[PATCH 2/2] difftool --dir-diff: symlink all files matching the working tree

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

Some users like to edit files in their diff tool when using "git
difftool --dir-diff --symlink" to compare against the working tree but
difftool currently only created symlinks when a file contains unstaged
changes.

Change this behaviour so that symlinks are created whenever the
right-hand side of the comparison has the same SHA1 as the file in the
working tree.

Note that textconv filters are handled in the same way as by git-diff
and if a clean filter is not the inverse of its smudge filter we already
get a null SHA1 from "diff --raw" and will symlink the file without
going through the new hash-object based check.

Reported-by: Matt McClure <matthewlmcclure@gmail.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
---
 Documentation/git-difftool.txt |  4 +++-
 git-difftool.perl              | 21 ++++++++++++++++++---
 t/t7800-difftool.sh            | 14 ++++++++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index e575fea..8361e6e 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -72,7 +72,9 @@ with custom merge tool commands and has the same value as `$MERGED`.
 --symlinks::
 --no-symlinks::
 	'git difftool''s default behavior is create symlinks to the
-	working tree when run in `--dir-diff` mode.
+	working tree when run in `--dir-diff` mode and the right-hand
+	side of the comparison yields the same content as the file in
+	the working tree.
 +
 Specifying `--no-symlinks` instructs 'git difftool' to create copies
 instead.  `--no-symlinks` is the default on Windows.
diff --git a/git-difftool.perl b/git-difftool.perl
index 0a90de4..5f093ae 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -83,6 +83,21 @@ sub exit_cleanup
 	exit($status | ($status >> 8));
 }
 
+sub use_wt_file
+{
+	my ($repo, $workdir, $file, $sha1, $symlinks) = @_;
+	my $null_sha1 = '0' x 40;
+
+	if ($sha1 eq $null_sha1) {
+		return 1;
+	} elsif (not $symlinks) {
+		return 0;
+	}
+
+	my $wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
+	return $sha1 eq $wt_sha1;
+}
+
 sub setup_dir_diff
 {
 	my ($repo, $workdir, $symlinks) = @_;
@@ -159,10 +174,10 @@ EOF
 		}
 
 		if ($rmode ne $null_mode) {
-			if ($rsha1 ne $null_sha1) {
-				$rindex .= "$rmode $rsha1\t$dst_path\0";
-			} else {
+			if (use_wt_file($repo, $workdir, $dst_path, $rsha1, $symlinks)) {
 				push(@working_tree, $dst_path);
+			} else {
+				$rindex .= "$rmode $rsha1\t$dst_path\0";
 			}
 		}
 	}
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index eb1d3f8..8102ce1 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -370,6 +370,20 @@ test_expect_success PERL 'difftool --dir-diff' '
 	echo "$diff" | stdin_contains file
 '
 
+write_script .git/CHECK_SYMLINKS <<\EOF &&
+#!/bin/sh
+test -L "$2/file" &&
+test -L "$2/file2" &&
+test -L "$2/sub/sub"
+echo $?
+EOF
+
+test_expect_success PERL,SYMLINKS 'difftool --dir-diff --symlink without unstaged changes' '
+	result=$(git difftool --dir-diff --symlink \
+		--extcmd "./.git/CHECK_SYMLINKS" branch HEAD) &&
+	test "$result" = 0
+'
+
 test_expect_success PERL 'difftool --dir-diff ignores --prompt' '
 	diff=$(git difftool --dir-diff --prompt --extcmd ls branch) &&
 	echo "$diff" | stdin_contains sub &&
-- 
1.8.2.rc2.4.g7799588