1 files changed, 591 insertions, 61 deletions

diff --git a/submodule.c b/submodule.c
index 7d70c4f7bf..5294cef641 100644
--- a/submodule.c
+++ b/submodule.c
@@ -5,14 +5,30 @@
 #include "commit.h"
 #include "revision.h"
 #include "run-command.h"
+#include "diffcore.h"
+#include "refs.h"
+#include "string-list.h"
+
+static struct string_list config_name_for_path;
+static struct string_list config_fetch_recurse_submodules_for_name;
+static struct string_list config_ignore_for_name;
+static int config_fetch_recurse_submodules = RECURSE_SUBMODULES_ON_DEMAND;
+static struct string_list changed_submodule_paths;
 
 static int add_submodule_odb(const char *path)
 {
 	struct strbuf objects_directory = STRBUF_INIT;
 	struct alternate_object_database *alt_odb;
 	int ret = 0;
+	const char *git_dir;
 
-	strbuf_addf(&objects_directory, "%s/.git/objects/", path);
+	strbuf_addf(&objects_directory, "%s/.git", path);
+	git_dir = read_gitfile_gently(objects_directory.buf);
+	if (git_dir) {
+		strbuf_reset(&objects_directory);
+		strbuf_addstr(&objects_directory, git_dir);
+	}
+	strbuf_addstr(&objects_directory, "/objects/");
 	if (!is_directory(objects_directory.buf)) {
 		ret = -1;
 		goto done;
@@ -38,17 +54,182 @@ done:
 	return ret;
 }
 
