Optimize server connection close logic (#1310)

* server.go Make code more clear

Inline isHTTP11 var.
Use direct Header.SetConnectionClose() and Header.ConnectionClose()

* server.go Use direct SetConnectionClose()

The SetCanonical(strConnection, strClose) call internally will anyway call SetConnectionClose().
The "Connection: close" will be printed in the ResponseHeader.AppendBytes()

* server.go Simplify connectionClose evaluation

The conn limit check merged into connectionClose evaluation.
This improves performance for most cases:
1. If the connectionClose already true then the conn limit check won't be performed.
2. The SetConnectionClose() was duplicated
3. First check conn limit and only then check for resp.connClose because most users don't close conns manually.
4. We may optimize more: If the resp.connClose = true then SetConnectionClose() not needed but as mentioned above this is a rare case.

server.go

@@ -2127,7 +2127,6 @@ func (s *Server) serveConn(c net.Conn) (err error) {
 		hijackNoResponse bool
 
 		connectionClose bool
-		isHTTP11        bool
 
 		continueReadingRequest bool = true
 	)
@@ -2323,8 +2322,8 @@ func (s *Server) serveConn(c net.Conn) (err error) {
 			}
 		}
 
+		// store req.ConnectionClose so even if it was changed inside of handler
 		connectionClose = s.DisableKeepalive || ctx.Request.Header.ConnectionClose()
-		isHTTP11 = ctx.Request.Header.IsHTTP11()
 
 		if serverName != nil {
 			ctx.Response.Header.SetServerBytes(serverName)
@@ -2354,10 +2353,6 @@ func (s *Server) serveConn(c net.Conn) (err error) {
 		hijackNoResponse = ctx.hijackNoResponse && hijackHandler != nil
 		ctx.hijackNoResponse = false
 
-		if s.MaxRequestsPerConn > 0 && connRequestNum >= uint64(s.MaxRequestsPerConn) {
-			ctx.SetConnectionClose()
-		}
-
 		if writeTimeout > 0 {
 			if err := c.SetWriteDeadline(time.Now().Add(writeTimeout)); err != nil {
 				panic(fmt.Sprintf("BUG: error in SetWriteDeadline(%v): %v", writeTimeout, err))
@@ -2371,11 +2366,14 @@ func (s *Server) serveConn(c net.Conn) (err error) {
 			previousWriteTimeout = 0
 		}
 
-		connectionClose = connectionClose || ctx.Response.ConnectionClose() || (s.CloseOnShutdown && atomic.LoadInt32(&s.stop) == 1)
+		connectionClose = connectionClose ||
+			(s.MaxRequestsPerConn > 0 && connRequestNum >= uint64(s.MaxRequestsPerConn)) ||
+			ctx.Response.Header.ConnectionClose() ||
+			(s.CloseOnShutdown && atomic.LoadInt32(&s.stop) == 1)
 		if connectionClose {
-			ctx.Response.Header.SetCanonical(strConnection, strClose)
-		} else if !isHTTP11 {
-			// Set 'Connection: keep-alive' response header for non-HTTP/1.1 request.
+			ctx.Response.Header.SetConnectionClose()
+		} else if !ctx.Request.Header.IsHTTP11() {
+			// Set 'Connection: keep-alive' response header for HTTP/1.0 request.
 			// There is no need in setting this header for http/1.1, since in http/1.1
 			// connections are keep-alive by default.
 			ctx.Response.Header.SetCanonical(strConnection, strKeepAlive)