nghttpx: Reconnect h1 backend if it lost connection before sending headers

This is the second attempt.  The first attempt was
8a59ce6d37 and it failed.

src/shrpx_http2_upstream.cc

@@ -1262,8 +1262,11 @@ int Http2Upstream::downstream_read(DownstreamConnection *dconn) {
   } else {
     auto rv = downstream->on_read();
     if (rv == SHRPX_ERR_EOF) {
+      if (downstream->get_request_header_sent()) {
         return downstream_eof(dconn);
       }
+      return SHRPX_ERR_RETRY;
+    }
     if (rv == SHRPX_ERR_DCONN_CANCELED) {
       downstream->pop_downstream_connection();
       handler_->signal_write();

src/shrpx_http_downstream_connection.cc

@@ -138,26 +138,33 @@ void connect_timeoutcb(struct ev_loop *loop, ev_timer *w, int revents) {
 }
 } // namespace
 
+namespace {
+void backend_retry(Downstream *downstream) {
+  retry_downstream_connection(downstream, 502);
+}
+} // namespace
+
 namespace {
 void readcb(struct ev_loop *loop, ev_io *w, int revents) {
+  int rv;
   auto conn = static_cast<Connection *>(w->data);
   auto dconn = static_cast<HttpDownstreamConnection *>(conn->data);
   auto downstream = dconn->get_downstream();
   auto upstream = downstream->get_upstream();
   auto handler = upstream->get_client_handler();
 
-  if (upstream->downstream_read(dconn) != 0) {
+  rv = upstream->downstream_read(dconn);
+  if (rv != 0) {
+    if (rv == SHRPX_ERR_RETRY) {
+      backend_retry(downstream);
+      return;
+    }
+
     delete handler;
   }
 }
 } // namespace
 
-namespace {
-void backend_retry(Downstream *downstream) {
-  retry_downstream_connection(downstream, 502);
-}
-} // namespace
-
 namespace {
 void writecb(struct ev_loop *loop, ev_io *w, int revents) {
   int rv;

src/shrpx_https_upstream.cc

@@ -788,8 +788,11 @@ int HttpsUpstream::downstream_read(DownstreamConnection *dconn) {
   rv = downstream->on_read();
 
   if (rv == SHRPX_ERR_EOF) {
+    if (downstream->get_request_header_sent()) {
       return downstream_eof(dconn);
     }
+    return SHRPX_ERR_RETRY;
+  }
 
   if (rv == SHRPX_ERR_DCONN_CANCELED) {
     downstream->pop_downstream_connection();