submodule merge: update conflict error message

When attempting to merge in a superproject with conflicting submodule
pointers that cannot be fast-forwarded or trivially resolved, the merge
fails and Git prints an error message that accurately describes the
failure, but does not provide steps for the user to resolve the error.

Git is left in a conflicted state, which requires the user to:
 1. merge submodules or update submodules to an already existing
	commit that reflects the merge
 2. add submodules changes to the superproject
 3. finish merging superproject
These steps are non-obvious for newer submodule users to figure out
based on the error message and neither `git submodule status` nor `git
status` provide any useful pointers.

Update error message to provide steps to resolve submodule merge
conflict. Future work could involve adding an advice flag to the
message. Although the message is long, it also has the id of the
submodule commit that needs to be merged, which could be useful
information for the user.

Additionally, 5 merge failures that resulted in an early return have
been updated to reflect the status of the merge.
1. Null merge base (null o): CONFLICT_SUBMODULE_NULL_MERGE_BASE added
   as a new conflict type and will print updated error message.
2. Null merge side a (null a): BUG(). See [1] for discussion
3. Null merge side b (null b): BUG(). See [1] for discussion
4. Submodule not checked out: added NEEDSWORK bit
5. Submodule commits not present: added NEEDSWORK bit
The errors with a NEEDSWORK bit deserve a more detailed explanation of
how to resolve them. See [2] for more context.

[1] https://lore.kernel.org/git/CABPp-BE0qGwUy80dmVszkJQ+tcpfLRW0OZyErymzhZ9+HWY1mw@mail.gmail.com/
[2] https://lore.kernel.org/git/xmqqpmhjjwo9.fsf@gitster.g/

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 152 insertions, 9 deletions

diff --git a/merge-ort.c b/merge-ort.c
index 8b7de0fbd8..a52faf6e21 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -387,8 +387,24 @@ struct merge_options_internal {
 
 	/* call_depth: recursion level counter for merging merge bases */
 	int call_depth;
+
+	/* field that holds submodule conflict information */
+	struct string_list conflicted_submodules;
+};
+
+struct conflicted_submodule_item {
+	char *abbrev;
+	int flag;
 };
 
+static void conflicted_submodule_item_free(void *util, const char *str)
+{
+	struct conflicted_submodule_item *item = util;
+
+	free(item->abbrev);
+	free(item);
+}
+
 struct version_info {
 	struct object_id oid;
 	unsigned short mode;
@@ -517,6 +533,7 @@ enum conflict_and_info_types {
 	CONFLICT_SUBMODULE_NOT_INITIALIZED,
 	CONFLICT_SUBMODULE_HISTORY_NOT_AVAILABLE,
 	CONFLICT_SUBMODULE_MAY_HAVE_REWINDS,
+	CONFLICT_SUBMODULE_NULL_MERGE_BASE,
 
 	/* Keep this entry _last_ in the list */
 	NB_CONFLICT_TYPES,
@@ -570,6 +587,8 @@ static const char *type_short_descriptions[] = {
 		"CONFLICT (submodule history not available)",
 	[CONFLICT_SUBMODULE_MAY_HAVE_REWINDS] =
 		"CONFLICT (submodule may have rewinds)",
+	[CONFLICT_SUBMODULE_NULL_MERGE_BASE] =
+		"CONFLICT (submodule lacks merge base)"
 };
 
 struct logical_conflict_info {
@@ -686,6 +705,9 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 
 	mem_pool_discard(&opti->pool, 0);
 
+	string_list_clear_func(&opti->conflicted_submodules,
+					conflicted_submodule_item_free);
+
 	/* Clean out callback_data as well. */
 	FREE_AND_NULL(renames->callback_data);
 	renames->callback_data_nr = renames->callback_data_alloc = 0;
@@ -1744,24 +1766,32 @@ static int merge_submodule(struct merge_options *opt,
 
 	int i;
 	int search = !opt->priv->call_depth;
+	int sub_not_initialized = 1;
+	int sub_flag = -1;
 
