[PATCH v4] log: improve --follow following renames for non-linear history

[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.

What happens is that 'git log --follow' stores the followed path only in
opt->diffopt.pathspec, so in case the commit history is non-linear, and
multiple parents have renames to the followed path, then the end result
isn't really defined: the first commit that happens to be visited in one
of the parents update opt->diffopt.pathspec, and from that point, only
that updated path is visited.

Fix the problem by introducing a commit -> path map
(follow_pathspec_slab) that stores what will be a path to follow when
visiting that parent. At the top of log_tree_commit(), if the slab has
an entry for this commit, we replace opt->diffopt.pathspec with a path
from this entry, so the correct path is followed, even if an unrelated
sub-tree changed the path to be followed to something else. After
log_tree_diff() runs, we record each parent's path in the slab. As a
result, the walk order doesn't matter, which was exactly the source of
problems previously.

This helps with subtree merges (rename happens inside the merge commit),
but also fixes the general case when the rename happens in the history
of parents, not in the merge commit itself.

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

Hi Junio,

I noticed that merging 'mv/log-follow-mergy' to 'master' results in a
(simple) conflict since commit 42d960748e (line-log: integrate -L output
with the standard log-tree pipeline, 2026-05-28), this is an updated
version that resolves the conflict.

Please replace the content of that branch with this patch.

Thanks,

Miklos

 Documentation/config/log.adoc |   3 +-
 log-tree.c                    | 116 ++++++++++++++++++++++++++++++
 log-tree.h                    |   1 +
 revision.c                    |   2 +
 revision.h                    |   4 ++
 t/meson.build                 |   1 +
 t/t4219-log-follow-merge.sh   | 129 ++++++++++++++++++++++++++++++++++
 7 files changed, 254 insertions(+), 2 deletions(-)
 create mode 100755 t/t4219-log-follow-merge.sh

diff --git a/Documentation/config/log.adoc b/Documentation/config/log.adoc
index f20cc25cd7..757a7be196 100644
--- a/Documentation/config/log.adoc
+++ b/Documentation/config/log.adoc
@@ -53,8 +53,7 @@ This is the same as the `--decorate` option of the `git log`.
 `log.follow`::
 	If `true`, `git log` will act as if the `--follow` option was used when
 	a single <path> is given.  This has the same limitations as `--follow`,
-	i.e. it cannot be used to follow multiple files and does not work well
-	on non-linear history.
+	i.e. it cannot be used to follow multiple files.
 
 `log.graphColors`::
 	A list of colors, separated by commas, that can be used to draw
diff --git a/log-tree.c b/log-tree.c
index 88b3019293..83a3c4bf9b 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -3,6 +3,7 @@
 
 #include "git-compat-util.h"
 #include "commit-reach.h"
+#include "commit-slab.h"
 #include "config.h"
 #include "diff.h"
 #include "diffcore.h"
@@ -1089,6 +1090,96 @@ static int do_remerge_diff(struct rev_info *opt,
 	return !opt->loginfo;
 }
 
+/* Per-commit path storage for --follow across merges */
+define_commit_slab(follow_pathspec_slab, char *);
+
+static const char *pathspec_single_path(const struct pathspec *ps)
+{
+	if (ps->nr != 1)
+		return NULL;
+	return ps->items[0].match;
+}
+
+static void set_pathspec_to_single_path(struct pathspec *ps, const char *path)
+{
+	const char *paths[2] = { path, NULL };
+
+	clear_pathspec(ps);
+	parse_pathspec(ps,
+		       PATHSPEC_ALL_MAGIC & ~PATHSPEC_LITERAL,
+		       PATHSPEC_LITERAL_PATH, "", paths);
+}
+
+static void remember_follow_pathspec(struct rev_info *opt,
+				     struct commit *c, const char *path)
+{
+	char **slot;
+
+	if (!path)
+		return;
+	if (!opt->follow_pathspec_slab) {
+		opt->follow_pathspec_slab = xmalloc(sizeof(*opt->follow_pathspec_slab));
+		init_follow_pathspec_slab(opt->follow_pathspec_slab);
+	}
+	slot = follow_pathspec_slab_at(opt->follow_pathspec_slab, c);
+	if (*slot && !strcmp(*slot, path))
+		return;
+	free(*slot);
+	*slot = xstrdup(path);
+}
+
+static const char *recall_follow_pathspec(struct rev_info *opt,
+					  struct commit *c)
+{
+	char **slot;
+
+	if (!opt->follow_pathspec_slab)
+		return NULL;
+	slot = follow_pathspec_slab_peek(opt->follow_pathspec_slab, c);
+	return slot ? *slot : NULL;
+}
+
+static void free_follow_pathspec_slot(char **slot)
+{
+	FREE_AND_NULL(*slot);
+}
+
+void release_follow_pathspec_slab(struct rev_info *opt)
+{
+	if (!opt->follow_pathspec_slab)
+		return;
+	deep_clear_follow_pathspec_slab(opt->follow_pathspec_slab,
+					free_follow_pathspec_slot);
+	FREE_AND_NULL(opt->follow_pathspec_slab);
+}
+
+/* Compute a path to follow in parent, if there is one */
+static void propagate_follow_pathspec_to_parent(struct rev_info *opt,
+						struct commit *commit,
+						struct commit *parent)
+{
+	struct diff_options diff_opts;
+	const char *path;
+
+	parse_commit_or_die(parent);
+	repo_diff_setup(opt->diffopt.repo, &diff_opts);
+	copy_pathspec(&diff_opts.pathspec, &opt->diffopt.pathspec);
+	diff_opts.flags.recursive = 1;
+	diff_opts.flags.follow_renames = 1;
+	diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
+	diff_setup_done(&diff_opts);
+	diff_tree_oid(get_commit_tree_oid(parent),
+		      get_commit_tree_oid(commit),
+		      "", &diff_opts);
+
+	path = pathspec_single_path(&diff_opts.pathspec);
+	if (path)
+		remember_follow_pathspec(opt, parent, path);
+
+	diff_queue_clear(&diff_queued_diff);
+	diff_free(&diff_opts);
+}
+
 /*
  * Show the diff of a commit.
  *
@@ -1185,6 +1276,16 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
 	opt->loginfo = &log;
 	opt->diffopt.no_free = 1;
 
