transport/http2: use HTTP 400 for bad requests instead of 500

Non-servicable HTTP requests from clients previously resulted in an HTTP
500 response, but no error exists within the server; the client simply
sent a malformed request. Detect bad requests and return HTTP 400 Bad
Request instead of HTTP 500 Internal Server Error.

fixes #5802

internal/transport/handler_server.go

@@ -46,6 +46,12 @@ import (
 	"google.golang.org/grpc/status"
 )
 
+// ErrInvalidWriter indicates that the provided http.ResponseWriter doesn't
+// support http.Flusher. Since this is typically a server-side error,
+// ErrInvalidWriter is intended for use in differentiating bad requests from
+// internal errors.
+var ErrInvalidWriter = errors.New("gRPC requires a ResponseWriter supporting http.Flusher")
+
 // NewServerHandlerTransport returns a ServerTransport handling gRPC
 // from inside an http.Handler. It requires that the http Server
 // supports HTTP/2.
@@ -63,7 +69,7 @@ func NewServerHandlerTransport(w http.ResponseWriter, r *http.Request, stats []s
 		return nil, errors.New("invalid gRPC request content-type")
 	}
 	if _, ok := w.(http.Flusher); !ok {
-		return nil, errors.New("gRPC requires a ResponseWriter supporting http.Flusher")
+		return nil, ErrInvalidWriter
 	}
 
 	st := &serverHandlerTransport{

server.go

@@ -1008,7 +1008,11 @@ var _ http.Handler = (*Server)(nil)
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	st, err := transport.NewServerHandlerTransport(w, r, s.opts.statsHandlers)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
+		var status = http.StatusBadRequest
+		if err == transport.ErrInvalidWriter {
+			status = http.StatusInternalServerError
+		}
+		http.Error(w, err.Error(), status)
 		return
 	}
 	if !s.addConn(listenerAddressForServeHTTP, st) {