From 1dc413ebe52bd5dffed63d69a0a239b7fb6ef699 Mon Sep 17 00:00:00 2001
From: Jeff King <peff@peff.net>
Date: Tue, 19 Jan 2016 17:07:22 -0500
Subject: [PATCH] filter-branch: resolve $commit^{tree} in no-index case

Commit 348d4f2 (filter-branch: skip index read/write when
possible, 2015-11-06) taught filter-branch to optimize out
the final "git write-tree" when we know we haven't touched
the tree with any of our filters. It does by simply putting
the literal text "$commit^{tree}" into the "$tree" variable,
avoiding a useless rev-parse call.

However, when we pass this to git_commit_non_empty_tree(),
it gets confused; it resolves "$commit^{tree}" itself, and
compares our string to the 40-hex sha1, which obviously
doesn't match. As a result, "--prune-empty" (or any custom
filter using git_commit_non_empty_tree) will fail to drop
an empty commit (when filter-branch is used without a tree
or index filter).

Let's resolve $tree to the 40-hex ourselves, so that
git_commit_non_empty_tree can work. Unfortunately, this is a
bit slower due to the extra process overhead:

  $ cd t/perf && ./run 348d4f2 HEAD p7000-filter-branch.sh
  [...]
  Test                  348d4f2           HEAD
  --------------------------------------------------------------
  7000.2: noop filter   3.76(0.24+0.26)   4.54(0.28+0.24) +20.7%

We could try to make git_commit_non_empty_tree more clever.
However, the value of $tree here is technically
user-visible. The user can provide arbitrary shell code at
this stage, which could itself have a similar assumption to
what is in git_commit_non_empty_tree. So the conservative
choice to fix this regression is to take the 20% hit and
give the pre-348d4f2 behavior. We still end up much faster
than before the optimization:

  $ cd t/perf && ./run 348d4f2^ HEAD p7000-filter-branch.sh
  [...]
  Test                  348d4f2^          HEAD
  --------------------------------------------------------------
  7000.2: noop filter   9.51(4.32+0.40)   4.51(0.28+0.23) -52.6%

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 git-filter-branch.sh     | 2 +-
 t/t7003-filter-branch.sh | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index d61f9bace..5e094ce18 100755
--- a/git-filter-branch.sh
+++ b/git-filter-branch.sh
@@ -404,7 +404,7 @@ while read commit parents; do
 	then
 		tree=$(git write-tree)
 	else
-		tree="$commit^{tree}"
+		tree=$(git rev-parse "$commit^{tree}")
 	fi
 	workdir=$workdir @SHELL_PATH@ -c "$filter_commit" "git commit-tree" \
 		"$tree" $parentstr < ../message > ../map/$commit ||
diff --git a/t/t7003-filter-branch.sh b/t/t7003-filter-branch.sh
index 377c648e0..97c23c2e3 100755
--- a/t/t7003-filter-branch.sh
+++ b/t/t7003-filter-branch.sh
@@ -333,6 +333,14 @@ test_expect_success 'prune empty collapsed merges' '
 	test_cmp expect actual
 '
 
+test_expect_success 'prune empty works even without index/tree filters' '
+	git rev-list HEAD >expect &&
+	git commit --allow-empty -m empty &&
+	git filter-branch -f --prune-empty HEAD &&
+	git rev-list HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '--remap-to-ancestor with filename filters' '
 	git checkout master &&
 	git reset --hard A &&