From 0bad611b1ebacf170976a10690bd0169f8d372bf Mon Sep 17 00:00:00 2001
From: Heiko Voigt <hvoigt@hvoigt.net>
Date: Wed, 7 Jul 2010 15:39:11 +0200
Subject: Teach ref iteration module about submodules

We will use this in a later patch to extend setup_revisions() to
load revisions directly from a submodule.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 cache.h |   3 ++
 path.c  |  38 +++++++++++++++++++++
 refs.c  | 118 ++++++++++++++++++++++++++++++++++++++++++++++------------------
 3 files changed, 127 insertions(+), 32 deletions(-)

diff --git a/cache.h b/cache.h
index c9fa3df7f..32932e87b 100644
--- a/cache.h
+++ b/cache.h
@@ -641,6 +641,9 @@ extern char *git_pathdup(const char *fmt, ...)
 /* Return a statically allocated filename matching the sha1 signature */
 extern char *mkpath(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
 extern char *git_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
+extern char *git_path_submodule(const char *path, const char *fmt, ...)
+	__attribute__((format (printf, 2, 3)));
+
 extern char *sha1_file_name(const unsigned char *sha1);
 extern char *sha1_pack_name(const unsigned char *sha1);
 extern char *sha1_pack_index_name(const unsigned char *sha1);
diff --git a/path.c b/path.c
index b4c8d9172..e32c61c2f 100644
--- a/path.c
+++ b/path.c
@@ -122,6 +122,44 @@ char *git_path(const char *fmt, ...)
 	return cleanup_path(pathname);
 }
 
+char *git_path_submodule(const char *path, const char *fmt, ...)
+{
+	char *pathname = get_pathname();
+	struct strbuf buf = STRBUF_INIT;
+	const char *git_dir;
+	va_list args;
+	unsigned len;
+
+	len = strlen(path);
+	if (len > PATH_MAX-100)
+		return bad_path;
+
+	strbuf_addstr(&buf, path);
+	if (len && path[len-1] != '/')
+		strbuf_addch(&buf, '/');
+	strbuf_addstr(&buf, ".git");
+
+	git_dir = read_gitfile_gently(buf.buf);
+	if (git_dir) {
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, git_dir);
+	}
+	strbuf_addch(&buf, '/');
+
+	if (buf.len >= PATH_MAX)
+		return bad_path;
+	memcpy(pathname, buf.buf, buf.len + 1);
+
+	strbuf_release(&buf);
+	len = strlen(pathname);
+
+	va_start(args, fmt);
+	len += vsnprintf(pathname + len, PATH_MAX - len, fmt, args);
+	va_end(args);
+	if (len >= PATH_MAX)
+		return bad_path;
+	return cleanup_path(pathname);
+}
 
 /* git_mkstemp() - create tmp file honoring TMPDIR variable */
 int git_mkstemp(char *path, size_t len, const char *template)
diff --git a/refs.c b/refs.c
index 6f486ae62..c8649b130 100644
--- a/refs.c
+++ b/refs.c
@@ -157,7 +157,7 @@ static struct cached_refs {
 	char did_packed;
 	struct ref_list *loose;
 	struct ref_list *packed;
-} cached_refs;
+} cached_refs, submodule_refs;
 static struct ref_list *current_ref;
 
 static struct ref_list *extra_refs;
@@ -229,23 +229,45 @@ void clear_extra_refs(void)
 	extra_refs = NULL;
 }
 
-static struct ref_list *get_packed_refs(void)
+static struct ref_list *get_packed_refs(const char *submodule)
 {
-	if (!cached_refs.did_packed) {
-		FILE *f = fopen(git_path("packed-refs"), "r");
-		cached_refs.packed = NULL;
+	const char *packed_refs_file;
+	struct cached_refs *refs;
+
+	if (submodule) {
+		packed_refs_file = git_path_submodule(submodule, "packed-refs");
+		refs = &submodule_refs;
+		free_ref_list(refs->packed);
+	} else {
+		packed_refs_file = git_path("packed-refs");
+		refs = &cached_refs;
+	}
+
+	if (!refs->did_packed || submodule) {
+		FILE *f = fopen(packed_refs_file, "r");
+		refs->packed = NULL;
 		if (f) {
-			read_packed_refs(f, &cached_refs);
+			read_packed_refs(f, refs);
 			fclose(f);
 		}
-		cached_refs.did_packed = 1;
+		refs->did_packed = 1;
 	}
-	return cached_refs.packed;
+	return refs->packed;
 }
 
