dial: add DialContext function

In order to replace timeouts with contexts in `Connect` instance
creation (go-tarantool), I need a `DialContext` function.
It accepts context, and cancels, if context is canceled by user.

Part of tarantool/go-tarantool#136

net.go

@@ -15,6 +15,7 @@
 package openssl
 
 import (
+	"context"
 	"errors"
 	"net"
 	"time"
@@ -90,7 +91,19 @@ func Dial(network, addr string, ctx *Ctx, flags DialFlags) (*Conn, error) {
 func DialTimeout(network, addr string, timeout time.Duration, ctx *Ctx,
 	flags DialFlags) (*Conn, error) {
 	d := net.Dialer{Timeout: timeout}
-	return dialSession(d, network, addr, ctx, flags, nil)
+	return dialSession(d, context.Background(), network, addr, ctx, flags, nil)
+}
+
+// DialContext acts like Dial but takes a context for network dial.
+//
+// The context includes only network dial. It does not include OpenSSL calls.
+//
+// See func Dial for a description of the network, addr, ctx and flags
+// parameters.
+func DialContext(context context.Context, network, addr string,
+	ctx *Ctx, flags DialFlags) (*Conn, error) {
+	d := net.Dialer{}
+	return dialSession(d, context, network, addr, ctx, flags, nil)
 }
 
 // DialSession will connect to network/address and then wrap the corresponding
@@ -109,11 +122,11 @@ func DialTimeout(network, addr string, timeout time.Duration, ctx *Ctx,
 func DialSession(network, addr string, ctx *Ctx, flags DialFlags,
 	session []byte) (*Conn, error) {
 	var d net.Dialer
-	return dialSession(d, network, addr, ctx, flags, session)
+	return dialSession(d, context.Background(), network, addr, ctx, flags, session)
 }
 
-func dialSession(d net.Dialer, network, addr string, ctx *Ctx, flags DialFlags,
-	session []byte) (*Conn, error) {
+func dialSession(d net.Dialer, context context.Context, network, addr string,
+	ctx *Ctx, flags DialFlags, session []byte) (*Conn, error) {
 	host, _, err := net.SplitHostPort(addr)
 	if err != nil {
 		return nil, err
@@ -127,7 +140,7 @@ func dialSession(d net.Dialer, network, addr string, ctx *Ctx, flags DialFlags,
 		// TODO: use operating system default certificate chain?
 	}
 
-	c, err := d.Dial(network, addr)
+	c, err := d.DialContext(context, network, addr)
 	if err != nil {
 		return nil, err
 	}