+void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
+					     const char *path)
+{
+	struct string_list_item *path_option, *ignore_option;
+	path_option = unsorted_string_list_lookup(&config_name_for_path, path);
+	if (path_option) {
+		ignore_option = unsorted_string_list_lookup(&config_ignore_for_name, path_option->util);
+		if (ignore_option)
+			handle_ignore_submodules_arg(diffopt, ignore_option->util);
+	}
+}
+
+int submodule_config(const char *var, const char *value, void *cb)
+{
+	if (!prefixcmp(var, "submodule."))
+		return parse_submodule_config_option(var, value);
+	else if (!strcmp(var, "fetch.recursesubmodules")) {
+		config_fetch_recurse_submodules = parse_fetch_recurse_submodules_arg(var, value);
+		return 0;
+	}
+	return 0;
+}
+
+void gitmodules_config(void)
+{
+	const char *work_tree = get_git_work_tree();
+	if (work_tree) {
+		struct strbuf gitmodules_path = STRBUF_INIT;
+		strbuf_addstr(&gitmodules_path, work_tree);
+		strbuf_addstr(&gitmodules_path, "/.gitmodules");
+		git_config_from_file(submodule_config, gitmodules_path.buf, NULL);
+		strbuf_release(&gitmodules_path);
+	}
+}
+
+int parse_submodule_config_option(const char *var, const char *value)
+{
+	int len;
+	struct string_list_item *config;
+	struct strbuf submodname = STRBUF_INIT;
+
+	var += 10;		/* Skip "submodule." */
+
+	len = strlen(var);
+	if ((len > 5) && !strcmp(var + len - 5, ".path")) {
+		strbuf_add(&submodname, var, len - 5);
+		config = unsorted_string_list_lookup(&config_name_for_path, value);
+		if (config)
+			free(config->util);
+		else
+			config = string_list_append(&config_name_for_path, xstrdup(value));
+		config->util = strbuf_detach(&submodname, NULL);
+		strbuf_release(&submodname);
+	} else if ((len > 23) && !strcmp(var + len - 23, ".fetchrecursesubmodules")) {
+		strbuf_add(&submodname, var, len - 23);
+		config = unsorted_string_list_lookup(&config_fetch_recurse_submodules_for_name, submodname.buf);
+		if (!config)
+			config = string_list_append(&config_fetch_recurse_submodules_for_name,
+						    strbuf_detach(&submodname, NULL));
+		config->util = (void *)(intptr_t)parse_fetch_recurse_submodules_arg(var, value);
+		strbuf_release(&submodname);
+	} else if ((len > 7) && !strcmp(var + len - 7, ".ignore")) {
+		if (strcmp(value, "untracked") && strcmp(value, "dirty") &&
+		    strcmp(value, "all") && strcmp(value, "none")) {
+			warning("Invalid parameter \"%s\" for config option \"submodule.%s.ignore\"", value, var);
+			return 0;
+		}
+
+		strbuf_add(&submodname, var, len - 7);
+		config = unsorted_string_list_lookup(&config_ignore_for_name, submodname.buf);
+		if (config)
+			free(config->util);
+		else
+			config = string_list_append(&config_ignore_for_name,
+						    strbuf_detach(&submodname, NULL));
+		strbuf_release(&submodname);
+		config->util = xstrdup(value);
+		return 0;
+	}
+	return 0;
+}
+
+void handle_ignore_submodules_arg(struct diff_options *diffopt,
+				  const char *arg)
+{
+	DIFF_OPT_CLR(diffopt, IGNORE_SUBMODULES);
+	DIFF_OPT_CLR(diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
+	DIFF_OPT_CLR(diffopt, IGNORE_DIRTY_SUBMODULES);
+
+	if (!strcmp(arg, "all"))
+		DIFF_OPT_SET(diffopt, IGNORE_SUBMODULES);
+	else if (!strcmp(arg, "untracked"))
+		DIFF_OPT_SET(diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
+	else if (!strcmp(arg, "dirty"))
+		DIFF_OPT_SET(diffopt, IGNORE_DIRTY_SUBMODULES);
+	else if (strcmp(arg, "none"))
+		die("bad --ignore-submodules argument: %s", arg);
+}
+
+static int prepare_submodule_summary(struct rev_info *rev, const char *path,
+		struct commit *left, struct commit *right,
+		int *fast_forward, int *fast_backward)
+{
+	struct commit_list *merge_bases, *list;
+
+	init_revisions(rev, NULL);
+	setup_revisions(0, NULL, rev, NULL);
+	rev->left_right = 1;
+	rev->first_parent_only = 1;
+	left->object.flags |= SYMMETRIC_LEFT;
+	add_pending_object(rev, &left->object, path);
+	add_pending_object(rev, &right->object, path);
+	merge_bases = get_merge_bases(left, right, 1);
+	if (merge_bases) {
+		if (merge_bases->item == left)
+			*fast_forward = 1;
+		else if (merge_bases->item == right)
+			*fast_backward = 1;
+	}
+	for (list = merge_bases; list; list = list->next) {
+		list->item->object.flags |= UNINTERESTING;
+		add_pending_object(rev, &list->item->object,
+			sha1_to_hex(list->item->object.sha1));
+	}
+	return prepare_revision_walk(rev);
+}
+
+static void print_submodule_summary(struct rev_info *rev, FILE *f,
+		const char *del, const char *add, const char *reset)
+{
+	static const char format[] = "  %m %s";
+	struct strbuf sb = STRBUF_INIT;
+	struct commit *commit;
+
+	while ((commit = get_revision(rev))) {
+		struct pretty_print_context ctx = {0};
+		ctx.date_mode = rev->date_mode;
+		strbuf_setlen(&sb, 0);
+		if (commit->object.flags & SYMMETRIC_LEFT) {
+			if (del)
+				strbuf_addstr(&sb, del);
+		}
+		else if (add)
+			strbuf_addstr(&sb, add);
+		format_commit_message(commit, format, &sb, &ctx);
+		if (reset)
+			strbuf_addstr(&sb, reset);
+		strbuf_addch(&sb, '\n');
+		fprintf(f, "%s", sb.buf);
+	}
+	strbuf_release(&sb);
+}
+
+int parse_fetch_recurse_submodules_arg(const char *opt, const char *arg)
+{
+	switch (git_config_maybe_bool(opt, arg)) {
+	case 1:
+		return RECURSE_SUBMODULES_ON;
+	case 0:
+		return RECURSE_SUBMODULES_OFF;
+	default:
+		if (!strcmp(arg, "on-demand"))
+			return RECURSE_SUBMODULES_ON_DEMAND;
+		die("bad %s argument: %s", opt, arg);
+	}
+}
+
 void show_submodule_summary(FILE *f, const char *path,
 		unsigned char one[20], unsigned char two[20],
 		unsigned dirty_submodule,
 		const char *del, const char *add, const char *reset)
 {
 	struct rev_info rev;
-	struct commit *commit, *left = left, *right = right;
-	struct commit_list *merge_bases, *list;
+	struct commit *left = left, *right = right;
 	const char *message = NULL;
 	struct strbuf sb = STRBUF_INIT;
-	static const char *format = "  %m %s";
 	int fast_forward = 0, fast_backward = 0;
 