-static struct ref_list *get_ref_dir(const char *base, struct ref_list *list)
+static struct ref_list *get_ref_dir(const char *submodule, const char *base,
+				    struct ref_list *list)
 {
-	DIR *dir = opendir(git_path("%s", base));
+	DIR *dir;
+	const char *path;
+
+	if (submodule)
+		path = git_path_submodule(submodule, "%s", base);
+	else
+		path = git_path("%s", base);
+
+
+	dir = opendir(path);
 
 	if (dir) {
 		struct dirent *de;
@@ -261,6 +283,7 @@ static struct ref_list *get_ref_dir(const char *base, struct ref_list *list)
 			struct stat st;
 			int flag;
 			int namelen;
+			const char *refdir;
 
 			if (de->d_name[0] == '.')
 				continue;
@@ -270,16 +293,27 @@ static struct ref_list *get_ref_dir(const char *base, struct ref_list *list)
 			if (has_extension(de->d_name, ".lock"))
 				continue;
 			memcpy(ref + baselen, de->d_name, namelen+1);
-			if (stat(git_path("%s", ref), &st) < 0)
+			refdir = submodule
+				? git_path_submodule(submodule, "%s", ref)
+				: git_path("%s", ref);
+			if (stat(refdir, &st) < 0)
 				continue;
 			if (S_ISDIR(st.st_mode)) {
-				list = get_ref_dir(ref, list);
+				list = get_ref_dir(submodule, ref, list);
 				continue;
 			}
-			if (!resolve_ref(ref, sha1, 1, &flag)) {
+			if (submodule) {
 				hashclr(sha1);
-				flag |= REF_BROKEN;
-			}
+				flag = 0;
+				if (resolve_gitlink_ref(submodule, ref, sha1) < 0) {
+					hashclr(sha1);
+					flag |= REF_BROKEN;
+				}
+			} else
+				if (!resolve_ref(ref, sha1, 1, &flag)) {
+					hashclr(sha1);
+					flag |= REF_BROKEN;
+				}
 			list = add_ref(ref, sha1, flag, list, NULL);
 		}
 		free(ref);
@@ -322,10 +356,16 @@ void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname)
 	for_each_rawref(warn_if_dangling_symref, &data);
 }
 
-static struct ref_list *get_loose_refs(void)
+static struct ref_list *get_loose_refs(const char *submodule)
 {
+	if (submodule) {
+		free_ref_list(submodule_refs.loose);
+		submodule_refs.loose = get_ref_dir(submodule, "refs", NULL);
+		return submodule_refs.loose;
+	}
+
 	if (!cached_refs.did_loose) {
-		cached_refs.loose = get_ref_dir("refs", NULL);
+		cached_refs.loose = get_ref_dir(NULL, "refs", NULL);
 		cached_refs.did_loose = 1;
 	}
 	return cached_refs.loose;
@@ -459,7 +499,7 @@ const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *
 		git_snpath(path, sizeof(path), "%s", ref);
 		/* Special case: non-existing file. */
 		if (lstat(path, &st) < 0) {
-			struct ref_list *list = get_packed_refs();
+			struct ref_list *list = get_packed_refs(NULL);
 			while (list) {
 				if (!strcmp(ref, list->name)) {
 					hashcpy(sha1, list->sha1);
@@ -588,7 +628,7 @@ int peel_ref(const char *ref, unsigned char *sha1)
 		return -1;
 
 	if ((flag & REF_ISPACKED)) {
-		struct ref_list *list = get_packed_refs();
+		struct ref_list *list = get_packed_refs(NULL);
 
 		while (list) {
 			if (!strcmp(list->name, ref)) {
@@ -615,12 +655,12 @@ fallback:
 	return -1;
 }
 
