describe: error if blob not found

If describe_blob() does not find the blob in question, it returns an
empty strbuf, and we print an empty line. This differs from
describe_commit(), which always either returns an answer or calls die()
itself. As the blob function was bolted onto the command afterwards, I
think its behavior is not intentional, and it is just a bug that it does
not report an error.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

builtin/describe.c

@@ -535,6 +535,9 @@ static void describe_blob(const struct object_id *oid, struct strbuf *dst)
 	reset_revision_walk();
 	release_revisions(&revs);
 	strvec_clear(&args);
+
+	if (!dst->len)
+		die(_("blob '%s' not reachable from HEAD"), oid_to_hex(oid));
 }
 
 static void describe(const char *arg, int last_one)

t/t6120-describe.sh

@@ -409,6 +409,12 @@ test_expect_success 'describe tag object' '
 	test_grep "fatal: test-blob-1 is neither a commit nor blob" actual
 '
 
+test_expect_success 'describe an unreachable blob' '
+	blob=$(echo not-found-anywhere | git hash-object -w --stdin) &&
+	test_must_fail git describe $blob 2>actual &&
+	test_grep "blob .$blob. not reachable from HEAD" actual
+'
+
 test_expect_success ULIMIT_STACK_SIZE 'name-rev works in a deep repo' '
 	i=1 &&
 	while test $i -lt 8000