[PATCH v2] difftool: fix dir-diff when file does not exist in working tree

[John Keeping <john@keeping.me.uk>]

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>
---
On Fri, May 17, 2013 at 11:10:40AM -0700, Junio C Hamano wrote:
> 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).

It can't ever happen because we only call this if the mode is non-zero
in which case the SHA-1 is only null if there are unstaged changes.

However, I think this revised version is much clearer.

 git-difftool.perl   | 6 ++++++
 t/t7800-difftool.sh | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/git-difftool.perl b/git-difftool.perl
index 6780292..8a75205 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -92,6 +92,12 @@ sub use_wt_file
 		return 0;
 	}
 
+	if (! -e "$workdir/$file") {
+		# If the file doesn't exist in the working tree, we cannot
+		# use it.
+		return (0, $null_sha1);
+	}
+
 	my $wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
 	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
-- 
1.8.3.rc2.285.gfc18c2c