-static int do_for_each_ref(const char *base, each_ref_fn fn, int trim,
-			   int flags, void *cb_data)
+static int do_for_each_ref(const char *submodule, const char *base, each_ref_fn fn,
+			   int trim, int flags, void *cb_data)
 {
 	int retval = 0;
-	struct ref_list *packed = get_packed_refs();
-	struct ref_list *loose = get_loose_refs();
+	struct ref_list *packed = get_packed_refs(submodule);
+	struct ref_list *loose = get_loose_refs(submodule);
 
 	struct ref_list *extra;
 
@@ -657,24 +697,38 @@ end_each:
 	return retval;
 }
 
-int head_ref(each_ref_fn fn, void *cb_data)
+
+static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
 {
 	unsigned char sha1[20];
 	int flag;
 
+	if (submodule) {
+		if (resolve_gitlink_ref(submodule, "HEAD", sha1) == 0)
+			return fn("HEAD", sha1, 0, cb_data);
+
+		return 0;
+	}
+
 	if (resolve_ref("HEAD", sha1, 1, &flag))
 		return fn("HEAD", sha1, flag, cb_data);
+
 	return 0;
 }
 
+int head_ref(each_ref_fn fn, void *cb_data)
+{
+	return do_head_ref(NULL, fn, cb_data);
+}
+
 int for_each_ref(each_ref_fn fn, void *cb_data)
 {
-	return do_for_each_ref("refs/", fn, 0, 0, cb_data);
+	return do_for_each_ref(NULL, "refs/", fn, 0, 0, cb_data);
 }
 
 int for_each_ref_in(const char *prefix, each_ref_fn fn, void *cb_data)
 {
-	return do_for_each_ref(prefix, fn, strlen(prefix), 0, cb_data);
+	return do_for_each_ref(NULL, prefix, fn, strlen(prefix), 0, cb_data);
 }
 
 int for_each_tag_ref(each_ref_fn fn, void *cb_data)
@@ -694,7 +748,7 @@ int for_each_remote_ref(each_ref_fn fn, void *cb_data)
 
 int for_each_replace_ref(each_ref_fn fn, void *cb_data)
 {
-	return do_for_each_ref("refs/replace/", fn, 13, 0, cb_data);
+	return do_for_each_ref(NULL, "refs/replace/", fn, 13, 0, cb_data);
 }
 
 int for_each_glob_ref_in(each_ref_fn fn, const char *pattern,
@@ -734,7 +788,7 @@ int for_each_glob_ref(each_ref_fn fn, const char *pattern, void *cb_data)
 
 int for_each_rawref(each_ref_fn fn, void *cb_data)
 {
-	return do_for_each_ref("refs/", fn, 0,
+	return do_for_each_ref(NULL, "refs/", fn, 0,
 			       DO_FOR_EACH_INCLUDE_BROKEN, cb_data);
 }
 
@@ -958,7 +1012,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char
 	 * name is a proper prefix of our refname.
 	 */
 	if (missing &&
-	     !is_refname_available(ref, NULL, get_packed_refs(), 0)) {
+	     !is_refname_available(ref, NULL, get_packed_refs(NULL), 0)) {
 		last_errno = ENOTDIR;
 		goto error_return;
 	}
@@ -1021,7 +1075,7 @@ static int repack_without_ref(const char *refname)
 	int fd;
 	int found = 0;
 
-	packed_ref_list = get_packed_refs();
+	packed_ref_list = get_packed_refs(NULL);
 	for (list = packed_ref_list; list; list = list->next) {
 		if (!strcmp(refname, list->name)) {
 			found = 1;
@@ -1110,10 +1164,10 @@ int rename_ref(const char *oldref, const char *newref, const char *logmsg)
 	if (!symref)
 		return error("refname %s not found", oldref);
 
-	if (!is_refname_available(newref, oldref, get_packed_refs(), 0))
+	if (!is_refname_available(newref, oldref, get_packed_refs(NULL), 0))
 		return 1;
 
