merge: remember conflict labels

When recreating merge conflicts with "git checkout -m <path>" the
original conflict labels are lost. For commands like "git merge" and
"git cherry-pick" we could use the presence of the related root
ref (MERGE_HEAD and CHERRY_PICK_HEAD respectively) to recreate the
labels. However, if the conflicts are from "git stash pop" or "git
checkout -m <branch>", then there is no ref to deduce the labels from. To
ensure the labels are always available, the merge machinery is updated to
write ".git/MERGE_LABELS" when it updates the worktree and
there are conflicts. The labels are then read from that file by "git
checkout -m <path>" when recreating the conflicts.

As "git checkout -m <branch>" calls remove_branch_state() which
ordinarily removes the labels file, we need to pass a flag down
to optionally prevent that so that the labels are available for any
subsequent "git checkout -m <path>". Note that merge_switch_to_result()
we assign "result->priv" to "opt->priv" and later clear "opt->priv" in
order to get a pointer to the private struct as result->priv is void*.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -860,9 +860,11 @@ out:
 	free(branch_point);
 }
 
-void remove_merge_branch_state(struct repository *r)
+static void do_remove_merge_branch_state(struct repository *r, unsigned flags)
 {
 	unlink(git_path_merge_head(r));
+	if (!(flags & REMOVE_BRANCH_STATE_PRESERVE_CONFLICT_LABELS))
+		unlink(git_path_merge_labels(r));
 	unlink(git_path_merge_rr(r));
 	unlink(git_path_merge_msg(r));
 	unlink(git_path_merge_mode(r));
@@ -871,11 +873,16 @@ void remove_merge_branch_state(struct repository *r)
 	save_autostash_ref(r, "MERGE_AUTOSTASH");
 }
 
+void remove_merge_branch_state(struct repository *r)
+{
+	do_remove_merge_branch_state(r, 0);
+}
+
 void remove_branch_state(struct repository *r, unsigned flags)
 {
 	sequencer_post_commit_cleanup(r, flags & REMOVE_BRANCH_STATE_VERBOSE);
 	unlink(git_path_squash_msg(r));
-	remove_merge_branch_state(r);
+	do_remove_merge_branch_state(r, flags);
 }
 
 void die_if_checked_out(const char *branch, int ignore_current_worktree)

branch.h

@@ -128,6 +128,7 @@ int validate_branchname(const char *name, struct strbuf *ref);
 int validate_new_branchname(const char *name, struct strbuf *ref, int force);
 
 #define REMOVE_BRANCH_STATE_VERBOSE (1u << 0)
+#define REMOVE_BRANCH_STATE_PRESERVE_CONFLICT_LABELS (1u << 1)
 /*
  * Remove information about the merge state on the current
  * branch. (E.g., MERGE_HEAD)

builtin/checkout.c

@@ -15,6 +15,7 @@
 #include "hex.h"
 #include "hook.h"
 #include "merge-ll.h"
+#include "merge.h"
 #include "lockfile.h"
 #include "mem-pool.h"
 #include "object-file.h"
@@ -313,14 +314,28 @@ static int checkout_stage(int stage, const struct cache_entry *ce, int pos,
 struct checkout_merged_data {
 	int conflict_style;
 	int renormalize;
+	char *base_label, *ours_label, *theirs_label;
 };
 
+static void checkout_merged_release(struct checkout_merged_data *data)
+{
+	free(data->base_label);
+	free(data->ours_label);
+	free(data->theirs_label);
+}
+
 static void checkout_merged_prepare(struct checkout_merged_data *data)
 {
 	int renormalize = 0;
 
 	repo_config_get_bool(the_repository, "merge.renormalize", &renormalize);
 	data->renormalize = renormalize;
+	if (read_merge_labels(the_repository, &data->base_label, &data->ours_label,
+			      &data->theirs_label)) {
+		data->base_label = xstrdup("base");
+		data->ours_label = xstrdup("ours");
+		data->theirs_label = xstrdup("theirs");
+	}
 }
 
 static int checkout_merged(int pos, const struct checkout *state,
@@ -359,8 +374,8 @@ static int checkout_merged(int pos, const struct checkout *state,
 
 	ll_opts.renormalize = data->renormalize;
 	ll_opts.conflict_style = data->conflict_style;
-	merge_status = ll_merge(&result_buf, path, &ancestor, "base",
-				&ours, "ours", &theirs, "theirs",
+	merge_status = ll_merge(&result_buf, path, &ancestor, data->base_label,
+				&ours, data->ours_label, &theirs, data->theirs_label,
 				state->istate, &ll_opts);
 	free(ancestor.ptr);
 	free(ours.ptr);
@@ -499,6 +514,10 @@ static int checkout_worktree(const struct checkout_opts *opts,
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
+
+	if (opts->merge)
+		checkout_merged_release(&checkout_merged_data);
+
 	if (pc_workers > 1)
 		errs |= run_parallel_checkout(&state, pc_workers, pc_threshold,
 					      NULL, NULL);
@@ -962,7 +981,8 @@ static void report_tracking(struct branch_info *new_branch_info)
 