 	/* store fallback answer in result in case we fail */
 	oidcpy(result, opt->priv->call_depth ? o : a);
 
 	/* we can not handle deletion conflicts */
-	if (is_null_oid(o))
-		return 0;
-	if (is_null_oid(a))
-		return 0;
-	if (is_null_oid(b))
-		return 0;
+	if (is_null_oid(a) || is_null_oid(b))
+		BUG("submodule deleted on one side; this should be handled outside of merge_submodule()");
 
-	if (repo_submodule_init(&subrepo, opt->repo, path, null_oid())) {
+	if ((sub_not_initialized = repo_submodule_init(&subrepo,
+		opt->repo, path, null_oid()))) {
 		path_msg(opt, CONFLICT_SUBMODULE_NOT_INITIALIZED, 0,
 			 path, NULL, NULL, NULL,
 			 _("Failed to merge submodule %s (not checked out)"),
 			 path);
-		return 0;
+		sub_flag = CONFLICT_SUBMODULE_NOT_INITIALIZED;
+		goto cleanup;
+	}
+
+	if (is_null_oid(o)) {
+		path_msg(opt, CONFLICT_SUBMODULE_NULL_MERGE_BASE, 0,
+			 path, NULL, NULL, NULL,
+			 _("Failed to merge submodule %s (no merge base)"),
+			 path);
+		goto cleanup;
 	}
 
 	if (!(commit_o = lookup_commit_reference(&subrepo, o)) ||
@@ -1771,6 +1801,7 @@ static int merge_submodule(struct merge_options *opt,
 			 path, NULL, NULL, NULL,
 			 _("Failed to merge submodule %s (commits not present)"),
 			 path);
+		sub_flag = CONFLICT_SUBMODULE_HISTORY_NOT_AVAILABLE;
 		goto cleanup;
 	}
 
@@ -1849,7 +1880,23 @@ static int merge_submodule(struct merge_options *opt,
 
 	object_array_clear(&merges);
 cleanup:
-	repo_clear(&subrepo);
+	if (!opt->priv->call_depth && !ret) {
+		struct string_list *csub = &opt->priv->conflicted_submodules;
+		struct conflicted_submodule_item *util;
+		const char *abbrev;
+
+		util = xmalloc(sizeof(*util));
+		util->flag = sub_flag;
+		util->abbrev = NULL;
+		if (!sub_not_initialized) {
+			abbrev = repo_find_unique_abbrev(&subrepo, b, DEFAULT_ABBREV);
+			util->abbrev = xstrdup(abbrev);
+		}
+		string_list_append(csub, path)->util = util;
+	}
+
+	if (!sub_not_initialized)
+		repo_clear(&subrepo);
 	return ret;
 }
 
@@ -4434,6 +4481,99 @@ static int record_conflicted_index_entries(struct merge_options *opt)
 	return errs;
 }
 