+	/* Any recorded path for this commit? If so, restore it */
+	if (opt->diffopt.flags.follow_renames) {
+		const char *stored = recall_follow_pathspec(opt, commit);
+		if (stored) {
+			const char *current = pathspec_single_path(&opt->diffopt.pathspec);
+			if (!current || strcmp(current, stored))
+				set_pathspec_to_single_path(&opt->diffopt.pathspec, stored);
+		}
+	}
+
 	if (opt->track_linear && !opt->linear && !opt->reverse_output_stage)
 		fprintf(opt->diffopt.file, "\n%s\n", opt->break_bar);
 	shown = log_tree_diff(opt, commit, &log);
@@ -1197,6 +1298,21 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
 		fprintf(opt->diffopt.file, "\n%s\n", opt->break_bar);
 	if (shown)
 		show_diff_of_diff(opt);
+
+	/* Record what path each parent of this commit should use */
+	if (opt->diffopt.flags.follow_renames) {
+		struct commit_list *parents = get_saved_parents(opt, commit);
+		if (parents && parents->next) {
+			struct commit_list *p;
+			for (p = parents; p; p = p->next)
+				propagate_follow_pathspec_to_parent(opt, commit,
+								    p->item);
+		} else if (parents) {
+			remember_follow_pathspec(opt, parents->item,
+				pathspec_single_path(&opt->diffopt.pathspec));
+		}
+	}
+
 	opt->loginfo = NULL;
 	maybe_flush_or_die(opt->diffopt.file, "stdout");
 	opt->diffopt.no_free = no_free;
diff --git a/log-tree.h b/log-tree.h
index 07924be8bc..e8679b6c4a 100644
--- a/log-tree.h
+++ b/log-tree.h
@@ -26,6 +26,7 @@ struct decoration_options {
 int parse_decorate_color_config(const char *var, const char *slot_name, const char *value);
 int log_tree_diff_flush(struct rev_info *);
 int log_tree_commit(struct rev_info *, struct commit *);
+void release_follow_pathspec_slab(struct rev_info *);
 void show_log(struct rev_info *opt);
 void format_decorations(struct strbuf *sb, const struct commit *commit,
 			enum git_colorbool use_color, const struct decoration_options *opts);
diff --git a/revision.c b/revision.c
index e91d7e1f11..0c95edef59 100644
--- a/revision.c
+++ b/revision.c
@@ -26,6 +26,7 @@
 #include "decorate.h"
 #include "string-list.h"
 #include "line-log.h"
+#include "log-tree.h"
 #include "mailmap.h"
 #include "commit-slab.h"
 #include "cache-tree.h"
@@ -3304,6 +3305,7 @@ void release_revisions(struct rev_info *revs)
 	line_log_free(revs);
 	oidset_clear(&revs->missing_commits);
 	release_revisions_bloom_keyvecs(revs);
+	release_follow_pathspec_slab(revs);
 }
 
 static void add_child(struct rev_info *revs, struct commit *parent, struct commit *child)
diff --git a/revision.h b/revision.h
index 00c392be37..569b3fa1cb 100644
--- a/revision.h
+++ b/revision.h
@@ -66,6 +66,7 @@ struct repository;
 struct rev_info;
 struct string_list;
 struct saved_parents;
+struct follow_pathspec_slab;
 struct bloom_keyvec;
 struct bloom_filter_settings;
 struct option;
@@ -363,6 +364,9 @@ struct rev_info {
 	/* copies of the parent lists, for --full-diff display */
 	struct saved_parents *saved_parents_slab;
 
