[PATCH v7] submodule merge: update conflict error message

[Calvin Wan <calvinwan@google.com>]

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, 4 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: Still returns early, but added a
   NEEDSWORK bit since current error message does not reflect the
   correct resolution

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

Signed-off-by: Calvin Wan <calvinwan@google.com>
---

For Elijah: Cleaned up the small nits and updated resolutions for
those 4 cases we discussed.

For Ævar: Apologies for misunderstanding your suggestions to make
my messages easier for translators to work with. I have reformatted
all of the messages to separate text vs formatting translations. Let
me know if this is what you were expecting.

 merge-ort.c                 | 112 ++++++++++++++++++++++++++++++++++--
 t/t6437-submodule-merge.sh  |  78 +++++++++++++++++++++++--
 t/t7402-submodule-rebase.sh |   9 ++-
 3 files changed, 185 insertions(+), 14 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 01f150ef3b..4302e900ee 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -387,6 +387,9 @@ 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 version_info {
@@ -517,6 +520,7 @@ 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 +574,8 @@ 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 no merge base)"
 };
 
 struct logical_conflict_info {
@@ -686,6 +692,8 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 
 	mem_pool_discard(&opti->pool, 0);
 
+	string_list_clear(&opti->conflicted_submodules, 1);
+
 	/* Clean out callback_data as well. */
 	FREE_AND_NULL(renames->callback_data);
 	renames->callback_data_nr = renames->callback_data_alloc = 0;
@@ -1744,26 +1752,40 @@ static int merge_submodule(struct merge_options *opt,
 
 	int i;
 	int search = !opt->priv->call_depth;
+	int sub_initialized = 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;
+		BUG("submodule deleted on one side; this should be handled outside of merge_submodule()"); 
 	if (is_null_oid(b))
-		return 0;
+		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_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);
+		/*
+		 * NEEDSWORK: Since the steps to resolve this error are
+		 * more involved than what is currently in 
+		 * print_submodule_conflict_suggestion(), we return
+		 * immediately rather than generating an error message
+		 */
 		return 0;
 	}
 
+	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)) ||
 	    !(commit_a = lookup_commit_reference(&subrepo, a)) ||
 	    !(commit_b = lookup_commit_reference(&subrepo, b))) {
@@ -1849,7 +1871,15 @@ 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;
+
+		string_list_append(csub, path)->util =
+				xstrdup(repo_find_unique_abbrev(&subrepo, b, DEFAULT_ABBREV));
+	}
+
+	if (!sub_initialized)
+		repo_clear(&subrepo);
 	return ret;
 }
 
@@ -4412,6 +4442,73 @@ static int record_conflicted_index_entries(struct merge_options *opt)
 	return errs;
 }
 
