[PATCH 7 of 7 V3] util: enable hardlink for copyfile

[Augie Fackler]

On Sun, Mar 12, 2017 at 11:00:05AM -0700, Mads Kiilerich wrote:
> On 03/12/2017 01:23 AM, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1489309403 28800
> > #      Sun Mar 12 01:03:23 2017 -0800
> > # Node ID 9da40a9e54c419490a2ff23b9dda7acc109f81cd
> > # Parent  de28b62236a7d47a896bc4aba2bd95dcd8defc87
> > # Available At https://bitbucket.org/quark-zju/hg-draft
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft -r 9da40a9e54c4
> > util: enable hardlink for copyfile
> >
> > Because why not, if we are sure that the filesystem has good hardlink
> > support (and we are - see "posix: implement a method to get filesystem type
> > for Linux").
> >
> > This patch removes the global variable "allowhardlinks" added by "util: add
> > allowhardlinks module variable". Third party extensions wanting to enable
> > hardlink support unconditionally can replace "_hardlinkfswhitelist".
>
> How about using the approximation that all case sensitive filesystems are
> safe to try to use hardlinks? Or to be kind to macOS: All filesystems that
> support symlinks?
>
> e5ce49a30146 was a last minute change and *very* safe, disabling it on all
> platforms.
>
> It seems like the problem only is seen when running Windows on the client
> side - perhaps only disable these hardlinks on Windows? It seems like there
> really is no need for detecting any file systems on Linux?

It's actually believed the problem is in the Windows CIFS *server*, if
you read the bug attached to the previous change.

I'm hesitant to turn on hardlinks at all because of the way problems
manifest (unrecoverable data corruption for users, at some random
point in the future), so if we're going to do it we need to be very
conservative to avoid the bug again.

> The infrastructure for detecting file system seems cool, but also quite low
> level and high-maintenance for each platform. If we need to detect CIFS,
> could we perhaps just detect CIFS through generic file system operations
> instead of whitelisting everything that isn't CIFS?
>
> /Mads
>
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