 	if (is_null_sha1(two))
@@ -61,28 +242,19 @@ void show_submodule_summary(FILE *f, const char *path,
 		 !(right = lookup_commit_reference(two)))
 		message = "(commits not present)";
 
-	if (!message) {
-		init_revisions(&rev, NULL);
-		setup_revisions(0, NULL, &rev, NULL);
-		rev.left_right = 1;
-		rev.first_parent_only = 1;
-		left->object.flags |= SYMMETRIC_LEFT;
-		add_pending_object(&rev, &left->object, path);
-		add_pending_object(&rev, &right->object, path);
-		merge_bases = get_merge_bases(left, right, 1);
-		if (merge_bases) {
-			if (merge_bases->item == left)
-				fast_forward = 1;
-			else if (merge_bases->item == right)
-				fast_backward = 1;
-		}
-		for (list = merge_bases; list; list = list->next) {
-			list->item->object.flags |= UNINTERESTING;
-			add_pending_object(&rev, &list->item->object,
-				sha1_to_hex(list->item->object.sha1));
-		}
-		if (prepare_revision_walk(&rev))
-			message = "(revision walker failed)";
+	if (!message &&
+	    prepare_submodule_summary(&rev, path, left, right,
+					&fast_forward, &fast_backward))
+		message = "(revision walker failed)";
+
+	if (dirty_submodule & DIRTY_SUBMODULE_UNTRACKED)
+		fprintf(f, "Submodule %s contains untracked content\n", path);
+	if (dirty_submodule & DIRTY_SUBMODULE_MODIFIED)
+		fprintf(f, "Submodule %s contains modified content\n", path);
+
+	if (!hashcmp(one, two)) {
+		strbuf_release(&sb);
+		return;
 	}
 
 	strbuf_addf(&sb, "Submodule %s %s..", path,
@@ -90,8 +262,6 @@ void show_submodule_summary(FILE *f, const char *path,
 	if (!fast_backward && !fast_forward)
 		strbuf_addch(&sb, '.');
 	strbuf_addf(&sb, "%s", find_unique_abbrev(two, DEFAULT_ABBREV));
-	if (dirty_submodule)
-		strbuf_add(&sb, "-dirty", 6);
 	if (message)
 		strbuf_addf(&sb, " %s\n", message);
 	else
@@ -99,42 +269,230 @@ void show_submodule_summary(FILE *f, const char *path,
 	fwrite(sb.buf, sb.len, 1, f);
 
 	if (!message) {
-		while ((commit = get_revision(&rev))) {
-			struct pretty_print_context ctx = {0};
-			ctx.date_mode = rev.date_mode;
-			strbuf_setlen(&sb, 0);
-			if (commit->object.flags & SYMMETRIC_LEFT) {
-				if (del)
-					strbuf_addstr(&sb, del);
-			}
-			else if (add)
-				strbuf_addstr(&sb, add);
-			format_commit_message(commit, format, &sb, &ctx);
-			if (reset)
-				strbuf_addstr(&sb, reset);
-			strbuf_addch(&sb, '\n');
-			fprintf(f, "%s", sb.buf);
-		}
+		print_submodule_summary(&rev, f, del, add, reset);
 		clear_commit_marks(left, ~0);
 		clear_commit_marks(right, ~0);
 	}
+
 	strbuf_release(&sb);
 }
 
