https://issues.dlang.org/show_bug.cgi?id=21627

          Issue ID: 21627
           Summary: macOS:  std.stdio.File.sync does not guarantee to be
                    written to disk
           Product: D
           Version: D2
          Hardware: All
                OS: Mac OS X
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: kubo39@gmail.com

The documentation(https://dlang.org/phobos/std_stdio.html#.File.sync) says:

> Forces any data buffered by the OS to be written to disk.
> Call flush before calling this function to flush the C FILE buffers first.

Accoriding to macOS's manapge, however, fsync(2) does not guarantee to be
written to disk.
https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/fsync.2.html

> Note that while fsync() will flush all data from the host to the drive
> (i.e. the "permanent storage device"), the drive itself may not physically
> write the data to the platters for quite some time and it may be
> written in an out-of-order sequence.
>
> Specifically, if the drive loses power or the OS crashes, the application may find that only some or none of their data was written.  The disk drive may also re-order the data so that later writes may be present, while earlier writes are not.
>
> This is not a theoretical edge case.  This scenario is easily reproduced with real world workloads and drive power failures.

The manpage also says that the F_FULLFSYNC fcntl would be useful in this case.

> For applications that require tighter guarantees about the integrity of their data, Mac OS X provides the F_FULLFSYNC fcntl.  The F_FULLFSYNC fcntl asks the drive to flush all buffered data to permanent storage.

I think std.stdio.File.sync should use F_FULLSYNC fcntl.

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