-	if (!is_refname_available(newref, oldref, get_loose_refs(), 0))
+	if (!is_refname_available(newref, oldref, get_loose_refs(NULL), 0))
 		return 1;
 
 	lock = lock_ref_sha1_basic(renamed_ref, NULL, 0, NULL);
-- 
cgit v1.2.1


From 9ef6aeb09ffe0650ca86dca53de93fd85333b02d Mon Sep 17 00:00:00 2001
From: Heiko Voigt <hvoigt@hvoigt.net>
Date: Wed, 7 Jul 2010 15:39:12 +0200
Subject: setup_revisions(): Allow walking history in a submodule

By passing the path to a submodule in opt->submodule, the function can
be used to walk history in the named submodule repository, instead of
the toplevel repository.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 refs.c     | 31 +++++++++++++++++++++++++++++++
 refs.h     |  8 ++++++++
 revision.c | 32 ++++++++++++++++++--------------
 revision.h |  1 +
 4 files changed, 58 insertions(+), 14 deletions(-)

diff --git a/refs.c b/refs.c
index c8649b130..37e279468 100644
--- a/refs.c
+++ b/refs.c
@@ -721,31 +721,62 @@ int head_ref(each_ref_fn fn, void *cb_data)
 	return do_head_ref(NULL, fn, cb_data);
 }
 
+int head_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+{
+	return do_head_ref(submodule, fn, cb_data);
+}
+
 int for_each_ref(each_ref_fn fn, void *cb_data)
 {
 	return do_for_each_ref(NULL, "refs/", fn, 0, 0, cb_data);
 }
 
+int for_each_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+{
+	return do_for_each_ref(submodule, "refs/", fn, 0, 0, cb_data);
+}
+
 int for_each_ref_in(const char *prefix, each_ref_fn fn, void *cb_data)
 {
 	return do_for_each_ref(NULL, prefix, fn, strlen(prefix), 0, cb_data);
 }
 
+int for_each_ref_in_submodule(const char *submodule, const char *prefix,
+		each_ref_fn fn, void *cb_data)
+{
+	return do_for_each_ref(submodule, prefix, fn, strlen(prefix), 0, cb_data);
+}
+
 int for_each_tag_ref(each_ref_fn fn, void *cb_data)
 {
 	return for_each_ref_in("refs/tags/", fn, cb_data);
 }
 
+int for_each_tag_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+{
+	return for_each_ref_in_submodule(submodule, "refs/tags/", fn, cb_data);
+}
+
 int for_each_branch_ref(each_ref_fn fn, void *cb_data)
 {
 	return for_each_ref_in("refs/heads/", fn, cb_data);
 }
 
+int for_each_branch_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+{
+	return for_each_ref_in_submodule(submodule, "refs/heads/", fn, cb_data);
+}
+
 int for_each_remote_ref(each_ref_fn fn, void *cb_data)
 {
 	return for_each_ref_in("refs/remotes/", fn, cb_data);
 }
 
+int for_each_remote_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+{
+	return for_each_ref_in_submodule(submodule, "refs/remotes/", fn, cb_data);
+}
+
 int for_each_replace_ref(each_ref_fn fn, void *cb_data)
 {
 	return do_for_each_ref(NULL, "refs/replace/", fn, 13, 0, cb_data);
diff --git a/refs.h b/refs.h
index 762ce504b..5e7a9a59f 100644
--- a/refs.h
+++ b/refs.h
@@ -28,6 +28,14 @@ extern int for_each_replace_ref(each_ref_fn, void *);
 extern int for_each_glob_ref(each_ref_fn, const char *pattern, void *);
 extern int for_each_glob_ref_in(each_ref_fn, const char *pattern, const char* prefix, void *);
 
+extern int head_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
+extern int for_each_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
+extern int for_each_ref_in_submodule(const char *submodule, const char *prefix,
+		each_ref_fn fn, void *cb_data);
+extern int for_each_tag_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
+extern int for_each_branch_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
+extern int for_each_remote_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data);
+
 static inline const char *has_glob_specials(const char *pattern)
 {
 	return strpbrk(pattern, "?*[");
diff --git a/revision.c b/revision.c
index 7e82efd93..5f2cf1e84 100644
--- a/revision.c
+++ b/revision.c
@@ -820,12 +820,12 @@ static void init_all_refs_cb(struct all_refs_cb *cb, struct rev_info *revs,
 	cb->all_flags = flags;
 }
 
