Bugfix detecting if match() was not called before backref()

src/tests/backref_basic.vtc

@@ -54,3 +54,43 @@ client c1 -repeat 2 {
 	expect resp.http.baz2 == "quux"
 	expect resp.http.frap == "_barf_frap_"
 } -run
+
+# backref failure
+varnish v1 -vcl {
+	import pcre2 from "${vmod_topbuild}/src/.libs/libvmod_pcre2.so";
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		new frobnitz = pcre2.regex("(frob)(nitz)");
+		new barbaz = pcre2.regex("(bar)(baz)");
+		new azbc = pcre2.regex("(a|(z))(bc)");
+	}
+
+	sub vcl_recv {
+		return(synth(200));
+	}
+
+	sub vcl_synth {
+		# Call to backref() before match()
+		set resp.http.nomatch = barbaz.backref(0, "fallback");
+
+		# match() and backref() now re-use the object's task scope
+		if (barbaz.match("barbaz")) {
+			set resp.http.bar0 = barbaz.backref(0, "error0");
+		}
+	}
+}
+
+client c1 -repeat 2 {
+	txreq
+	rxresp
+	expect resp.status == "200"
+	expect resp.http.nomatch == "fallback"
+	expect resp.http.bar0 == "barbaz"
+} -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error "^vmod pcre2 error: barbaz.backref.0. called without prior match$"
+	expect * = End
+} -run

src/vmod_pcre2.c

@@ -536,7 +536,8 @@ vmod_regex_backref(VRT_CTX, struct vmod_pcre2_regex *regex, VCL_INT ref,
 	CHECK_UINT32_RANGE(ref, regex->vcl_name, ".backref()", fallback);
 
 	match_task = VRT_priv_task(ctx, regex);
-	if (match_task == NULL) {
+	AN(match_task);
+	if (match_task->priv == NULL) {
 		VERR(ctx, "%s.backref(%ld) called without prior match",
 		     regex->vcl_name, ref);
 		return fallback;