proxy: don't allow mutating mutated objects

proxy_mutator.c

@@ -960,6 +960,12 @@ static int mcp_mut_run(struct mcp_mut_run *run) {
     // ensure space and/or allocate memory then seed our destination pointer.
     if (mut->type == MUT_REQ) {
         mcp_request_t *rq = run->arg;
+        if (rq->pr.vbuf) {
+            // FIXME: maybe NULL rq->pr.request in cleanup phase and test that
+            // instead? this check will only fire if req had a vbuf.
+            proxy_lua_error(run->L, "mutator: request has already been rendered");
+            return 0;
+        }
         // future.. should be able to dynamically assign request buffer.
         if (total > MCP_REQUEST_MAXLEN) {
             proxy_lua_error(run->L, "mutator: new request is too long");
@@ -989,6 +995,10 @@ static int mcp_mut_run(struct mcp_mut_run *run) {
         }
     } else {
         mcp_resp_t *rs = run->arg;
+        if (rs->buf) {
+            proxy_lua_error(run->L, "mutator: result has already been rendered");
+            return 0;
+        }
 
         // value is inlined in result buffers. future intention to allow more
         // complex objects so we can refcount values.