backref() method and function always fail if the fallback is NULL,

for example if set from an unset header

src/tests/backref_fail.vtc

@@ -41,6 +41,11 @@ varnish v1 -vcl+backend {
 			set resp.http.foo0 = barbaz.backref(0, "error0");
 			set resp.http.foo1 = barbaz.backref(1, "error1");
 			set resp.http.foo2 = barbaz.backref(2, "error2");
+
+			# Even if a match succeeded, backref() fails if
+			# the fallback is undefined.
+			set resp.http.undeffallback
+				= barbaz.backref(0, resp.http.undef);
 		}
 		/* match fails */
 		if (barbaz.match(resp.http.barf)) {
@@ -71,6 +76,9 @@ varnish v1 -vcl+backend {
 		set resp.http.never0 = never.backref(0, "fallback0");
 		set resp.http.never1 = never.backref(1, "fallback1");
 		set resp.http.never2 = never.backref(2, "fallback2");
+
+		/* Fallback default */
+		set resp.http.nofallback = never.backref(0);
 	}
 } -start
 
@@ -87,6 +95,7 @@ client c1 {
 	expect resp.http.foo0 == "barbaz"
 	expect resp.http.foo1 == "bar"
 	expect resp.http.foo2 == "baz"
+	expect resp.http.undeffallback ~ "^..BACKREF (METHOD|FUNCTION) FAILED..$"
 	expect resp.http.puke == <undef>
 	expect resp.http.barf0 == "fallback0"
 	expect resp.http.barf1 == "fallback1"
@@ -99,6 +108,7 @@ client c1 {
 	expect resp.http.never0 == "fallback0"
 	expect resp.http.never1 == "fallback1"
 	expect resp.http.never2 == "fallback2"
+	expect resp.http.nofallback ~ "^..BACKREF (METHOD|FUNCTION) FAILED..$"
 } -run
 
 logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
@@ -111,6 +121,8 @@ logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
         # would have also failed due to failing prior match
 	expect * = VCL_Error "^vmod re2 error: frobnitz.backref.ref=3, fallback=.fallback3..: backref out of range .max 2.$"
 
+	expect * = VCL_Error "^vmod re2 error: barbaz.backref.ref=0, fallback=.<undefined>..: fallback is undefined$"
+
 	expect * = VCL_Error "^vmod re2 error: never.backref.ref=0, fallback=.fallback0..: never_capture is true for object never$"
 	expect * = VCL_Error "^vmod re2 error: never.backref.ref=1, fallback=.fallback1..: never_capture is true for object never$"
 	expect * = VCL_Error "^vmod re2 error: never.backref.ref=2, fallback=.fallback2..: never_capture is true for object never$"
@@ -235,6 +247,8 @@ varnish v1 -vcl+backend {
 			set resp.http.foo0 = re2.backref(0, "error0");
 			set resp.http.foo1 = re2.backref(1, "error1");
 			set resp.http.foo2 = re2.backref(2, "error2");
+			set resp.http.undeffallback
+				= re2.backref(0, resp.http.undef);
 		}
 		if (re2.match("(bar)(baz)", resp.http.barf)) {
 			set resp.http.puke = "match";
@@ -258,6 +272,8 @@ varnish v1 -vcl+backend {
 		set resp.http.never0 = re2.backref(0, "fallback0");
 		set resp.http.never1 = re2.backref(1, "fallback1");
 		set resp.http.never2 = re2.backref(2, "fallback2");
+
+		set resp.http.nofallback = re2.backref(0);
 	}
 }
 
@@ -265,6 +281,7 @@ logexpect l3 -v v1 -d 0 -g vxid -q "VCL_Error" {
 	expect 0 * Begin req
 	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=0, fallback=.fallback..: called without previous match$"
 	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=3, fallback=.fallback3..: backref out of range .max 2.$"
+	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=0, fallback=.<undefined>..: fallback is undefined$"
 	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=0, fallback=.fallback0..: never_capture was true in previous match$"
 	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=1, fallback=.fallback1..: never_capture was true in previous match$"
 	expect * = VCL_Error "^vmod re2 error: re2.backref.ref=2, fallback=.fallback2..: never_capture was true in previous match$"

src/vmod_re2.c

@@ -362,8 +362,11 @@ vmod_regex_backref(VRT_CTX, struct vmod_re2_regex *re, VCL_INT refnum,
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(re, VMOD_RE2_REGEX_MAGIC);
 	assert(refnum >= 0);
-	if (fallback == NULL)
-		fallback = "";
+	if (fallback == NULL) {
+		VERR(ctx, ERR_PREFIX "fallback is undefined", re->vcl_name,
+		     refnum, "<undefined>");
+		return "**BACKREF METHOD FAILED**";
+	}
 
 	if (re->never_capture) {
 		VERR(ctx, ERR_PREFIX "never_capture is true for object %s",
@@ -643,8 +646,11 @@ vmod_backref(VRT_CTX, struct vmod_priv *priv, VCL_INT refnum,
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	AN(priv);
 	assert(refnum >= 0);
-	if (fallback == NULL)
-		fallback = "";
+	if (fallback == NULL) {
+		VERR(ctx, ERR_PREFIX "fallback is undefined",  refnum,
+		     "<undefined>");
+		return "**BACKREF FUNCTION FAILED**";
+	}
 	if (priv->priv == NULL) {
 		VERR(ctx, ERR_PREFIX "called without previous match", refnum,
 		     fallback);