Enforce the order of .add() and .compile().

src/tests/match.vtc

@@ -888,6 +888,30 @@ varnish v1 -errvcl {vmod selector failure: t.compile(): "foo" added more than on
 	}
 }
 
+varnish v1 -errvcl {vmod selector failure: s.compile(): set was already compiled} {
+	import ${vmod_selector};
+	backend b None;
+
+	sub vcl_init {
+		new s = selector.set();
+		s.add("foo");
+		s.compile();
+		s.compile();
+	}
+}
+
+varnish v1 -errvcl {vmod selector failure: s.add(): set was already compiled} {
+	import ${vmod_selector};
+	backend b None;
+
+	sub vcl_init {
+		new s = selector.set();
+		s.add("foo");
+		s.compile();
+		s.add("bar");
+	}
+}
+
 varnish v1 -vcl {
 	import ${vmod_selector};
 	backend b None;

src/vmod_selector.c

@@ -269,7 +269,10 @@ vmod_set_add(VRT_CTX, struct vmod_selector_set *set,
 		      set->vcl_name);
 		return;
 	}
-
+	if (set->hash != NULL) {
+		VFAIL(ctx, "%s.add(): set was already compiled", set->vcl_name);
+		return;
+	}
 	if (args->arg1 == NULL) {
 		VFAIL(ctx, "%s.add(): string to be added is NULL",
 		      set->vcl_name);
@@ -358,6 +361,11 @@ vmod_set_compile(VRT_CTX, struct VPFX(selector_set) *set)
 		      set->vcl_name);
 		return;
 	}
+	if (set->hash != NULL) {
+		VFAIL(ctx, "%s.compile(): set was already compiled",
+		      set->vcl_name);
+		return;
+	}
 
 	members = set->members;
 	if (!set->case_sensitive)