NOT UPDATE FILE ON DISK

Description:- When the server variable, "myisam_use_mmap" is
enabled, MyISAM tables on windows are not updating the file
on disk even when the server variable "flush" is set to 1.
This is inturn making the table corrupted when encountering
a power failure.

Analysis:- When the server variable "myisam_use_mmap" is set,
files of MyISAM tables will be memory mapped using the OS
APIs mmap()/munmap()/msync() on Unix and CreateFileMapping()
/UnmapViewOfFile()/FlushViewOfFile() on Windows. msync() and
FlushViewOfFile() is responsible for flushing the changes
made to the in-core copy of a file that was mapped into
memory using mmap()/CreateFileMapping() back to the
file system.  FLUSH is determined by the OS unless
explicitly called using msync()/FlushViewOfFile().

When the server variables "myisam_use_mmap" and "flush" are
enabled, MyISAM is only flushing the files from file system
cache to disc using "mysql_file_sync()" and not the memory
mapped file from memory to FS cache using "my_msync()".
["my_msync()" inturn calls  msync() on Unix and
FlushViewOfFile() on Windows.

Fix:- As part of the fix, if server variable
"myisam_use_mmap" is enabled along with  "flush",
"my_msync()" is invoked to flush the data in memory to file
system cache and followed by "mysql_file_sync()" which will
flush the data from file system cache to disk.