sub() method and function fail if rewrite is NULL

src/tests/sub.vtc

@@ -69,6 +69,12 @@ varnish v1 -vcl {
 			= b.sub(req.http.undef, "x", "text undef");
 		set resp.http.undeftextf
 			= re2.sub("b", req.http.undef, "x", "text undef");
+
+		# Undefined rewrite
+		set resp.http.undefrewrite
+			= b.sub("b", req.http.undef, "rewrite undef");
+		set resp.http.undefrewritef
+			= re2.sub("b", "b", req.http.undef, "rewrite undef");
 	}
 
 } -start
@@ -108,6 +114,9 @@ client c1 {
 
 	expect resp.http.undeftext == "text undef"
 	expect resp.http.undeftextf == "text undef"
+
+	expect resp.http.undefrewrite == "rewrite undef"
+	expect resp.http.undefrewritef == "rewrite undef"
 } -run
 
 logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
@@ -119,5 +128,9 @@ logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
 
 	expect * = VCL_Error "^vmod re2 error: b.sub.text=<undefined>, fallback=.text undef..: text is undefined$"
 	expect * = VCL_Error "^vmod re2 error: re2.sub.pattern=.b., text=<undefined>, fallback=.text undef..: text is undefined$"
+
+	expect * = VCL_Error "^vmod re2 error: b.sub.text=.b., rewrite=<undefined>, fallback=.rewrite undef..: rewrite is undefined$"
+	expect * = VCL_Error "^vmod re2 error: re2.sub.pattern=.b., text=.b., rewrite=<undefined>, fallback=.rewrite undef..: rewrite is undefined$"
+
 	expect * = End
 } -run

src/vmod_re2.c

@@ -418,8 +418,6 @@ vmod_regex_namedref(VRT_CTX, struct vmod_re2_regex *re, VCL_STRING name,
 
 #undef ERR_PREFIX
 
-#define ERR_PREFIX "%s.sub(text=\"%s\", rewrite=\"%s\", fallback=\"%s\"): "
-
 VCL_STRING
 vmod_regex_sub(VRT_CTX, struct vmod_re2_regex *re, VCL_STRING text,
 	       VCL_STRING rewrite, VCL_STRING fallback)
@@ -435,14 +433,16 @@ vmod_regex_sub(VRT_CTX, struct vmod_re2_regex *re, VCL_STRING text,
 		     "text is undefined", re->vcl_name, fallback);
 		return fallback;
 	}
-	if (rewrite == NULL)
-		rewrite = "";
+	if (rewrite == NULL) {
+		VERR(ctx, "%s.sub(text=\"%s\", rewrite=<undefined>, "
+		     "fallback=\"%s\"): rewrite is undefined", re->vcl_name,
+		     text, fallback);
+		return fallback;
+	}
 
 	return sub(ctx, re->vre2, text, rewrite, fallback);
 }
 
-#undef ERR_PREFIX
-
 /* Object set */
 
 VCL_VOID
@@ -755,8 +755,12 @@ vmod_sub(VRT_CTX, VCL_STRING pattern, VCL_STRING text, VCL_STRING rewrite,
 		     "fallback=\"%s\"): text is undefined", pattern, fallback);
 		return fallback;
 	}
-	if (rewrite == NULL)
-		rewrite = "";
+	if (rewrite == NULL) {
+		VERR(ctx, "re2.sub(pattern=\"%s\", text=\"%s\", "
+		     "rewrite=<undefined>, fallback=\"%s\"): "
+		     "rewrite is undefined", pattern, text, fallback);
+		return fallback;
+	}
 
 	if ((err = vre2_init(&vre2, pattern, utf8, posix_syntax, longest_match,
 			     max_mem, literal, never_nl, dot_nl, never_capture,