Enable TCP Keepalive by default.

GODRIVER-714

Change-Id: I080f3bbab626161adbd51085ce56cfc3164e6355

Author: Divjot Arora

mongo/options/clientoptions.go

@@ -117,6 +117,7 @@ func (c *ClientOptions) SetConnectTimeout(d time.Duration) *ClientOptions {
 }
 
 // SetDialer specifies a custom dialer used to dial new connections to a server.
+// If a custom dialer is not set, a net.Dialer with a 300 second keepalive time will be used by default.
 func (c *ClientOptions) SetDialer(d ContextDialer) *ClientOptions {
 	c.TopologyOptions = append(
 		c.TopologyOptions,

x/network/connection/keepalive_300.go

@@ -0,0 +1,13 @@
+// Copyright (C) MongoDB, Inc. 2017-present.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License. You may obtain
+// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+
+//+build !go1.12
+
+package connection
+
+import "time"
+
+const tcpKeepalive = 300 * time.Second

x/network/connection/keepalive_default.go

@@ -0,0 +1,11 @@
+// Copyright (C) MongoDB, Inc. 2017-present.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License. You may obtain
+// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+
+//+build go1.12
+
+package connection
+
+const tcpKeepalive = 0 // will be set by default on Go 1.12 and higher

x/network/connection/options.go

@@ -44,7 +44,10 @@ func newConfig(opts ...Option) (*config, error) {
 	}
 
 	if cfg.dialer == nil {
-		cfg.dialer = &net.Dialer{Timeout: cfg.connectTimeout}
+		cfg.dialer = &net.Dialer{
+			KeepAlive: tcpKeepalive,
+			Timeout:   cfg.connectTimeout,
+		}
 	}
 
 	return cfg, nil