Send an http client request's head and body in one write

request.end(body) on a request whose head was not sent yet wrote the
head and then the body. On a socket that was already connected (a
reused keep-alive connection), each write is sent at once, so the two
went out as separate segments, and the server could handle the head and
answer before the body arrived. Node sends the head together with the
body. When the client and the server run in one program, the server's
'end' and 'close' for the request and the client's reading of the
response then depended on which socket the poller reported first: corpus
5409-http-held-request-body printed the client's line before the
request's 'end' and 'close' in a macOS dev build.

The head and a body known at end() now go out as one write, as Node
sends them (the server's response already did).

packages/runtime/src/scr_http.c

@@ -4323,14 +4323,32 @@ static void scr_http_client_end_raw(ScrHttpClientReq *c, const char *data, size_
   }
   if (!c->head_sent) {
     /* whole body known NOW: Content-Length framing (or none — the head
-     * serializer's empty-body method split) */
+     * serializer's empty-body method split). Head and body go out as one
+     * write, as Node sends them: on a connected (reused) socket two writes
+     * would be two segments, and the server could answer before the body
+     * arrives. */
+    c->hold_head = true;
     scr_http_client_send_head(c, (long long)len);
+    c->hold_head = false;
     if (scr_exc_pending()) return;
     if (c->chunked) {
       scr_http_client_write_raw(c, data, len);
       if (c->sock) scr_http_send_chunk_end(c->sock, &c->trailers);
-    } else if (c->sock && len > 0) {
-      scr_net_sock_write_native(c->sock, data, len);
+    } else if (c->held_head != NULL) {
+      char *head = c->held_head;
+      size_t head_len = c->held_head_len;
+      c->held_head = NULL;
+      c->held_head_len = c->held_head_cap = 0;
+      if (c->sock) {
+        if (len > 0) {
+          char *joined = realloc(head, head_len + len);
+          if (!joined) scr_http_oom();
+          head = joined;
+          memcpy(head + head_len, data, len);
+        }
+        scr_net_sock_write_native(c->sock, head, head_len + len);
+      }
+      free(head);
     }
   } else {
     scr_http_client_write_raw(c, data, len);