[PATCH 3 of 5 V2] localrepo: handle rename with hardlinks properly

[Augie Fackler]

On Thu, Mar 02, 2017 at 10:13:29PM -0800, Jun Wu wrote:
> # HG changeset patch
> # User Jun Wu <quark at fb.com>
> # Date 1488520170 28800
> #      Thu Mar 02 21:49:30 2017 -0800
> # Node ID 1cf153ec3faaef92c9ad3515372a6d8591195d6e
> # Parent  406f94842e3372f241a151a3ee6ea5f39c04758d
> # Available At https://bitbucket.org/quark-zju/hg-draft
> #              hg pull https://bitbucket.org/quark-zju/hg-draft -r 1cf153ec3faa
> localrepo: handle rename with hardlinks properly

I've taken patches 1-3, and will try to find time to reflect on the
nuances of 4 and 5 soon (but will try and unblock other series first.)

>
> In "aftertrans", we rename "journal.*" to "undo.*". We expect "journal.*"
> files to disappear after renaming.
>
> However, if "journal.foo" and "undo.foo" refer to a same file (hardlink),
> rename may be a no-op, leaving both files on disk, according to Linux
> manpage [1]:
>
>     If oldpath and newpath are existing hard links referring to the same
>     file, then rename() does nothing, and returns  a  suc‐ cess status.
>
> The POSIX specification [2] is not very clear about what to do.
>
> To be safe, remove "undo.*" before the rename so "journal.*" cannot be left
> on disk.
>
> [1]: http://man7.org/linux/man-pages/man2/rename.2.html
> [2]: http://pubs.opengroup.org/onlinepubs/9699919799/
>
> diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
> --- a/mercurial/localrepo.py
> +++ b/mercurial/localrepo.py
> @@ -2000,4 +2000,12 @@ def aftertrans(files):
>          for vfs, src, dest in renamefiles:
>              try:
> +                # if src and dest refer to a same file, vfs.rename is a no-op,
> +                # leaving both src and dest on disk. delete dest to make sure
> +                # the rename couldn't be such a no-op.
> +                vfs.unlink(dest)
> +            except OSError as ex:
> +                if ex.errno != errno.ENOENT:
> +                    raise
> +            try:
>                  vfs.rename(src, dest)
>              except OSError: # journal file does not yet exist
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