+	/* per-commit pathspec for --follow across merges */
+	struct follow_pathspec_slab *follow_pathspec_slab;
+
 	struct commit_list *previous_parents;
 	struct commit_list *ancestry_path_bottoms;
 	const char *break_bar;
diff --git a/t/meson.build b/t/meson.build
index 3219264fe7..b6ac49b443 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -576,6 +576,7 @@ integration_tests = [
   't4215-log-skewed-merges.sh',
   't4216-log-bloom.sh',
   't4217-log-limit.sh',
+  't4219-log-follow-merge.sh',
   't4252-am-options.sh',
   't4253-am-keep-cr-dos.sh',
   't4254-am-corrupt.sh',
diff --git a/t/t4219-log-follow-merge.sh b/t/t4219-log-follow-merge.sh
new file mode 100755
index 0000000000..e370f82955
--- /dev/null
+++ b/t/t4219-log-follow-merge.sh
@@ -0,0 +1,129 @@
+#!/bin/sh
+
+test_description='Test --follow follows renames across 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_expect_success 'setup merge of two branches that both renamed a file to README' '
+	git init foo &&
+	mkdir foo/foo &&
+	echo "foo readme" >foo/foo/README &&
+	git -C foo add foo/README &&
+	git -C foo commit -m "add foo README" &&
+
+	git -C foo mv foo/README README &&
+	git -C foo commit -m "promote foo README to toplevel" &&
+
+	echo "foo c" >foo/foo.c &&
+	git -C foo add foo.c &&
+	git -C foo commit -m "add foo C impl" &&
+
+	git init bar &&
+	mkdir bar/bar &&
+	echo "bar readme" >bar/bar/README &&
+	git -C bar add bar/README &&
+	git -C bar commit -m "add bar README" &&
+
+	git -C bar mv bar/README README &&
+	git -C bar commit -m "promote bar README to toplevel" &&
+
+	echo "bar c" >bar/bar.c &&
+	git -C bar add bar.c &&
+	git -C bar commit -m "add bar C impl" &&
+
+	git -C foo fetch ../bar master &&
+	git -C foo merge -s ours --no-commit --allow-unrelated-histories \
+		FETCH_HEAD &&
+	git -C foo checkout FETCH_HEAD -- bar.c &&
+	git -C foo commit -m "merge bar into foo"
+'
+
+test_expect_success '--follow follows renames across both sides of a merge' '
+	git -C foo log --follow --pretty=tformat:%s README >actual &&
+	sort actual >actual.sorted &&
+	cat >expect <<-\EOF &&
+	add bar README
+	add foo README
+	promote bar README to toplevel
+	promote foo README to toplevel
+	EOF
+	test_cmp expect actual.sorted
+'
+
+test_expect_success 'setup diamond with renames on both sides of a fork' '
+	git init diamond &&
+	test_lines="line 1\nline 2\nline 3\nline 4\nline 5\n" &&
+
+	printf "$test_lines" >diamond/path0 &&
+	git -C diamond add path0 &&
+	git -C diamond commit -m "A: add path0" &&
+
+	git -C diamond checkout -b upper &&
+	printf "line 1\nline 2\nline 3 modified by B\nline 4\nline 5\n" \
+		>diamond/path0 &&
+	git -C diamond commit -am "B: modify path0 on upper" &&
+	git -C diamond mv path0 path1 &&
+	git -C diamond commit -m "X: rename path0 to path1" &&
+
+	git -C diamond checkout -b lower master &&
+	printf "line 1\nline 2\nline 3 modified by C\nline 4\nline 5\n" \
+		>diamond/path0 &&
+	git -C diamond commit -am "C: modify path0 on lower" &&
+	git -C diamond mv path0 path2 &&
+	git -C diamond commit -m "Y: rename path0 to path2" &&
+
+	git -C diamond checkout upper &&
+	git -C diamond merge -s ours --no-commit lower &&
+	git -C diamond rm path1 &&
+	printf "line 1\nline 2\nline 3 merged\nline 4\nline 5\n" \
+		>diamond/path &&
+	git -C diamond add path &&
+	git -C diamond commit -m "M: merge with rename to path" &&
+
+	printf "line 1\nline 2\nline 3 merged again\nline 4\nline 5\n" \
+		>diamond/path &&
+	git -C diamond commit -am "Z: modify path"
+'
+
+test_expect_success '--follow follows renames through a fork in a single history' '
+	git -C diamond log --follow --pretty=tformat:%s path >actual &&
+	sort actual >actual.sorted &&
+	cat >expect <<-\EOF &&
+	A: add path0
+	B: modify path0 on upper
+	C: modify path0 on lower
+	X: rename path0 to path1
+	Y: rename path0 to path2
+	Z: modify path
+	EOF
+	test_cmp expect actual.sorted
+'
+
+test_done
-- 
2.51.0