }

storage/myisam/mi_packrec.c

@@ -1502,7 +1502,9 @@ my_bool _mi_memmap_file(MI_INFO *info)
       DBUG_PRINT("warning",("File isn't extended for memmap"));
       DBUG_RETURN(0);
     }
-    if (mi_dynmap_file(info, share->state.state.data_file_length))
+    if (mi_dynmap_file(info,
+                       share->state.state.data_file_length + 
+                         MEMMAP_EXTRA_MARGIN))
       DBUG_RETURN(0);
   }
   info->opt_flag|= MEMMAP_USED;