util: add null string test handling to strv_join

Make it return NULL for a string array in the form of [ NULL ], like the docs
say. This also adds an extra safety check for the joiner to be of a reasonable
length to avoid overflows.

Found in
https://gitlab.freedesktop.org/libinput/libinput/issues/26#note_6320

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

src/libinput-util.c

@@ -586,11 +586,15 @@ strv_join(char **strv, const char *joiner)
 	if (!strv || !joiner)
 		return NULL;
 
+	if (strv[0] == NULL)
+		return NULL;
+
 	for (s = strv, count = 0; *s; s++, count++) {
 		slen += strlen(*s);
 	}
 
 	assert(slen < 1000);
+	assert(strlen(joiner) < 1000);
 
 	slen += (count - 1) * strlen(joiner);
 

test/test-misc.c

@@ -1366,6 +1366,7 @@ START_TEST(strjoin_test)
 		{ { NULL }, NULL, NULL }
 	};
 	struct strjoin_test *t = tests;
+	struct strjoin_test nulltest = { {NULL}, "x", NULL };
 
 	while (t->strv[0]) {
 		char *str;
@@ -1377,6 +1378,8 @@ START_TEST(strjoin_test)
 		free(str);
 		t++;
 	}
+
+	ck_assert(strv_join(nulltest.strv, "x") == NULL);
 }
 END_TEST