From 02e63b0f2a607d0d0f2aa335d24547b509bd1e8f Mon Sep 17 00:00:00 2001
From: Xing Xin <xingxin.xx@bytedance.com>
Date: Thu, 8 Aug 2024 20:13:29 +0800
Subject: [PATCH] diff-tree: fix crash when used with --remerge-diff

When using "git-diff-tree" to get the tree diff for merge commits with
the diff format set to `remerge`, a bug is triggered as shown below:

  $ git diff-tree -r --remerge-diff 363337e6eb
  363337e6eb812d0c0d785ed4261544f35559ff8b
  BUG: log-tree.c:1006: did a remerge diff without remerge_objdir?!?

This bug is reported by `log-tree.c:do_remerge_diff`, where a bug check
added in commit 7b90ab467a (log: clean unneeded objects during log
--remerge-diff, 2022-02-02) detects the absence of `remerge_objdir` when
attempting to clean up temporary objects generated during the remerge
process.

After some further digging, I find that the remerge-related diff options
were introduced in db757e8b8d (show, log: provide a --remerge-diff
capability, 2022-02-02), which also affect the setup of `rev_info` for
"git-diff-tree", but were not accounted for in the original
implementation (inferred from the commit message).

Elijah Newren (author of the remerge diff feature) also notes that other
callers of `log-tree.c:log_tree_commit` (which is the only caller of
above mentioned `log-tree.c:do_remerge_diff`) also exist, but:

  builtin/am.c: manually sets all flags; remerge_diff is not among them
  sequencer.c: manually sets all flags; remerge_diff is not among them

so "builtin/diff-tree.c" really is the only caller that was overlooked
when remerge-diff functionality was added.

This commit fixes the bug by adding the setup logic for `remerge_objdir`
in "builtin/diff-tree.c", mirroring the logic in
`builtin/log.c:cmd_log_walk_no_free`. And a final cleanup for
`remerge_objdir` is also included.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Xing Xin <xingxin.xx@bytedance.com>
---
 builtin/diff-tree.c     | 13 +++++++++++++
 t/t4069-remerge-diff.sh | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
index 0d3c611aac07ab..b8df1d4b79b981 100644
--- a/builtin/diff-tree.c
+++ b/builtin/diff-tree.c
@@ -8,6 +8,7 @@
 #include "read-cache-ll.h"
 #include "repository.h"
 #include "revision.h"
+#include "tmp-objdir.h"
 #include "tree.h"
 
 static struct rev_info log_tree_opt;
@@ -166,6 +167,13 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 
 	opt->diffopt.rotate_to_strict = 1;
 
+	if (opt->remerge_diff) {
+		opt->remerge_objdir = tmp_objdir_create("remerge-diff");
+		if (!opt->remerge_objdir)
+			die(_("unable to create temporary object directory"));
+		tmp_objdir_replace_primary_odb(opt->remerge_objdir, 1);
+	}
+
 	/*
 	 * NOTE!  We expect "a..b" to expand to "^a b" but it is
 	 * perfectly valid for revision range parser to yield "b ^a",
@@ -230,5 +238,10 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		diff_free(&opt->diffopt);
 	}
 
+	if (opt->remerge_diff) {
+		tmp_objdir_destroy(opt->remerge_objdir);
+		opt->remerge_objdir = NULL;
+	}
+
 	return diff_result_code(&opt->diffopt);
 }
diff --git a/t/t4069-remerge-diff.sh b/t/t4069-remerge-diff.sh
index 07323ebafe0d0c..ca8f999caba54d 100755
--- a/t/t4069-remerge-diff.sh
+++ b/t/t4069-remerge-diff.sh
@@ -110,6 +110,41 @@ test_expect_success 'can filter out additional headers with pickaxe' '
 	test_must_be_empty actual
 '
 
+test_expect_success 'remerge-diff also works for git-diff-tree' '
+	# With a clean merge
+	git diff-tree -r -p --remerge-diff --no-commit-id bc_resolution >actual &&
+	test_must_be_empty actual &&
+
+	# With both a resolved conflict and an unrelated change
+	cat <<-EOF >tmp &&
+	diff --git a/numbers b/numbers
+	remerge CONFLICT (content): Merge conflict in numbers
+	index a1fb731..6875544 100644
+	--- a/numbers
+	+++ b/numbers
+	@@ -1,13 +1,9 @@
+	 1
+	 2
+	-<<<<<<< b0ed5cb (change_a)
+	-three
+	-=======
+	-tres
+	->>>>>>> 6cd3f82 (change_b)
+	+drei
+	 4
+	 5
+	 6
+	 7
+	-eight
+	+acht
+	 9
+	EOF
+	sed -e "s/[0-9a-f]\{7,\}/HASH/g" tmp >expect &&
+	git diff-tree -r -p --remerge-diff --no-commit-id ab_resolution >tmp &&
+	sed -e "s/[0-9a-f]\{7,\}/HASH/g" tmp >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'setup non-content conflicts' '
 	git switch --orphan base &&