 static void update_refs_for_switch(const struct checkout_opts *opts,
 				   struct branch_info *old_branch_info,
-				   struct branch_info *new_branch_info)
+				   struct branch_info *new_branch_info,
+				   bool merge_conflicts)
 {
 	struct strbuf msg = STRBUF_INIT;
 	const char *old_desc, *reflog_msg;
@@ -1064,6 +1084,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 	}
 	if (!opts->quiet)
 		flags |= REMOVE_BRANCH_STATE_VERBOSE;
+	if (merge_conflicts)
+		flags |= REMOVE_BRANCH_STATE_PRESERVE_CONFLICT_LABELS;
 	remove_branch_state(the_repository, flags);
 	strbuf_release(&msg);
 	if (!opts->quiet &&
@@ -1278,7 +1300,9 @@ static int switch_branches(const struct checkout_opts *opts,
 
 	if (autostash_res == STASH_APPLY_CONFLICT && !opts->quiet)
 		fputc('\n', stderr);
-	update_refs_for_switch(opts, &old_branch_info, new_branch_info);
+
+	update_refs_for_switch(opts, &old_branch_info, new_branch_info,
+			       autostash_res == STASH_APPLY_CONFLICT);
 
 	if (created_autostash) {
 		discard_index(the_repository->index);

builtin/commit.c

@@ -1950,6 +1950,7 @@ int cmd_commit(int argc,
 
 	sequencer_post_commit_cleanup(the_repository, 0);
 	unlink(git_path_merge_head(the_repository));
+	unlink(git_path_merge_labels(the_repository));
 	unlink(git_path_merge_msg(the_repository));
 	unlink(git_path_merge_mode(the_repository));
 	unlink(git_path_squash_msg(the_repository));

merge-ort.c

@@ -35,6 +35,7 @@
 #include "hex.h"
 #include "entry.h"
 #include "merge-ll.h"
+#include "merge.h"
 #include "match-trees.h"
 #include "mem-pool.h"
 #include "object-file.h"
@@ -418,6 +419,9 @@ struct merge_options_internal {
 
 	/* field that holds submodule conflict information */
 	struct string_list conflicted_submodules;
+
+	/* Copies of the labels used for conflict markers */
+	const char *labels[3];
 };
 
 struct conflicted_submodule_item {
@@ -4969,6 +4973,14 @@ void merge_switch_to_result(struct merge_options *opt,
 			return;
 		}
 		trace2_region_leave("merge", "write_auto_merge", opt->repo);
+
+		if (!result->clean) {
+			struct merge_options_internal *priv = result->priv;
+
+			trace2_region_enter("merge", "write_merge_labels", opt->repo);
+			write_merge_labels(opt->repo, priv->labels);
+			trace2_region_leave("merge", "write_merge_labels", opt->repo);
+		}
 	}
 	if (display_update_msgs)
 		merge_display_update_messages(opt, /* detailed */ 0, result);
@@ -5234,6 +5246,14 @@ static void move_opt_priv_to_result_priv(struct merge_options *opt,
 	 * to move it.
 	 */
 	assert(opt->priv && !result->priv);
+	if (!result->clean) {
+		opt->priv->labels[0] =
+			mem_pool_strdup(&opt->priv->pool, opt->ancestor);
+		opt->priv->labels[1] =
+			mem_pool_strdup(&opt->priv->pool, opt->branch1);
+		opt->priv->labels[2] =
+			mem_pool_strdup(&opt->priv->pool, opt->branch2);
+	}
 	result->priv = opt->priv;
 	result->_properly_initialized = RESULT_INITIALIZED;
 	opt->priv = NULL;

