From 0165ca40e00d8bc25b29edd145d394a4f78692dc Mon Sep 17 00:00:00 2001
From: Sebastian Harl <sh@tokkee.org>
Date: Thu, 20 Nov 2014 23:35:27 +0100
Subject: [PATCH] Make sure to not use a client connection multiple times in
 parallel.

That's not supported by the client at the moment. Instead, use multiple
connections. That's a bit of a hack but the server can easily handle many idle
connections.
---
 main.go          | 12 ++++++++----
 server/server.go | 28 ++++++++++++++++++++++------
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/main.go b/main.go
index 9d099da..20bb732 100644
--- a/main.go
+++ b/main.go
@@ -50,13 +50,17 @@ func main() {
 	flag.Parse()
 
 	log.Printf("Connecting to SysDB at %s.", *addr)
-	conn, err := client.Connect(*addr, *user)
-	if err != nil {
-		fatalf("Failed to connect to SysDB at %q: %v", *addr, err)
+	var conns []*client.Conn
+	for i := 0; i < 10; i++ {
+		conn, err := client.Connect(*addr, *user)
+		if err != nil {
+			fatalf("Failed to connect to SysDB at %q: %v", *addr, err)
+		}
+		conns = append(conns, conn)
 	}
 
 	srv, err := server.New(server.Config{
-		Conn:         conn,
+		Conns:        conns,
 		TemplatePath: *tmpl,
 		StaticPath:   *static,
 	})
diff --git a/server/server.go b/server/server.go
index 995f255..706dac2 100644
--- a/server/server.go
+++ b/server/server.go
@@ -44,8 +44,11 @@ import (
 
 // A Config specifies configuration values for a SysDB web server.
 type Config struct {
-	// Conn specifies a connection to a SysDB server instance.
-	Conn *client.Conn
+	// Conns is a slice of connections to a SysDB server instance. The number of
+	// elements specifies the maximum number of parallel queries to the backend.
+	// Note that a client connection is not thread-safe but multiple idle
+	// connections don't impose any load on the server.
+	Conns []*client.Conn
 
 	// TemplatePath specifies the relative or absolute location of template files.
 	TemplatePath string
@@ -56,7 +59,7 @@ type Config struct {
 
 // A Server implements an http.Handler that serves the SysDB user interface.
 type Server struct {
-	c *client.Conn
+	conns chan *client.Conn
 
 	// Templates:
 	main    *template.Template
@@ -68,7 +71,17 @@ type Server struct {
 
 // New constructs a new SysDB web server using the specified configuration.
 func New(cfg Config) (*Server, error) {
-	s := &Server{c: cfg.Conn, results: make(map[string]*template.Template)}
+	if len(cfg.Conns) == 0 {
+		return nil, errors.New("need at least one client connection")
+	}
+
+	s := &Server{
+		conns:   make(chan *client.Conn, len(cfg.Conns)),
+		results: make(map[string]*template.Template),
+	}
+	for _, c := range cfg.Conns {
+		s.conns <- c
+	}
 
 	var err error
 	s.main, err = cfg.parse("main.tmpl")
@@ -251,16 +264,19 @@ func html(s string) template.HTML {
 }
 
 func (s *Server) query(cmd string) (interface{}, error) {
+	c := <-s.conns
+	defer func() { s.conns <- c }()
+
 	m := &proto.Message{
 		Type: proto.ConnectionQuery,
 		Raw:  []byte(cmd),
 	}
-	if err := s.c.Send(m); err != nil {
+	if err := c.Send(m); err != nil {
 		return nil, fmt.Errorf("Query %q: %v", cmd, err)
 	}
 
 	for {
-		m, err := s.c.Receive()
+		m, err := c.Receive()
 		if err != nil {
 			return nil, fmt.Errorf("Failed to receive server response: %v", err)
 		}
-- 
2.30.2