-static void handle_refs(struct rev_info *revs, unsigned flags,
-		int (*for_each)(each_ref_fn, void *))
+static void handle_refs(const char *submodule, struct rev_info *revs, unsigned flags,
+		int (*for_each)(const char *, each_ref_fn, void *))
 {
 	struct all_refs_cb cb;
 	init_all_refs_cb(&cb, revs, flags);
-	for_each(handle_one_ref, &cb);
+	for_each(submodule, handle_one_ref, &cb);
 }
 
 static void handle_one_reflog_commit(unsigned char *sha1, void *cb_data)
@@ -1417,14 +1417,14 @@ void parse_revision_opt(struct rev_info *revs, struct parse_opt_ctx_t *ctx,
 	ctx->argc -= n;
 }
 
-static int for_each_bad_bisect_ref(each_ref_fn fn, void *cb_data)
+static int for_each_bad_bisect_ref(const char *submodule, each_ref_fn fn, void *cb_data)
 {
-	return for_each_ref_in("refs/bisect/bad", fn, cb_data);
+	return for_each_ref_in_submodule(submodule, "refs/bisect/bad", fn, cb_data);
 }
 
-static int for_each_good_bisect_ref(each_ref_fn fn, void *cb_data)
+static int for_each_good_bisect_ref(const char *submodule, each_ref_fn fn, void *cb_data)
 {
-	return for_each_ref_in("refs/bisect/good", fn, cb_data);
+	return for_each_ref_in_submodule(submodule, "refs/bisect/good", fn, cb_data);
 }
 
 static void append_prune_data(const char ***prune_data, const char **av)
@@ -1466,6 +1466,10 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 {
 	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0;
 	const char **prune_data = NULL;
+	const char *submodule = NULL;
+
+	if (opt)
+		submodule = opt->submodule;
 
 	/* First, search for "--" */
 	seen_dashdash = 0;
@@ -1490,26 +1494,26 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 			int opts;
 
 			if (!strcmp(arg, "--all")) {
-				handle_refs(revs, flags, for_each_ref);
-				handle_refs(revs, flags, head_ref);
+				handle_refs(submodule, revs, flags, for_each_ref_submodule);
+				handle_refs(submodule, revs, flags, head_ref_submodule);
 				continue;
 			}
 			if (!strcmp(arg, "--branches")) {
-				handle_refs(revs, flags, for_each_branch_ref);
+				handle_refs(submodule, revs, flags, for_each_branch_ref_submodule);
 				continue;
 			}
 			if (!strcmp(arg, "--bisect")) {
-				handle_refs(revs, flags, for_each_bad_bisect_ref);
-				handle_refs(revs, flags ^ UNINTERESTING, for_each_good_bisect_ref);
+				handle_refs(submodule, revs, flags, for_each_bad_bisect_ref);
+				handle_refs(submodule, revs, flags ^ UNINTERESTING, for_each_good_bisect_ref);
 				revs->bisect = 1;
 				continue;
 			}
 			if (!strcmp(arg, "--tags")) {
-				handle_refs(revs, flags, for_each_tag_ref);
+				handle_refs(submodule, revs, flags, for_each_tag_ref_submodule);
 				continue;
 			}
 			if (!strcmp(arg, "--remotes")) {
-				handle_refs(revs, flags, for_each_remote_ref);
+				handle_refs(submodule, revs, flags, for_each_remote_ref_submodule);
 				continue;
 			}
 			if (!prefixcmp(arg, "--glob=")) {
diff --git a/revision.h b/revision.h
index 36fdf22b2..05659c64a 100644
--- a/revision.h
+++ b/revision.h
@@ -151,6 +151,7 @@ extern volatile show_early_output_fn_t show_early_output;
 struct setup_revision_opt {
 	const char *def;
 	void (*tweak)(struct rev_info *, struct setup_revision_opt *);
+	const char *submodule;
 };
 
 extern void init_revisions(struct rev_info *revs, const char *prefix);
-- 
cgit v1.2.1


From 68d03e4a6e448aa557f52adef92595ac4d6cd4bd Mon Sep 17 00:00:00 2001
From: Heiko Voigt <hvoigt@hvoigt.net>
Date: Wed, 7 Jul 2010 15:39:13 +0200
Subject: Implement automatic fast-forward merge for submodules

This implements a simple merge strategy for submodule hashes. We check
whether one side of the merge candidates is already contained in the
other and then merge automatically.

