[PATCH] log: let --follow follow renames in merge commits

[Miklos Vajna <vmiklos@collabora.com>]

Have a repo with a subtree merge, do a 'git log --follow prefix/test.c',
the output only contains history in the outer repo, not commits that
were merged via a subtree merge.

This is inconsistent, since doing a 'git blame prefix/test.c' does find
the original commits. This works because find_rename() in blame.c is
invoked for each parent, and there diff_tree_oid() is used, which uses
try_to_follow_renames(). This means that in case a rename happens as
part of a merge commit, git blame can follow that rename.

Fix the problem in a similar way for the 'git log --follow' case: in
case log_tree_diff() finds a merge commit and it would return early,
then do some extra work in the follow_renames case first. Check each
parent, use diff_tree_oid() and if found_follow is set, then work with
that parent instead of returning.

This means that users examining the history of a repo with subtree
merges can see all commits to a file with a single 'git log --follow'
invocation, instead of one invocation for the outer repo and one for the
history before the subtree merge.

Signed-off-by: Miklos Vajna <vmiklos@collabora.com>
---

Hi Junio,

I sent this out a week ago at
<https://lore.kernel.org/git/afmfSa-p-9vuDL3E@collabora.com/T/#u>, I
didn't get any reply to it -- so I'm somewhat optimistic that the patch
itself is a good idea, seeing no negative comments.

So this is a resend, this time to you, CC'ing the list, rather than the
other way around.

Could you please review this?

Thanks,

Miklos

 log-tree.c                          | 20 ++++++++++++++++-
 t/meson.build                       |  1 +
 t/t4218-log-follow-subtree-merge.sh | 34 +++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+), 1 deletion(-)
 create mode 100755 t/t4218-log-follow-subtree-merge.sh

diff --git a/log-tree.c b/log-tree.c
index 7e048701d0..bce09c7dac 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -1142,8 +1142,26 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 				/* Show parent info for multiple diffs */
 				log->parent = parents->item;
 			}
-		} else
+		} else {
+			if (opt->diffopt.flags.follow_renames) {
+				/*
+				 * Detect a rename across one of the parents.
+				 * Check each parent till we find a follow.
+				 */
+				struct commit_list *p;
+				for (p = parents; p; p = p->next) {
+					parse_commit_or_die(p->item);
+					diff_tree_oid(get_commit_tree_oid(p->item),
+						      oid, "", &opt->diffopt);
+					diff_queue_clear(&diff_queued_diff);
+					if (opt->diffopt.found_follow) {
+						opt->diffopt.found_follow = 0;
+						break;
+					}
+				}
+			}
 			return 0;
+		}
 	}
 
 	showed_log = 0;
diff --git a/t/meson.build b/t/meson.build
index 7528e5cda5..b4ae8d76d8 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -574,6 +574,7 @@ integration_tests = [
   't4215-log-skewed-merges.sh',
   't4216-log-bloom.sh',
   't4217-log-limit.sh',
+  't4218-log-follow-subtree-merge.sh',
   't4252-am-options.sh',
   't4253-am-keep-cr-dos.sh',
   't4254-am-corrupt.sh',
diff --git a/t/t4218-log-follow-subtree-merge.sh b/t/t4218-log-follow-subtree-merge.sh
new file mode 100755
index 0000000000..7ca607cbb8
--- /dev/null
+++ b/t/t4218-log-follow-subtree-merge.sh
@@ -0,0 +1,34 @@
+#!/bin/sh
+
+test_description='Test --follow follows renames across subtree merges'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+test_expect_success 'setup subtree-merged repository' '
+	git init inner &&
+	echo inner >inner/inner.txt &&
+	git -C inner add inner.txt &&
+	git -C inner commit -m "inner init" &&
+
+	git init outer &&
+	echo outer >outer/outer.txt &&
+	git -C outer add outer.txt &&
+	git -C outer commit -m "outer init" &&
+
+	git -C outer fetch ../inner master &&
+	git -C outer merge -s ours --no-commit --allow-unrelated-histories \
+		FETCH_HEAD &&
+	git -C outer read-tree --prefix=inner/ -u FETCH_HEAD &&
+	git -C outer commit -m "Merge inner repo into inner/ subdirectory"
+'
+
+test_expect_success '--follow finds the pre-merge commit through a subtree merge' '
+	git -C outer log --follow --pretty=tformat:%s inner/inner.txt >actual &&
+	echo "inner init" >expect &&
+	test_cmp expect actual
+'
+
+test_done
-- 
2.51.0