feat server: flush headers right away in streaming

In case of response stream it makes sense to flush headers completely before starting to produce the body parts,
so a client can react to that earlier.

Tests: протестировано CI
0220c2f6b0019bb6621003e0cd9f4c759b082fb3

Pull Request resolved: userver-framework#488

core/src/server/http/http_response.cpp

@@ -358,9 +358,8 @@ std::size_t HttpResponse::SetBodyStreamed(
   impl::OutputHeader(
       header, USERVER_NAMESPACE::http::headers::kTransferEncoding, "chunked");
 
-  if (is_body_forbidden) {
-    header.append(kCrlf);
-  }
+  // headers end marker
+  header.append(kCrlf);
 
   // send HTTP headers
   size_t sent_bytes = socket.WriteAll(header.data(), header.size(), {});
@@ -373,19 +372,27 @@ std::size_t HttpResponse::SetBodyStreamed(
 
   // Transmit HTTP response body
   std::string body_part;
+  // First chunk must be sent without kCrlf
+  // because kCrlf was sent with headers
+  bool first_chunk_processed = false;
   while (body_stream_->Pop(body_part)) {
     if (body_part.empty()) {
       LOG_DEBUG() << "Zero size body_part in http_response.cpp";
       continue;
     }
 
-    auto size = fmt::format("\r\n{:x}\r\n", body_part.size());
+    auto size = first_chunk_processed
+                    ? fmt::format("\r\n{:x}\r\n", body_part.size())
+                    : fmt::format("{:x}\r\n", body_part.size());
     sent_bytes += socket.WriteAll(
         {{size.data(), size.size()}, {body_part.data(), body_part.size()}},
         engine::Deadline{});
+
+    first_chunk_processed = true;
   }
 
-  const constexpr std::string_view terminating_chunk{"\r\n0\r\n\r\n"};
+  const std::string_view terminating_chunk{
+      first_chunk_processed ? "\r\n0\r\n\r\n" : "0\r\n\r\n"};
   sent_bytes +=
       socket.WriteAll(terminating_chunk.data(), terminating_chunk.size(), {});