Re: [PATCH v2 2/3] t7800: fix tests when difftool uses --no-symlinks

Subject: [PATCH] difftool: don't overwrite modified files

After running the user's diff tool, git-difftool will copy any files
that differ between the working tree and the temporary tree.  This is
useful when the user edits the file in their diff tool but is wrong if
they edit the working tree file while examining the diff.

Instead of copying unconditionally when the files differ, store the
initial hash of the working tree file and only copy the temporary file
back if it was modified and the working tree file was not.  If both
files have been modified, print a warning and exit with an error.

Signed-off-by: John Keeping <redacted>
---
 git-difftool.perl   | 35 +++++++++++++++++++++--------------
 t/t7800-difftool.sh | 26 ++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 14 deletions(-)

diff --git a/git-difftool.perl b/git-difftool.perl
index c433e86..be82b5a 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl

@@ -15,7 +15,6 @@ use strict;
 use warnings;
 use File::Basename qw(dirname);
 use File::Copy;
-use File::Compare;
 use File::Find;
 use File::stat;
 use File::Path qw(mkpath rmtree);

@@ -123,7 +122,7 @@ sub setup_dir_diff
 	my $rindex = '';
 	my %submodule;
 	my %symlink;
-	my @working_tree = ();
+	my %working_tree;
 	my @rawdiff = split('\0', $diffrtn);
 
 	my $i = 0;

@@ -175,7 +174,9 @@ EOF
 
 		if ($rmode ne $null_mode) {
 			if (use_wt_file($repo, $workdir, $dst_path, $rsha1, $symlinks)) {
-				push(@working_tree, $dst_path);
+				$working_tree{$dst_path} =
+					$repo->command_oneline('hash-object',
+						"$workdir/$dst_path");
 			} else {
 				$rindex .= "$rmode $rsha1\t$dst_path\0";
 			}

@@ -227,7 +228,7 @@ EOF
 	# not part of the index. Remove any trailing slash from $workdir
 	# before starting to avoid double slashes in symlink targets.
 	$workdir =~ s|/$||;
-	for my $file (@working_tree) {
+	for my $file (keys %working_tree) {
 		my $dir = dirname($file);
 		unless (-d "$rdir/$dir") {
 			mkpath("$rdir/$dir") or

@@ -278,7 +279,7 @@ EOF
 		exit_cleanup($tmpdir, 1) if not $ok;
 	}
 
-	return ($ldir, $rdir, $tmpdir, @working_tree);
+	return ($ldir, $rdir, $tmpdir, %working_tree);
 }
 
 sub write_to_file

@@ -376,7 +377,7 @@ sub dir_diff
 	my $error = 0;
 	my $repo = Git->repository();
 	my $workdir = find_worktree($repo);
-	my ($a, $b, $tmpdir, @worktree) =
+	my ($a, $b, $tmpdir, %worktree) =
 		setup_dir_diff($repo, $workdir, $symlinks);
 
 	if (defined($extcmd)) {

@@ -390,19 +391,25 @@ sub dir_diff
 	# should be copied back to the working tree.
 	# Do not copy back files when symlinks are used and the
 	# external tool did not replace the original link with a file.
-	for my $file (@worktree) {
+	for my $file (keys %worktree) {
 		next if $symlinks && -l "$b/$file";
 		next if ! -f "$b/$file";
 
-		my $diff = compare("$b/$file", "$workdir/$file");
-		if ($diff == 0) {
-			next;
-		} elsif ($diff == -1) {
-			my $errmsg = "warning: Could not compare ";
-			$errmsg += "'$b/$file' with '$workdir/$file'\n";
+		my $wt_hash = $repo->command_oneline('hash-object',
+			"$workdir/$file");
+		my $tmp_hash = $repo->command_oneline('hash-object',
+			"$b/$file");