crypto/tls: add Dialer

Fixes #18482

Change-Id: I99d65dc5d824c00093ea61e7445fc121314af87f
Reviewed-on: https://go-review.googlesource.com/c/go/+/214977
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

tls_test.go

@@ -6,6 +6,7 @@ package tls
 
 import (
 	"bytes"
+	"context"
 	"crypto"
 	"crypto/x509"
 	"encoding/json"
@@ -272,6 +273,47 @@ func TestDeadlineOnWrite(t *testing.T) {
 	}
 }
 
+type readerFunc func([]byte) (int, error)
+
+func (f readerFunc) Read(b []byte) (int, error) { return f(b) }
+
+// TestDialer tests that tls.Dialer.DialContext can abort in the middle of a handshake.
+// (The other cases are all handled by the existing dial tests in this package, which
+// all also flow through the same code shared code paths)
+func TestDialer(t *testing.T) {
+	ln := newLocalListener(t)
+	defer ln.Close()
+
+	unblockServer := make(chan struct{}) // close-only
+	defer close(unblockServer)
+	go func() {
+		conn, err := ln.Accept()
+		if err != nil {
+			return
+		}
+		defer conn.Close()
+		<-unblockServer
+	}()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	d := Dialer{Config: &Config{
+		Rand: readerFunc(func(b []byte) (n int, err error) {
+			// By the time crypto/tls wants randomness, that means it has a TCP
+			// connection, so we're past the Dialer's dial and now blocked
+			// in a handshake. Cancel our context and see if we get unstuck.
+			// (Our TCP listener above never reads or writes, so the Handshake
+			// would otherwise be stuck forever)
+			cancel()
+			return len(b), nil
+		}),
+		ServerName: "foo",
+	}}
+	_, err := d.DialContext(ctx, "tcp", ln.Addr().String())
+	if err != context.Canceled {
+		t.Errorf("err = %v; want context.Canceled", err)
+	}
+}
+
 func isTimeoutError(err error) bool {
 	if ne, ok := err.(net.Error); ok {
 		return ne.Timeout()