format-patch: add '--base' option to record base tree info

Maintainers or third party testers may want to know the exact base tree
the patch series applies to. Teach git format-patch a '--base' option
to record the base tree info and append it at the end of the first
message (either the cover letter or the first patch in the series).

The base tree info consists of the "base commit", which is a well-known
commit that is part of the stable part of the project history everybody
else works off of, and zero or more "prerequisite patches", which are
well-known patches in flight that is not yet part of the "base commit"
that need to be applied on top of "base commit" in topological order
before the patches can be applied.

The "base commit" is shown as "base-commit: " followed by the 40-hex of
the commit object name.  A "prerequisite patch" is shown as
"prerequisite-patch-id: " followed by the 40-hex "patch id", which can
be obtained by passing the patch through the "git patch-id --stable"
command.

Imagine that on top of the public commit P, you applied well-known
patches X, Y and Z from somebody else, and then built your three-patch
series A, B, C, the history would be like:

---P---X---Y---Z---A---B---C

With "git format-patch --base=P -3 C" (or variants thereof, e.g. with
"--cover-letter" of using "Z..C" instead of "-3 C" to specify the
range), the base tree information block is shown at the end of the
first message the command outputs (either the first patch, or the
cover letter), like this:

base-commit: P
prerequisite-patch-id: X
prerequisite-patch-id: Y
prerequisite-patch-id: Z

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 139 insertions, 0 deletions

diff --git a/builtin/log.c b/builtin/log.c
index 0d738d6dd..c9ee33393 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1185,6 +1185,131 @@ static int from_callback(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
+struct base_tree_info {
+	struct object_id base_commit;
+	int nr_patch_id, alloc_patch_id;
+	struct object_id *patch_id;
+};
+
+static struct commit *get_base_commit(const char *base_commit,
+				      struct commit **list,
+				      int total)
+{
+	struct commit *base = NULL;
+	struct commit **rev;
+	int i = 0, rev_nr = 0;
+
+	base = lookup_commit_reference_by_name(base_commit);
+	if (!base)
+		die(_("Unknown commit %s"), base_commit);
+
+	ALLOC_ARRAY(rev, total);
+	for (i = 0; i < total; i++)
+		rev[i] = list[i];
+
+	rev_nr = total;
+	/*
+	 * Get merge base through pair-wise computations
+	 * and store it in rev[0].
+	 */
+	while (rev_nr > 1) {
+		for (i = 0; i < rev_nr / 2; i++) {
+			struct commit_list *merge_base;
+			merge_base = get_merge_bases(rev[2 * i], rev[2 * i + 1]);
+			if (!merge_base || merge_base->next)
+				die(_("Failed to find exact merge base"));
+
+			rev[i] = merge_base->item;
+		}
+
+		if (rev_nr % 2)
+			rev[i] = rev[2 * i];
+		rev_nr = (rev_nr + 1) / 2;
+	}
+
+	if (!in_merge_bases(base, rev[0]))
+		die(_("base commit should be the ancestor of revision list"));
+
+	for (i = 0; i < total; i++) {
+		if (base == list[i])
+			die(_("base commit shouldn't be in revision list"));
+	}
+
+	free(rev);
+	return base;
+}
+
+static void prepare_bases(struct base_tree_info *bases,
+			  struct commit *base,
+			  struct commit **list,
+			  int total)
+{
+	struct commit *commit;
+	struct rev_info revs;
+	struct diff_options diffopt;
+	int i;
+
+	if (!base)
+		return;
+
+	diff_setup(&diffopt);
+	DIFF_OPT_SET(&diffopt, RECURSIVE);
+	diff_setup_done(&diffopt);
+
+	oidcpy(&bases->base_commit, &base->object.oid);
+
+	init_revisions(&revs, NULL);
+	revs.max_parents = 1;
+	revs.topo_order = 1;
+	for (i = 0; i < total; i++) {
+		list[i]->object.flags &= ~UNINTERESTING;
+		add_pending_object(&revs, &list[i]->object, "rev_list");
+		list[i]->util = (void *)1;
+	}
+	base->object.flags |= UNINTERESTING;
+	add_pending_object(&revs, &base->object, "base");
+
+	if (prepare_revision_walk(&revs))
+		die(_("revision walk setup failed"));
+	/*
+	 * Traverse the commits list, get prerequisite patch ids
+	 * and stuff them in bases structure.
+	 */
+	while ((commit = get_revision(&revs)) != NULL) {
+		unsigned char sha1[20];
+		struct object_id *patch_id;
+		if (commit->util)
+			continue;
+		if (commit_patch_id(commit, &diffopt, sha1))
+			die(_("cannot get patch id"));
+		ALLOC_GROW(bases->patch_id, bases->nr_patch_id + 1, bases->alloc_patch_id);
+		patch_id = bases->patch_id + bases->nr_patch_id;
+		hashcpy(patch_id->hash, sha1);
+		bases->nr_patch_id++;
+	}
+}
+
+static void print_bases(struct base_tree_info *bases)
+{
+	int i;
+
+	/* Only do this once, either for the cover or for the first one */
+	if (is_null_oid(&bases->base_commit))
+		return;
+
+	/* Show the base commit */
+	printf("base-commit: %s\n", oid_to_hex(&bases->base_commit));
+
+	/* Show the prerequisite patches */
+	for (i = bases->nr_patch_id - 1; i >= 0; i--)
+		printf("prerequisite-patch-id: %s\n", oid_to_hex(&bases->patch_id[i]));
+
+	free(bases->patch_id);
+	bases->nr_patch_id = 0;
+	bases->alloc_patch_id = 0;
+	oidclr(&bases->base_commit);
+}
+
 int cmd_format_patch(int argc, const char **argv, const char *prefix)
 {
 	struct commit *commit;
@@ -1209,6 +1334,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	int reroll_count = -1;
 	char *branch_name = NULL;
 	char *from = NULL;
+	char *base_commit = NULL;
+	struct base_tree_info bases;
+
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
 			    N_("use [PATCH n/m] even with a single patch"),
@@ -1271,6 +1399,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			    PARSE_OPT_OPTARG, thread_callback },
 		OPT_STRING(0, "signature", &signature, N_("signature"),
 			    N_("add a signature")),
+		OPT_STRING(0, "base", &base_commit, N_("base-commit"),
+			   N_("add prerequisite tree info to the patch series")),
 		OPT_FILENAME(0, "signature-file", &signature_file,
 				N_("add a signature from a file")),
 		OPT__QUIET(&quiet, N_("don't print the patch filenames")),
@@ -1507,6 +1637,13 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		signature = strbuf_detach(&buf, NULL);
 	}
 
+	memset(&bases, 0, sizeof(bases));
+	if (base_commit) {
+		struct commit *base = get_base_commit(base_commit, list, nr);
+		reset_revision_walk();
+		prepare_bases(&bases, base, list, nr);
+	}
+
 	if (in_reply_to || thread || cover_letter)
 		rev.ref_message_ids = xcalloc(1, sizeof(struct string_list));
 	if (in_reply_to) {
@@ -1520,6 +1657,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			gen_message_id(&rev, "cover");
 		make_cover_letter(&rev, use_stdout,
 				  origin, nr, list, branch_name, quiet);
+		print_bases(&bases);
 		total++;
 		start_number--;
 	}
@@ -1585,6 +1723,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 				       rev.mime_boundary);
 			else
 				print_signature();
+			print_bases(&bases);
 		}
 		if (!use_stdout)
 			fclose(stdout);