[PATCH] submodule init: fail gracefully with a missing .gitmodules file

[Stefan Beller <sbeller@google.com>]

When there is no .gitmodules file availabe to initialize a submodule
from, `submodule_from_path` just returns NULL. We need to check for
that and abort gracefully. When `submodule init` was implemented in shell,
a missing .gitmodules file would result in an error message

    No url found for submodule path '%s' in .gitmodules

While that is technically true, I think we can broaden the error message
and just say there is no section for the submodule in the .gitmodules file.

Signed-off-by: Stefan Beller <sbeller@google.com>
---

 This goes on top of sb/submodule-init (I added the base-commit
 below, just in case anyone uses that feature already. Though I did
 that manually :)
 
 Thanks,
 Stefan

 builtin/submodule--helper.c | 10 +++++++---
 t/t7400-submodule-basic.sh  |  8 ++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index b6d4f27..af5406e 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -314,13 +314,17 @@ static void init_submodule(const char *path, const char *prefix, int quiet)
 	/* Only loads from .gitmodules, no overlay with .git/config */
 	gitmodules_config();
 
-	sub = submodule_from_path(null_sha1, path);
-
 	if (prefix) {
 		strbuf_addf(&sb, "%s%s", prefix, path);
 		displaypath = strbuf_detach(&sb, NULL);
 	} else
-		displaypath = xstrdup(sub->path);
+		displaypath = xstrdup(path);
+
+	sub = submodule_from_path(null_sha1, path);
+
+	if (!sub)
+		die(_("No section found for submodule path '%s' in .gitmodules"),
+			displaypath);
 
 	/*
 	 * Copy url setting when it is not set yet.
diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh
index f99f674..f2b3519 100755
--- a/t/t7400-submodule-basic.sh
+++ b/t/t7400-submodule-basic.sh
@@ -18,6 +18,14 @@ test_expect_success 'setup - initial commit' '
 	git branch initial
 '
 
+test_expect_success 'sane abort on missing .gitmodules file' '
+	test_when_finished "git update-index --remove sub" &&
+	git update-index --add --cacheinfo 160000,$(git rev-parse HEAD),sub &&
+	# missing the .gitmodules file here
+	test_must_fail git submodule init 2>actual &&
+	test_i18ngrep "No section found for submodule path" actual
+'
+
 test_expect_success 'configuration parsing' '
 	test_when_finished "rm -f .gitmodules" &&
 	cat >.gitmodules <<-\EOF &&
-- 
2.8.0.26.g3604242.dirty

base-commit: 3604242f080a813d6f20a7394def422d1e55b30e