merge.c

@@ -8,6 +8,7 @@
 #include "merge.h"
 #include "commit.h"
 #include "repository.h"
+#include "path.h"
 #include "run-command.h"
 #include "resolve-undo.h"
 #include "tree.h"
@@ -111,3 +112,68 @@ int checkout_fast_forward(struct repository *r,
 		return error(_("unable to write new index file"));
 	return 0;
 }
+
+int write_merge_labels(struct repository *r, const char *labels[3])
+{
+	FILE *f = fopen_or_warn(git_path_merge_labels(r), "w");
+
+	if (!f)
+		return -1;
+
+	fprintf(f, "%s\n%s\n%s\n", labels[0], labels[1], labels[2]);
+	if (fclose(f))
+		return error_errno("could not write '%s'",
+				   git_path_merge_labels(r));
+
+	return 0;
+}
+
+static char *parse_merge_label_line(FILE *fp)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	if (strbuf_getline(&buf, fp) == EOF) {
+		strbuf_release(&buf);
+		return NULL;
+	}
+
+	return strbuf_detach(&buf, NULL);
+}
+
+int read_merge_labels(struct repository *r,
+		      char **pbase, char **pours, char **ptheirs)
+{
+	char *base = NULL, *ours = NULL, *theirs = NULL;
+	int ret = -1;
+	FILE *fp = fopen(git_path_merge_labels(r), "r");
+
+	if (!fp)
+		return -1;
+
+	base = parse_merge_label_line(fp);
+	if (!base)
+		goto out;
+
+	ours = parse_merge_label_line(fp);
+	if (!ours)
+		goto out;
+
+	theirs = parse_merge_label_line(fp);
+	if (!theirs)
+		goto out;
+	/* We ignore any trailing lines */
+
+	ret = 0;
+	*pbase = base;
+	*pours = ours;
+	*ptheirs = theirs;
+out:
+	if (ret) {
+		free(base);
+		free(ours);
+		free(theirs);
+	}
+	fclose(fp);
+
+	return ret;
+}

merge.h

@@ -13,5 +13,8 @@ int checkout_fast_forward(struct repository *r,
 			  const struct object_id *from,
 			  const struct object_id *to,
 			  int overwrite_ignore);
+int write_merge_labels(struct repository *r, const char *labels[3]);
+int read_merge_labels(struct repository *r,
+		      char **base, char **ours, char **theirs);
 
 #endif /* MERGE_H */

path.c

@@ -1655,3 +1655,4 @@ REPO_GIT_PATH_FUNC(merge_mode, "MERGE_MODE")
 REPO_GIT_PATH_FUNC(merge_head, "MERGE_HEAD")
 REPO_GIT_PATH_FUNC(fetch_head, "FETCH_HEAD")
 REPO_GIT_PATH_FUNC(shallow, "shallow")
+REPO_GIT_PATH_FUNC(merge_labels, "MERGE_LABELS")

path.h

@@ -142,6 +142,7 @@ const char *git_path_merge_mode(struct repository *r);
 const char *git_path_merge_head(struct repository *r);
 const char *git_path_fetch_head(struct repository *r);
 const char *git_path_shallow(struct repository *r);
+const char *git_path_merge_labels(struct repository *r);
 
 int ends_with_path_components(const char *path, const char *components);
 

repository.c

@@ -367,6 +367,7 @@ static void repo_clear_path_cache(struct repo_path_cache *cache)
 	FREE_AND_NULL(cache->merge_head);
 	FREE_AND_NULL(cache->fetch_head);
 	FREE_AND_NULL(cache->shallow);
+	FREE_AND_NULL(cache->merge_labels);
 }
 
 void repo_clear(struct repository *repo)

repository.h

@@ -36,6 +36,7 @@ struct repo_path_cache {
 	char *merge_head;
 	char *fetch_head;
 	char *shallow;
+	char *merge_labels;
 };
 
 struct repository {

sequencer.c

@@ -5138,6 +5138,7 @@ static int pick_commits(struct repository *r,
 	unlink(rebase_path_stopped_sha());
 	unlink(rebase_path_amend());
 	unlink(rebase_path_patch());
+	unlink(git_path_merge_labels(r));
 
 	while (todo_list->current < todo_list->nr) {
 		struct todo_item *item = todo_list->items + todo_list->current;

t/t7201-co.sh

@@ -99,6 +99,7 @@ test_expect_success 'checkout -m with dirty tree' '
 
 	fill 0 1 2 3 4 5 6 7 8 >one &&
 	git checkout -m side >messages &&
+	test_path_is_missing .git/MERGE_LABELS &&
 
 	test "$(git symbolic-ref HEAD)" = "refs/heads/side" &&
 
@@ -183,6 +184,27 @@ test_expect_success 'format of merge conflict from checkout -m' '
 	d
 	>>>>>>> local
 	EOF
+	test_cmp expect two &&
+
+	test_path_is_file .git/MERGE_LABELS &&
+
+	git checkout --conflict=diff3 two &&
+	cat >expect <<-\EOF &&
+	<<<<<<< simple
+	a
+	c
+	e
+	||||||| main
+	a
+	b
+	c
+	d
+	e
+	=======
+	b
+	d
+	>>>>>>> local
+	EOF
 	test_cmp expect two
 '