If both sides contain changes we search for a merge in the submodule.
In case a single one exists we check that out and suggest it as the
merge resolution. A list of candidates is returned when we find multiple
merges that contain both sides of the changes.

This is useful for a workflow in which the developers can publish topic
branches in submodules and a separate maintainer merges them. In case
the developers always wait until their branch gets merged before tracking
them in the superproject all merges of branches that contain submodule
changes will be resolved automatically. If developers choose to track
their feature branch the maintainer might get a conflict but git will
search the submodule for a merge and suggest it/them as a resolution.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 merge-recursive.c          |   9 ++-
 submodule.c                | 161 +++++++++++++++++++++++++++++++++++++++++++++
 submodule.h                |   2 +
 t/t7405-submodule-merge.sh | 128 +++++++++++++++++++++++++++++++++--
 4 files changed, 291 insertions(+), 9 deletions(-)

diff --git a/merge-recursive.c b/merge-recursive.c
index 856e98c08..4dcf41758 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -20,6 +20,7 @@
 #include "attr.h"
 #include "merge-recursive.h"
 #include "dir.h"
+#include "submodule.h"
 
 static struct tree *shift_tree_object(struct tree *one, struct tree *two,
 				      const char *subtree_shift)
@@ -525,13 +526,15 @@ static void update_file_flags(struct merge_options *o,
 		void *buf;
 		unsigned long size;
 
-		if (S_ISGITLINK(mode))
+		if (S_ISGITLINK(mode)) {
 			/*
 			 * We may later decide to recursively descend into
 			 * the submodule directory and update its index
 			 * and/or work tree, but we do not do that now.
 			 */
+			update_wd = 0;
 			goto update_index;
+		}
 
 		buf = read_sha1_file(sha, &type, &size);
 		if (!buf)
@@ -716,8 +719,8 @@ static struct merge_file_info merge_file(struct merge_options *o,
 			free(result_buf.ptr);
 			result.clean = (merge_status == 0);
 		} else if (S_ISGITLINK(a->mode)) {
-			result.clean = 0;
-			hashcpy(result.sha, a->sha1);
+			result.clean = merge_submodule(result.sha, one->path, one->sha1,
+						       a->sha1, b->sha1);
 		} else if (S_ISLNK(a->mode)) {
 			hashcpy(result.sha, a->sha1);
 
diff --git a/submodule.c b/submodule.c
index 61cb6e21d..dcb4b5da7 100644
--- a/submodule.c
+++ b/submodule.c
@@ -6,6 +6,7 @@
 #include "revision.h"
 #include "run-command.h"
 #include "diffcore.h"
+#include "refs.h"
 
 static int add_submodule_odb(const char *path)
 {
@@ -218,3 +219,163 @@ unsigned is_submodule_modified(const char *path, int ignore_untracked)
 	strbuf_release(&buf);
 	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;
+}
diff --git a/submodule.h b/submodule.h
index 6fd3bb407..b0a571c7f 100644
--- a/submodule.h
+++ b/submodule.h
@@ -9,5 +9,7 @@ void show_submodule_summary(FILE *f, const char *path,
 		unsigned dirty_submodule,
 		const char *del, const char *add, const char *reset);
 unsigned is_submodule_modified(const char *path, int ignore_untracked);
+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]);
 
 #endif
diff --git a/t/t7405-submodule-merge.sh b/t/t7405-submodule-merge.sh
index 4a7b8933f..6ec559db0 100755
--- a/t/t7405-submodule-merge.sh
+++ b/t/t7405-submodule-merge.sh
@@ -54,13 +54,129 @@ test_expect_success setup '
 	git merge -s ours a
 '
 