+static void print_submodule_conflict_suggestion(struct string_list *csub) {
+	if (csub->nr > 0) {
+		struct string_list_item *item;
+		struct strbuf msg = STRBUF_INIT;
+		struct strbuf tmp = STRBUF_INIT;
+
+		strbuf_addf(&tmp, _("Recursive merging with submodules currently only supports trivial cases."));
+		strbuf_addf(&msg, "%s\n", tmp.buf);
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, _("Please manually handle the merging of each conflicted submodule."));
+		strbuf_addf(&msg, "%s\n", tmp.buf);
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, _("This can be accomplished with the following steps:"));
+		strbuf_addf(&msg, "%s\n", tmp.buf);
+		strbuf_release(&tmp);
+
+		for_each_string_list_item(item, csub) {
+			const char *abbrev= 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"),
+													item->string, abbrev);
+			strbuf_addf(&msg, _(" - %s"), tmp.buf);
+			strbuf_addf(&msg, "\n");
+			strbuf_release(&tmp);
+			strbuf_addf(&tmp, _("or update to an existing commit which has merged those changes"));
+			strbuf_addf(&msg, _("   %s"), tmp.buf);
+			strbuf_addf(&msg, "\n");
+			strbuf_release(&tmp);
+		}
+		strbuf_addf(&tmp, _("come back to superproject and run:"));
+		strbuf_addf(&msg, _(" - %s"), tmp.buf);
+		strbuf_addf(&msg, "\n\n");
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, "git add ");
+		strbuf_add_separated_string_list(&tmp, " ", csub);
+		strbuf_addf(&msg, _("       %s"), tmp.buf);
+		strbuf_addf(&msg, "\n\n");
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, _("to record the above merge or update"));
+		strbuf_addf(&msg, _("   %s"), tmp.buf);
+		strbuf_addf(&msg, "\n");
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, _("resolve any other conflicts in the superproject"));
+		strbuf_addf(&msg, _(" - %s"), tmp.buf);
+		strbuf_addf(&msg, "\n");
+		strbuf_release(&tmp);
+
+		strbuf_addf(&tmp, _("commit the resulting index in the superproject"));
+		strbuf_addf(&msg, _(" - %s"), tmp.buf);
+		strbuf_addf(&msg, "\n");
+		strbuf_release(&tmp);
+
+		printf("%s", msg.buf);
+		strbuf_release(&msg);
+	}
+}
+
 void merge_display_update_messages(struct merge_options *opt,
 				   int detailed,
 				   struct merge_result *result)
@@ -4461,6 +4558,8 @@ 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);
@@ -4657,6 +4756,7 @@ 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;
 	}
diff --git a/t/t6437-submodule-merge.sh b/t/t6437-submodule-merge.sh
index c253bf759a..414597a420 100755
--- a/t/t6437-submodule-merge.sh
+++ b/t/t6437-submodule-merge.sh
@@ -103,8 +103,25 @@ test_expect_success 'setup for merge search' '
 	 echo "file-c" > file-c &&
 	 git add file-c &&
 	 git commit -m "sub-c") &&
-	git commit -a -m "c" &&
+	git commit -a -m "c")
+'
 
+test_expect_success 'merging should conflict for non fast-forward' '
+	test_when_finished "git -C merge-search reset --hard" &&
+	(cd merge-search &&
+	 git checkout -b test-nonforward-a b &&
+	  if test "$GIT_TEST_MERGE_ALGORITHM" = ort
+	  then
+		test_must_fail git merge c >actual &&
+		sub_expect="go to submodule (sub), and either merge commit $(git -C sub rev-parse --short sub-c)" &&
+	 	grep "$sub_expect" actual
+	  else
+		test_must_fail git merge c 2> actual
+	  fi)
+'
+
+test_expect_success 'finish setup for merge-search' '
+	(cd merge-search &&
 	git checkout -b d a &&
 	(cd sub &&
 	 git checkout -b sub-d sub-b &&
@@ -129,14 +146,16 @@ test_expect_success 'merge with one side as a fast-forward of the other' '
 	 test_cmp expect actual)
 '
 
-test_expect_success 'merging should conflict for non fast-forward' '
+test_expect_success 'merging should conflict for non fast-forward (resolution exists)' '
 	(cd merge-search &&
-	 git checkout -b test-nonforward b &&
+	 git checkout -b test-nonforward-b b &&
 	 (cd sub &&
 	  git rev-parse --short sub-d > ../expect) &&
 	  if test "$GIT_TEST_MERGE_ALGORITHM" = ort
 	  then
-		test_must_fail git merge c >actual
+		test_must_fail git merge c >actual &&
+		sub_expect="go to submodule (sub), and either merge commit $(git -C sub rev-parse --short sub-c)" &&
+	 	grep "$sub_expect" actual
 	  else
 		test_must_fail git merge c 2> actual
 	  fi &&
@@ -161,7 +180,9 @@ test_expect_success 'merging should fail for ambiguous common parent' '
 	 ) &&
 	 if test "$GIT_TEST_MERGE_ALGORITHM" = ort
 	 then
-		test_must_fail git merge c >actual
+		test_must_fail git merge c >actual &&
+		sub_expect="go to submodule (sub), and either merge commit $(git -C sub rev-parse --short sub-c)" &&
+		grep "$sub_expect" actual
 	 else
 		test_must_fail git merge c 2> actual
 	 fi &&
@@ -205,7 +226,12 @@ test_expect_success 'merging should fail for changes that are backwards' '
 	git commit -a -m "f" &&
 