-int is_submodule_modified(const char *path)
+void set_config_fetch_recurse_submodules(int value)
 {
-	int len;
+	config_fetch_recurse_submodules = value;
+}
+
+static int is_submodule_commit_present(const char *path, unsigned char sha1[20])
+{
+	int is_present = 0;
+	if (!add_submodule_odb(path) && lookup_commit_reference(sha1)) {
+		/* Even if the submodule is checked out and the commit is
+		 * present, make sure it is reachable from a ref. */
+		struct child_process cp;
+		const char *argv[] = {"rev-list", "-n", "1", NULL, "--not", "--all", NULL};
+		struct strbuf buf = STRBUF_INIT;
+
+		argv[3] = sha1_to_hex(sha1);
+		memset(&cp, 0, sizeof(cp));
+		cp.argv = argv;
+		cp.env = local_repo_env;
+		cp.git_cmd = 1;
+		cp.no_stdin = 1;
+		cp.out = -1;
+		cp.dir = path;
+		if (!run_command(&cp) && !strbuf_read(&buf, cp.out, 1024))
+			is_present = 1;
+
+		close(cp.out);
+		strbuf_release(&buf);
+	}
+	return is_present;
+}
+
+static void submodule_collect_changed_cb(struct diff_queue_struct *q,
+					 struct diff_options *options,
+					 void *data)
+{
+	int i;
+	for (i = 0; i < q->nr; i++) {
+		struct diff_filepair *p = q->queue[i];
+		if (!S_ISGITLINK(p->two->mode))
+			continue;
+
+		if (S_ISGITLINK(p->one->mode)) {
+			/* NEEDSWORK: We should honor the name configured in
+			 * the .gitmodules file of the commit we are examining
+			 * here to be able to correctly follow submodules
+			 * being moved around. */
+			struct string_list_item *path;
+			path = unsorted_string_list_lookup(&changed_submodule_paths, p->two->path);
+			if (!path && !is_submodule_commit_present(p->two->path, p->two->sha1))
+				string_list_append(&changed_submodule_paths, xstrdup(p->two->path));
+		} else {
+			/* Submodule is new or was moved here */
+			/* NEEDSWORK: When the .git directories of submodules
+			 * live inside the superprojects .git directory some
+			 * day we should fetch new submodules directly into
+			 * that location too when config or options request
+			 * that so they can be checked out from there. */
+			continue;
+		}
+	}
+}
+
+void check_for_new_submodule_commits(unsigned char new_sha1[20])
+{
+	struct rev_info rev;
+	struct commit *commit;
+	const char *argv[] = {NULL, NULL, "--not", "--all", NULL};
+	int argc = ARRAY_SIZE(argv) - 1;
+
+	init_revisions(&rev, NULL);
+	argv[1] = xstrdup(sha1_to_hex(new_sha1));
+	setup_revisions(argc, argv, &rev, NULL);
+	if (prepare_revision_walk(&rev))
+		die("revision walk setup failed");
+
+	/*
+	 * Collect all submodules (whether checked out or not) for which new
+	 * commits have been recorded upstream in "changed_submodule_paths".
+	 */
+	while ((commit = get_revision(&rev))) {
+		struct commit_list *parent = commit->parents;
+		while (parent) {
+			struct diff_options diff_opts;
+			diff_setup(&diff_opts);
+			diff_opts.output_format |= DIFF_FORMAT_CALLBACK;
+			diff_opts.format_callback = submodule_collect_changed_cb;
+			if (diff_setup_done(&diff_opts) < 0)
+				die("diff_setup_done failed");
+			diff_tree_sha1(parent->item->object.sha1, commit->object.sha1, "", &diff_opts);
+			diffcore_std(&diff_opts);
+			diff_flush(&diff_opts);
+			parent = parent->next;
+		}
+	}
+	free((char *)argv[1]);
+}
+
+int fetch_populated_submodules(int num_options, const char **options,
+			       const char *prefix, int command_line_option,
+			       int quiet)
+{
+	int i, result = 0, argc = 0, default_argc;
+	struct child_process cp;
