Cross platform Replace function

Cross platform Replace function

Turns out that os.rename works as atomic file replacement on linux, but not on Windows. There's actually no way (as far as I can tell) to atomically replace a file with another one on windows using the go code as written.  Luckily, it's not hard to wrap the Windows API in go code and use that.... so that's what this code code.

This fixes problems where we try to replace existing files with new ones, which fails on windows (notably during bootstrap).

https://code.launchpad.net/~natefinch/juju-core/fix-win-bootstrap/+merge/190461

(do not edit description out of merge proposal)

[natefinch, 2013-10-10 19:57:16]

Please take a look.

[natefinch, 2013-10-10 20:34:46]

Please take a look.

[rog, 2013-10-10 20:38:56]

LGTM modulo the below; and I think it should be called utils.ReplaceFile, as
discussed online.

https://codereview.appspot.com/14502058/diff/1/utils/file_windows.go
File utils/file_windows.go (right):

https://codereview.appspot.com/14502058/diff/1/utils/file_windows.go#newcode11
utils/file_windows.go:11: movefile_replace_existing = 0x1
moveFileReplaceExisting ?

https://codereview.appspot.com/14502058/diff/4001/utils/file_unix.go
File utils/file_unix.go (right):

https://codereview.appspot.com/14502058/diff/4001/utils/file_unix.go#newcode11
utils/file_unix.go:11: // Replace atomically replaces the destination file or
directory with the source.
Perhaps document that the error return is exactly that of os.Rename?

https://codereview.appspot.com/14502058/diff/4001/utils/file_windows.go
File utils/file_windows.go (right):

https://codereview.appspot.com/14502058/diff/4001/utils/file_windows.go#newco...
utils/file_windows.go:18: // Replace atomically replaces the destination file or
directory with the source.
ditto

[natefinch, 2013-10-10 20:59:43]

On 2013/10/10 20:38:56, rog wrote:
> LGTM modulo the below; and I think it should be called utils.ReplaceFile, as
> discussed online.

Yep, done.

> https://codereview.appspot.com/14502058/diff/1/utils/file_windows.go
> File utils/file_windows.go (right):
> 
> https://codereview.appspot.com/14502058/diff/1/utils/file_windows.go#newcode11
> utils/file_windows.go:11: movefile_replace_existing = 0x1
> moveFileReplaceExisting ?

The underscored version is straight out of Microsoft's documentation (except
they capitalize it), so I'm trying to keep it as close to that as possible,
while not exposing them to the outside world.  this is also what syscall does
(though they do expose many of the values).
 
> https://codereview.appspot.com/14502058/diff/4001/utils/file_unix.go
> File utils/file_unix.go (right):
> 
> https://codereview.appspot.com/14502058/diff/4001/utils/file_unix.go#newcode11
> utils/file_unix.go:11: // Replace atomically replaces the destination file or
> directory with the source.
> Perhaps document that the error return is exactly that of os.Rename?

Done.

> https://codereview.appspot.com/14502058/diff/4001/utils/file_windows.go
> File utils/file_windows.go (right):
> 
>
https://codereview.appspot.com/14502058/diff/4001/utils/file_windows.go#newco...
> utils/file_windows.go:18: // Replace atomically replaces the destination file
or
> directory with the source.
> ditto

done.