 	git checkout -b test-backward e &&
-	test_must_fail git merge f)
+	test_must_fail git merge f >actual &&
+	if test "$GIT_TEST_MERGE_ALGORITHM" = ort
+    then
+		sub_expect="go to submodule (sub), and either merge commit $(git -C sub rev-parse --short sub-d)" &&
+		grep "$sub_expect" actual
+	fi)
 '
 
 
@@ -476,4 +502,44 @@ test_expect_failure 'directory/submodule conflict; merge --abort works afterward
 	)
 '
 
+# Setup:
+#   - Submodule has 2 commits: a and b
+#   - Superproject branch 'a' adds and commits submodule pointing to 'commit a'
+#   - Superproject branch 'b' adds and commits submodule pointing to 'commit b'
+# If these two branches are now merged, there is no merge base
+test_expect_success 'setup for null merge base' '
+	mkdir no-merge-base &&
+	(cd no-merge-base &&
+	git init &&
+	mkdir sub &&
+	(cd sub &&
+	 git init &&
+	 echo "file-a" > file-a &&
+	 git add file-a &&
+	 git commit -m "commit a") &&
+	git commit --allow-empty -m init &&
+	git branch init &&
+	git checkout -b a init &&
+	git add sub &&
+	git commit -m "a" &&
+	git switch main &&
+	(cd sub &&
+	 echo "file-b" > file-b &&
+	 git add file-b &&
+	 git commit -m "commit b"))
+'
+
+test_expect_success 'merging should fail with no merge base' '
+	(cd no-merge-base &&
+	git checkout -b b init &&
+	git add sub &&
+	git commit -m "b" &&
+	test_must_fail git merge a >actual &&
+	if test "$GIT_TEST_MERGE_ALGORITHM" = ort
+    then
+		sub_expect="go to submodule (sub), and either merge commit $(git -C sub rev-parse --short HEAD^1)" &&
+		grep "$sub_expect" actual
+	fi)
+'
+
 test_done
diff --git a/t/t7402-submodule-rebase.sh b/t/t7402-submodule-rebase.sh
index 8e32f19007..ebeca12a71 100755
--- a/t/t7402-submodule-rebase.sh
+++ b/t/t7402-submodule-rebase.sh
@@ -104,7 +104,7 @@ test_expect_success 'rebasing submodule that should conflict' '
 	test_tick &&
 	git commit -m fourth &&
 
-	test_must_fail git rebase --onto HEAD^^ HEAD^ HEAD^0 &&
+	test_must_fail git rebase --onto HEAD^^ HEAD^ HEAD^0 >actual_output &&
 	git ls-files -s submodule >actual &&
 	(
 		cd submodule &&
@@ -112,7 +112,12 @@ test_expect_success 'rebasing submodule that should conflict' '
 		echo "160000 $(git rev-parse HEAD^^) 2	submodule" &&
 		echo "160000 $(git rev-parse HEAD) 3	submodule"
 	) >expect &&
-	test_cmp expect actual
+	test_cmp expect actual &&
+	if test "$GIT_TEST_MERGE_ALGORITHM" = ort
+    then
+		sub_expect="go to submodule (submodule), and either merge commit $(git -C submodule rev-parse --short HEAD^0)" &&
+		grep "$sub_expect" actual_output
+	fi
 '
 
 test_done

base-commit: 9dd64cb4d310986dd7b8ca7fff92f9b61e0bd21a
-- 
2.37.1.455.g008518b4e5-goog