Re: [PATCH] difftool: fix dir-diff when file does not exist in working tree

[Junio C Hamano <gitster@pobox.com>]

John Keeping <john@keeping.me.uk> writes:

> Commit 02c5631 (difftool --dir-diff: symlink all files matching the
> working tree, 2013-03-14) does not handle the case where a file that is
> being compared does not exist in the working tree.  Fix this by checking
> for existence explicitly before running git-hash-object.
>
> Reported-by: Kevin Bracey <kevin@bracey.fi>
> Signed-off-by: John Keeping <john@keeping.me.uk>
> ---
> This fixes a regression in 1.8.3-rc0.
>
>  git-difftool.perl   | 9 ++++++++-
>  t/t7800-difftool.sh | 7 +++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/git-difftool.perl b/git-difftool.perl
> index 6780292..0a1cb0a 100755
> --- a/git-difftool.perl
> +++ b/git-difftool.perl
> @@ -92,7 +92,14 @@ sub use_wt_file
>  		return 0;
>  	}
>  
> -	my $wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
> +	my $wt_sha1;
> +	if (-e "$workdir/$file") {
> +		$wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
> +	} else {
> +		# If the file doesn't exist in the working tree, use something
> +		# that cannot match a SHA-1.
> +		$wt_sha1 = '';

Yuck.

"that cannot match" might be a good justification to say "this does
not break the next line to set $use and forces it to false", but
"when we return false in $use, the value of $wt_sha1 is not used"
needs to be said to convince why this is a safe change.

But if $sha1 is $null_sha1, we do end up setting $use to true and
the caller would stuff the empty $wt_sha1 to form:

	$wtindex .= "$rmode \$dst_path\0";

Is that what we want to do here, or is it a "will never happen"
condition?  If the latter, the reason need to be described in this
comment (and in the log).

Thanks.

> +	};
>  	my $use = ($sha1 eq $null_sha1) || ($sha1 eq $wt_sha1);
>  	return ($use, $wt_sha1);
>  }
> diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
> index a6bd99e..d46f041 100755
> --- a/t/t7800-difftool.sh
> +++ b/t/t7800-difftool.sh
> @@ -356,6 +356,13 @@ run_dir_diff_test 'difftool --dir-diff from subdirectory' '
>  	)
>  '
>  
> +run_dir_diff_test 'difftool --dir-diff when worktree file is missing' '
> +	test_when_finished git reset --hard &&
> +	rm file2 &&
> +	git difftool --dir-diff $symlinks --extcmd ls branch master >output &&
> +	grep file2 output
> +'
> +
>  write_script .git/CHECK_SYMLINKS <<\EOF
>  for f in file file2 sub/sub
>  do