-test_expect_success 'merging with modify/modify conflict' '
+# History setup
+#
+#      b
+#    /   \
+#   a     d
+#    \   /
+#      c
+#
+# a in the main repository records to sub-a in the submodule and
+# analogous b and c. d should be automatically found by merging c into
+# b in the main repository.
+test_expect_success 'setup for merge search' '
+	mkdir merge-search &&
+	cd merge-search &&
+	git init &&
+	mkdir sub &&
+	(cd sub &&
+	 git init &&
+	 echo "file-a" > file-a &&
+	 git add file-a &&
+	 git commit -m "sub-a" &&
+	 git branch sub-a) &&
+	git add sub &&
+	git commit -m "a" &&
+	git branch a &&
+
+	git checkout -b b &&
+	(cd sub &&
+	 git checkout -b sub-b &&
+	 echo "file-b" > file-b &&
+	 git add file-b &&
+	 git commit -m "sub-b") &&
+	git commit -a -m "b" &&
+
+	git checkout -b c a &&
+	(cd sub &&
+	 git checkout -b sub-c sub-a &&
+	 echo "file-c" > file-c &&
+	 git add file-c &&
+	 git commit -m "sub-c") &&
+	git commit -a -m "c" &&
+
+	git checkout -b d a &&
+	(cd sub &&
+	 git checkout -b sub-d sub-b &&
+	 git merge sub-c) &&
+	git commit -a -m "d" &&
+	git branch test b &&
+	cd ..
+'
+
+test_expect_success 'merge with one side as a fast-forward of the other' '
+	(cd merge-search &&
+	 git checkout -b test-forward b &&
+	 git merge d &&
+	 git ls-tree test-forward sub | cut -f1 | cut -f3 -d" " > actual &&
+	 (cd sub &&
+	  git rev-parse sub-d > ../expect) &&
+	 test_cmp actual expect)
+'
+
+test_expect_success 'merging should conflict for non fast-forward' '
+	(cd merge-search &&
+	 git checkout -b test-nonforward b &&
+	 (cd sub &&
+	  git rev-parse sub-d > ../expect) &&
+	 test_must_fail git merge c 2> actual  &&
+	 grep $(cat expect) actual > /dev/null &&
+	 git reset --hard)
+'
+
+test_expect_success 'merging should fail for ambiguous common parent' '
+	cd merge-search &&
+	git checkout -b test-ambiguous b &&
+	(cd sub &&
+	 git checkout -b ambiguous sub-b &&
+	 git merge sub-c &&
+	 git rev-parse sub-d > ../expect1 &&
+	 git rev-parse ambiguous > ../expect2) &&
+	test_must_fail git merge c 2> actual &&
+	grep $(cat expect1) actual > /dev/null &&
+	grep $(cat expect2) actual > /dev/null &&
+	git reset --hard &&
+	cd ..
+'
+
+# in a situation like this
+#
+# submodule tree:
+#
+#    sub-a --- sub-b --- sub-d
+#
+# main tree:
+#
+#    e (sub-a)
+#   /
+#  bb (sub-b)
+#   \
+#    f (sub-d)
+#
+# A merge between e and f should fail because one of the submodule
+# commits (sub-a) does not descend from the submodule merge-base (sub-b).
+#
+test_expect_success 'merging should fail for changes that are backwards' '
+	cd merge-search &&
+	git checkout -b bb a &&
+	(cd sub &&
+	 git checkout sub-b) &&
+	git commit -a -m "bb" &&
+
+	git checkout -b e bb &&
+	(cd sub &&
+	 git checkout sub-a) &&
+	git commit -a -m "e" &&
+
+	git checkout -b f bb &&
+	(cd sub &&
+	 git checkout sub-d) &&
+	git commit -a -m "f" &&
 
-	git checkout -b test1 a &&
-	test_must_fail git merge b &&
-	test -f .git/MERGE_MSG &&
-	git diff &&
-	test -n "$(git ls-files -u)"
+	git checkout -b test-backward e &&
+	test_must_fail git merge f &&
+	cd ..
 '
 
 test_expect_success 'merging with a modify/modify conflict between merge bases' '
-- 
cgit v1.2.1