Subject: Re: [PATCH] filter-branch: resolve $commit^{tree} in no-index case

From: Jonathan Nieder <[email-masked]>
Date: Tue, 19 Jan 2016 17:22:53 -0800

Jeff King wrote:
> On Tue, Jan 19, 2016 at 04:59:28PM -0500, Jeff King wrote:

>> We could get away "git diff --exit-code $1 $2" to do a single process
>> invocation (rather than two rev-parses), but I don't know if it is worth
>> the complexity.
>
> And here's that patch.
>
> I'm actually a little iffy on it because it switches to "diff-tree" from
> a raw-sha1 comparison. For a well-formed repo, that shouldn't matter.
> But what if you had a commit that was replacing a malformed tree object,
> but not otherwise changing the diff?  We might drop it as "empty", even
> though you'd prefer to keep it.

Mph.  We could get the best of both worlds by introducing a "git
rev-parse --compare <a> <b>" that compares object ids.  Actually...

How about something like this?

Signed-off-by: Jonathan Nieder <[email-masked]>
---
 git-filter-branch.sh | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index 86b2ff1..06f4e0f 100755
--- a/git-filter-branch.sh
+++ b/git-filter-branch.sh
@@ -44,7 +44,15 @@ skip_commit()
 # it will skip commits that leave the tree untouched, commit the other.
 git_commit_non_empty_tree()
 {
-	if test $# = 3 && test "$1" = $(git rev-parse "$3^{tree}"); then
+	if
+		test $# = 3 &&
+		git rev-parse "$1" "$3^{tree}" |
+		{
+			read a
+			read b
+			test "$a" = "$b"
+		}
+	then
 		map "$3"
 	else
 		git commit-tree "$@"
@@ -404,7 +412,7 @@ while read commit parents; do
 	then
 		tree=$(git write-tree)
 	else
-		tree=$(git rev-parse "$commit^{tree}")
+		tree="$commit^{tree}"
 	fi
 	workdir=$workdir @SHELL_PATH@ -c "$filter_commit" "git commit-tree" \
 		"$tree" $parentstr < ../message > ../map/$commit ||
-- 
2.7.0