+	const char **argv;
+	struct string_list_item *name_for_path;
+	const char *work_tree = get_git_work_tree();
+	if (!work_tree)
+		goto out;
+
+	if (!the_index.initialized)
+		if (read_cache() < 0)
+			die("index file corrupt");
+
+	/* 6: "fetch" (options) --recurse-submodules-default default "--submodule-prefix" prefix NULL */
+	argv = xcalloc(num_options + 6, sizeof(const char *));
+	argv[argc++] = "fetch";
+	for (i = 0; i < num_options; i++)
+		argv[argc++] = options[i];
+	argv[argc++] = "--recurse-submodules-default";
+	default_argc = argc++;
+	argv[argc++] = "--submodule-prefix";
+
+	memset(&cp, 0, sizeof(cp));
+	cp.argv = argv;
+	cp.env = local_repo_env;
+	cp.git_cmd = 1;
+	cp.no_stdin = 1;
+
+	for (i = 0; i < active_nr; i++) {
+		struct strbuf submodule_path = STRBUF_INIT;
+		struct strbuf submodule_git_dir = STRBUF_INIT;
+		struct strbuf submodule_prefix = STRBUF_INIT;
+		struct cache_entry *ce = active_cache[i];
+		const char *git_dir, *name, *default_argv;
+
+		if (!S_ISGITLINK(ce->ce_mode))
+			continue;
+
+		name = ce->name;
+		name_for_path = unsorted_string_list_lookup(&config_name_for_path, ce->name);
+		if (name_for_path)
+			name = name_for_path->util;
+
+		default_argv = "yes";
+		if (command_line_option == RECURSE_SUBMODULES_DEFAULT) {
+			struct string_list_item *fetch_recurse_submodules_option;
+			fetch_recurse_submodules_option = unsorted_string_list_lookup(&config_fetch_recurse_submodules_for_name, name);
+			if (fetch_recurse_submodules_option) {
+				if ((intptr_t)fetch_recurse_submodules_option->util == RECURSE_SUBMODULES_OFF)
+					continue;
+				if ((intptr_t)fetch_recurse_submodules_option->util == RECURSE_SUBMODULES_ON_DEMAND) {
+					if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+						continue;
+					default_argv = "on-demand";
+				}
+			} else {
+				if (config_fetch_recurse_submodules == RECURSE_SUBMODULES_OFF)
+					continue;
+				if (config_fetch_recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND) {
+					if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+						continue;
+					default_argv = "on-demand";
+				}
+			}
+		} else if (command_line_option == RECURSE_SUBMODULES_ON_DEMAND) {
+			if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+				continue;
+			default_argv = "on-demand";
+		}
+
+		strbuf_addf(&submodule_path, "%s/%s", work_tree, ce->name);
+		strbuf_addf(&submodule_git_dir, "%s/.git", submodule_path.buf);
+		strbuf_addf(&submodule_prefix, "%s%s/", prefix, ce->name);
+		git_dir = read_gitfile_gently(submodule_git_dir.buf);
+		if (!git_dir)
+			git_dir = submodule_git_dir.buf;
+		if (is_directory(git_dir)) {
+			if (!quiet)
+				printf("Fetching submodule %s%s\n", prefix, ce->name);
+			cp.dir = submodule_path.buf;
+			argv[default_argc] = default_argv;
+			argv[argc] = submodule_prefix.buf;
+			if (run_command(&cp))
+				result = 1;
+		}
+		strbuf_release(&submodule_path);
+		strbuf_release(&submodule_git_dir);
+		strbuf_release(&submodule_prefix);
+	}
+	free(argv);
+out:
+	string_list_clear(&changed_submodule_paths, 1);
+	return result;
+}
+
+unsigned is_submodule_modified(const char *path, int ignore_untracked)
+{
+	ssize_t len;
 	struct child_process cp;
 	const char *argv[] = {
 		"status",
 		"--porcelain",
 		NULL,
+		NULL,
 	};
-	char *env[4];
 	struct strbuf buf = STRBUF_INIT;
+	unsigned dirty_submodule = 0;
+	const char *line, *next_line;
+	const char *git_dir;
 
