[PATCH V1] util: move checknlink away from the dependency of a hard-coded filename

[Yuya Nishihara]

On Tue, 23 Aug 2016 10:39:29 -0700, ttung at fb.com wrote:
> # HG changeset patch
> # User Tony Tung <tonytung at merly.org>
> # Date 1471973931 25200
> #      Tue Aug 23 10:38:51 2016 -0700
> # Node ID 800bdaf50847ed85c913d8d8701d7c6738ee082c
> # Parent  2efc5a05d80a6d4253767f2ce0c2fb062ba83cb6
> util: move checknlink away from the dependency of a hard-coded filename
> 
> This somewhat insulates us against bugs in checknlink when a stale file
> left behind could result in checknlink always returning False.
> 
> diff --git a/mercurial/util.py b/mercurial/util.py
> --- a/mercurial/util.py
> +++ b/mercurial/util.py
> @@ -1305,34 +1305,46 @@
>  def checknlink(testfile):
>      '''check whether hardlink count reporting works properly'''
>  
> -    # testfile may be open, so we need a separate file for checking to
> -    # work around issue2543 (or testfile may get lost on Samba shares)
> -    f1 = testfile + ".hgtmp1"
> -    if os.path.lexists(f1):
> -        return False
> +    f1path, f2path, f1handle, f2handle = None, None, None, None

Nit: this could be f1path = f2path = ... = None.

>      try:
> -        posixfile(f1, 'w').close()
> -    except IOError:
> +        dirpath, filename = os.path.split(testfile)
> +
> +        # testfile may be open, so we need a separate file for checking to
> +        # work around issue2543 (or testfile may get lost on Samba shares)
> +        f1handle, f1path = tempfile.mkstemp(prefix=filename, dir=dirpath)
> +        os.close(f1handle)
> +        f1handle = None
> +
> +        f2handle, f2path = tempfile.mkstemp(prefix=filename, dir=dirpath)
> +        os.close(f2handle)
> +        f2handle = None
> +
> +        # there's a small race condition that another file can jam itself in
> +        # between the time we remove f2 and the time we create the hard link.
> +        # in the unlikely scenario that happens, we'll treat it as nlink being
> +        # unreliable.
>          try:
> -            os.unlink(f1)
> +            os.unlink(f2path)
> +            oslink(f1path, f2path)
> +            # nlinks() may behave differently for files on Windows shares if
> +            # the file is open.
> +            f2handle = posixfile(f2path)
> +            return nlinks(f2path) > 1
>          except OSError:
> -            pass
> -        return False
> -
> -    f2 = testfile + ".hgtmp2"
> -    fd = None
> -    try:
> -        oslink(f1, f2)
> -        # nlinks() may behave differently for files on Windows shares if
> -        # the file is open.
> -        fd = posixfile(f2)
> -        return nlinks(f2) > 1
> -    except OSError:
> -        return False
> +            return False
>      finally:
> -        if fd is not None:
> -            fd.close()
> -        for f in (f1, f2):
> +        for f in (f1handle, f2handle):
> +            try:
> +                if f is None:
> +                    continue
> +                elif isinstance(f, int):
> +                    os.close(f)
> +                else:
> +                    f.close()

I don't think there's a benefit to store file descriptor or file object
to the same variable. And IIRC, f.close() would raise IOError on failure,
whereas os.close() would OSError.

> +            except OSError:
> +                pass
> +        for f in (f1path, f2path):

f1path and f2path could be None.

>              try:
>                  os.unlink(f)