+static void format_submodule_conflict_suggestion(struct strbuf *msg) {
+	struct strbuf tmp = STRBUF_INIT;
+	struct string_list msg_list = STRING_LIST_INIT_DUP;
+	int i;
+
+	string_list_split(&msg_list, msg->buf, '\n', -1);
+	for (i = 0; i < msg_list.nr; i++) {
+		if (!i)
+			/*
+			 * TRANSLATORS: This is line item of submodule conflict message
+			 * from print_submodule_conflict_suggestion() below. For RTL
+			 * languages, the following swap is suggested:
+			 *      " - %s\n" -> "%s - \n"
+			 */
+			strbuf_addf(&tmp, _(" - %s\n"), msg_list.items[i].string);
+		else
+			/*
+			 * TRANSLATORS: This is line item of submodule conflict message
+			 * from print_submodule_conflict_suggestion() below. For RTL
+			 * languages, the following swap is suggested:
+			 *      "   %s\n" -> "%s   \n"
+			 */
+			strbuf_addf(&tmp, _("   %s\n"), msg_list.items[i].string);
+	}
+	strbuf_reset(msg);
+	strbuf_add(msg, tmp.buf, tmp.len);
+}
+
+static void print_submodule_conflict_suggestion(struct string_list *csub) {
+	struct string_list_item *item;
+	struct strbuf msg = STRBUF_INIT;
+	struct strbuf tmp = STRBUF_INIT;
+	struct strbuf subs = STRBUF_INIT;
+
+	if (!csub->nr)
+		return;
+
+	/*
+	 * NEEDSWORK: The steps to resolve these errors deserve a more
+	 * detailed explanation than what is currently printed below.
+	 */
+	for_each_string_list_item(item, csub) {
+		struct conflicted_submodule_item *util = item->util;
+
+		if (util->flag == CONFLICT_SUBMODULE_NOT_INITIALIZED ||
+			util->flag == CONFLICT_SUBMODULE_HISTORY_NOT_AVAILABLE)
+			return;
+	}
+
+	printf(_("Recursive merging with submodules currently only supports "
+		"trivial cases.\nPlease manually handle the merging of each "
+		"conflicted submodule.\nThis can be accomplished with the following "
+		"steps:"));
+	putchar('\n');
+
+	for_each_string_list_item(item, csub) {
+		struct conflicted_submodule_item *util = item->util;
+		/*
+		 * TRANSLATORS: This is a line of advice to resolve a merge conflict
+		 * in a submodule. The second argument is the abbreviated id of the
+		 * commit that needs to be merged.
+		 * E.g. - go to submodule (sub), and either merge commit abc1234"
+		 */
+		strbuf_addf(&tmp, _("go to submodule (%s), and either merge commit %s\n"
+			"or update to an existing commit which has merged those changes"),
+			item->string, util->abbrev);
+		format_submodule_conflict_suggestion(&tmp);
+		strbuf_add(&msg, tmp.buf, tmp.len);
+		strbuf_reset(&tmp);
+	}
+	strbuf_add_separated_string_list(&subs, " ", csub);
+	strbuf_addstr(&tmp, _("come back to superproject and run:"));
+	strbuf_addf(&tmp, "\n\ngit add %s\n\n", subs.buf);
+	strbuf_addstr(&tmp, _("to record the above merge or update"));
+	format_submodule_conflict_suggestion(&tmp);
+	strbuf_add(&msg, tmp.buf, tmp.len);
+	strbuf_reset(&tmp);
+
+	strbuf_addstr(&tmp, _("resolve any other conflicts in the superproject"));
+	format_submodule_conflict_suggestion(&tmp);
+	strbuf_add(&msg, tmp.buf, tmp.len);
+	strbuf_reset(&tmp);
+
+	strbuf_addstr(&tmp, _("commit the resulting index in the superproject"));
+	format_submodule_conflict_suggestion(&tmp);
+	strbuf_add(&msg, tmp.buf, tmp.len);
+
+	printf("%s", msg.buf);
+	strbuf_release(&subs);
+	strbuf_release(&tmp);
+	strbuf_release(&msg);
+}
+
 void merge_display_update_messages(struct merge_options *opt,
 				   int detailed,
 				   struct merge_result *result)
@@ -4483,6 +4623,8 @@ void merge_display_update_messages(struct merge_options *opt,
 	}
 	string_list_clear(&olist, 0);
 
+	print_submodule_conflict_suggestion(&opti->conflicted_submodules);
+
 	/* Also include needed rename limit adjustment now */
 	diff_warn_rename_limit("merge.renamelimit",
 			       opti->renames.needed_limit, 0);
@@ -4679,6 +4821,7 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
 	trace2_region_enter("merge", "allocate/init", opt->repo);
 	if (opt->priv) {
 		clear_or_reinit_internal_opts(opt->priv, 1);
+		string_list_init_nodup(&opt->priv->conflicted_submodules);
 		trace2_region_leave("merge", "allocate/init", opt->repo);
 		return;
 	}