-	strbuf_addf(&buf, "%s/.git/", path);
-	if (!is_directory(buf.buf)) {
+	strbuf_addf(&buf, "%s/.git", path);
+	git_dir = read_gitfile_gently(buf.buf);
+	if (!git_dir)
+		git_dir = buf.buf;
+	if (!is_directory(git_dir)) {
 		strbuf_release(&buf);
 		/* The submodule is not checked out, so it is not modified */
 		return 0;
@@ -142,32 +500,204 @@ int is_submodule_modified(const char *path)
 	}
 	strbuf_reset(&buf);
 
-	strbuf_addf(&buf, "GIT_WORK_TREE=%s", path);
-	env[0] = strbuf_detach(&buf, NULL);
-	strbuf_addf(&buf, "GIT_DIR=%s/.git", path);
-	env[1] = strbuf_detach(&buf, NULL);
-	strbuf_addf(&buf, "GIT_INDEX_FILE");
-	env[2] = strbuf_detach(&buf, NULL);
-	env[3] = NULL;
+	if (ignore_untracked)
+		argv[2] = "-uno";
 
 	memset(&cp, 0, sizeof(cp));
 	cp.argv = argv;
-	cp.env = (const char *const *)env;
+	cp.env = local_repo_env;
 	cp.git_cmd = 1;
 	cp.no_stdin = 1;
 	cp.out = -1;
+	cp.dir = path;
 	if (start_command(&cp))
 		die("Could not run git status --porcelain");
 
 	len = strbuf_read(&buf, cp.out, 1024);
+	line = buf.buf;
+	while (len > 2) {
+		if ((line[0] == '?') && (line[1] == '?')) {
+			dirty_submodule |= DIRTY_SUBMODULE_UNTRACKED;
+			if (dirty_submodule & DIRTY_SUBMODULE_MODIFIED)
+				break;
+		} else {
+			dirty_submodule |= DIRTY_SUBMODULE_MODIFIED;
+			if (ignore_untracked ||
+			    (dirty_submodule & DIRTY_SUBMODULE_UNTRACKED))
+				break;
+		}
+		next_line = strchr(line, '\n');
+		if (!next_line)
+			break;
+		next_line++;
+		len -= (next_line - line);
+		line = next_line;
+	}
 	close(cp.out);
 
 	if (finish_command(&cp))
 		die("git status --porcelain failed");
 
