sub() method and function fail if the fallback is NULL

src/tests/sub.vtc

@@ -54,6 +54,11 @@ varnish v1 -vcl {
 		# Match failure
 		set resp.http.bfail = b.sub("acd", "x", "fallbackb");
 		set resp.http.bfailf = re2.sub("b", "acd", "x", "fallbackf");
+
+		# Undefined fallback
+		set resp.http.undeffallback = b.sub("b", "x", req.http.undef);
+		set resp.http.undeffallbackf
+			= re2.sub("b", "b", "x", req.http.undef);
 	}
 
 } -start
@@ -85,4 +90,14 @@ client c1 {
 
 	expect resp.http.bfail == "fallbackb"
 	expect resp.http.bfailf == "fallbackf"
+
+	expect resp.http.undeffallback == "**SUB METHOD FAILED**"
+	expect resp.http.undeffallbackf == "**SUB FUNCTION FAILED**"
+} -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error "^vmod re2 error: b.sub..: fallback is undefined$"
+	expect * = VCL_Error "^vmod re2 error: re2.sub..: fallback is undefined$"
+	expect * = End
 } -run

src/vmod_re2.c

@@ -426,12 +426,14 @@ vmod_regex_sub(VRT_CTX, struct vmod_re2_regex *re, VCL_STRING text,
 {
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(re, VMOD_RE2_REGEX_MAGIC);
+	if (fallback == NULL) {
+		VERR(ctx, "%s.sub(): fallback is undefined", re->vcl_name);
+		return "**SUB METHOD FAILED**";
+	}
 	if (text == NULL)
 		text = "";
 	if (rewrite == NULL)
 		rewrite = "";
-	if (fallback == NULL)
-		fallback = "";
 
 	return sub(ctx, re->vre2, text, rewrite, fallback);
 }
@@ -736,14 +738,16 @@ vmod_sub(VRT_CTX, VCL_STRING pattern, VCL_STRING text, VCL_STRING rewrite,
 	const char *ret, *err;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	if (fallback == NULL) {
+		ERR(ctx, "re2.sub(): fallback is undefined");
+		return "**SUB FUNCTION FAILED**";
+	}
 	if (pattern == NULL)
 		pattern = "";
 	if (text == NULL)
 		text = "";
 	if (rewrite == NULL)
 		rewrite = "";
-	if (fallback == NULL)
-		fallback = "";
 
 	if ((err = vre2_init(&vre2, pattern, utf8, posix_syntax, longest_match,
 			     max_mem, literal, never_nl, dot_nl, never_capture,