-	free(env[0]);
-	free(env[1]);
-	free(env[2]);
 	strbuf_release(&buf);
-	return len != 0;
+	return dirty_submodule;
+}
+
+static int find_first_merges(struct object_array *result, const char *path,
+		struct commit *a, struct commit *b)
+{
+	int i, j;
+	struct object_array merges;
+	struct commit *commit;
+	int contains_another;
+
+	char merged_revision[42];
+	const char *rev_args[] = { "rev-list", "--merges", "--ancestry-path",
+				   "--all", merged_revision, NULL };
+	struct rev_info revs;
+	struct setup_revision_opt rev_opts;
+
+	memset(&merges, 0, sizeof(merges));
+	memset(result, 0, sizeof(struct object_array));
+	memset(&rev_opts, 0, sizeof(rev_opts));
+
+	/* get all revisions that merge commit a */
+	snprintf(merged_revision, sizeof(merged_revision), "^%s",
+			sha1_to_hex(a->object.sha1));
+	init_revisions(&revs, NULL);
+	rev_opts.submodule = path;
+	setup_revisions(sizeof(rev_args)/sizeof(char *)-1, rev_args, &revs, &rev_opts);
+
+	/* save all revisions from the above list that contain b */
+	if (prepare_revision_walk(&revs))
+		die("revision walk setup failed");
+	while ((commit = get_revision(&revs)) != NULL) {
+		struct object *o = &(commit->object);
+		if (in_merge_bases(b, &commit, 1))
+			add_object_array(o, NULL, &merges);
+	}
+
+	/* Now we've got all merges that contain a and b. Prune all
+	 * merges that contain another found merge and save them in
+	 * result.
+	 */
+	for (i = 0; i < merges.nr; i++) {
+		struct commit *m1 = (struct commit *) merges.objects[i].item;
+
+		contains_another = 0;
+		for (j = 0; j < merges.nr; j++) {
+			struct commit *m2 = (struct commit *) merges.objects[j].item;
+			if (i != j && in_merge_bases(m2, &m1, 1)) {
+				contains_another = 1;
+				break;
+			}
+		}
+
+		if (!contains_another)
+			add_object_array(merges.objects[i].item,
+					 merges.objects[i].name, result);
+	}
+
+	free(merges.objects);
+	return result->nr;
+}
+
+static void print_commit(struct commit *commit)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct pretty_print_context ctx = {0};
+	ctx.date_mode = DATE_NORMAL;
+	format_commit_message(commit, " %h: %m %s", &sb, &ctx);
+	fprintf(stderr, "%s\n", sb.buf);
+	strbuf_release(&sb);
+}
+
+#define MERGE_WARNING(path, msg) \
+	warning("Failed to merge submodule %s (%s)", path, msg);
+
+int merge_submodule(unsigned char result[20], const char *path,
+		    const unsigned char base[20], const unsigned char a[20],
+		    const unsigned char b[20])
+{
+	struct commit *commit_base, *commit_a, *commit_b;
+	int parent_count;
+	struct object_array merges;
+
+	int i;
+
+	/* store a in result in case we fail */
+	hashcpy(result, a);
+
+	/* we can not handle deletion conflicts */
+	if (is_null_sha1(base))
+		return 0;
+	if (is_null_sha1(a))
+		return 0;
+	if (is_null_sha1(b))
+		return 0;
+
+	if (add_submodule_odb(path)) {
+		MERGE_WARNING(path, "not checked out");
+		return 0;
+	}
+
+	if (!(commit_base = lookup_commit_reference(base)) ||
+	    !(commit_a = lookup_commit_reference(a)) ||
+	    !(commit_b = lookup_commit_reference(b))) {
+		MERGE_WARNING(path, "commits not present");
+		return 0;
+	}
+
+	/* check whether both changes are forward */
+	if (!in_merge_bases(commit_base, &commit_a, 1) ||
+	    !in_merge_bases(commit_base, &commit_b, 1)) {
+		MERGE_WARNING(path, "commits don't follow merge-base");
+		return 0;
+	}
+
+	/* Case #1: a is contained in b or vice versa */
+	if (in_merge_bases(commit_a, &commit_b, 1)) {
+		hashcpy(result, b);
+		return 1;
+	}
+	if (in_merge_bases(commit_b, &commit_a, 1)) {
+		hashcpy(result, a);
+		return 1;
+	}
+
+	/*
+	 * Case #2: There are one or more merges that contain a and b in
+	 * the submodule. If there is only one, then present it as a
+	 * suggestion to the user, but leave it marked unmerged so the
+	 * user needs to confirm the resolution.
+	 */
+
+	/* find commit which merges them */
+	parent_count = find_first_merges(&merges, path, commit_a, commit_b);
+	switch (parent_count) {
+	case 0:
+		MERGE_WARNING(path, "merge following commits not found");
+		break;
+
+	case 1:
+		MERGE_WARNING(path, "not fast-forward");
+		fprintf(stderr, "Found a possible merge resolution "
+				"for the submodule:\n");
+		print_commit((struct commit *) merges.objects[0].item);
+		fprintf(stderr,
+			"If this is correct simply add it to the index "
+			"for example\n"
+			"by using:\n\n"
+			"  git update-index --cacheinfo 160000 %s \"%s\"\n\n"
+			"which will accept this suggestion.\n",
+			sha1_to_hex(merges.objects[0].item->sha1), path);
+		break;
+
+	default:
+		MERGE_WARNING(path, "multiple merges found");
+		for (i = 0; i < merges.nr; i++)
+			print_commit((struct commit *) merges.objects[i].item);
+	}
+
+	free(merges.objects);
+	return 0;
 }