crypto/tls: add HandshakeContext method to Conn

Adds the (*tls.Conn).HandshakeContext method. This allows
us to pass the context provided down the call stack to
eventually reach the tls.ClientHelloInfo and
tls.CertificateRequestInfo structs.
These contexts are exposed to the user as read-only via Context()
methods.

This allows users of (*tls.Config).GetCertificate and
(*tls.Config).GetClientCertificate to use the context for
request scoped parameters and cancellation.

Replace uses of (*tls.Conn).Handshake with (*tls.Conn).HandshakeContext
where appropriate, to propagate existing contexts.

Fixes #32406

Change-Id: I33c228904fe82dcf57683b63627497d3eb841ff2
Reviewed-on: https://go-review.googlesource.com/c/go/+/246338
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

handshake_client_test.go

@@ -6,6 +6,7 @@ package tls
 
 import (
 	"bytes"
+	"context"
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/base64"
@@ -20,6 +21,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -2511,3 +2513,37 @@ func testResumptionKeepsOCSPAndSCT(t *testing.T, ver uint16) {
 			serverConfig.Certificates[0].SignedCertificateTimestamps, ccs.SignedCertificateTimestamps)
 	}
 }
+
+func TestClientHandshakeContextCancellation(t *testing.T) {
+	c, s := localPipe(t)
+	serverConfig := testConfig.Clone()
+	serverErr := make(chan error, 1)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go func() {
+		defer close(serverErr)
+		defer s.Close()
+		conn := Server(s, serverConfig)
+		_, err := conn.readClientHello(ctx)
+		cancel()
+		serverErr <- err
+	}()
+	cli := Client(c, testConfig)
+	err := cli.HandshakeContext(ctx)
+	if err == nil {
+		t.Fatal("Client handshake did not error when the context was canceled")
+	}
+	if err != context.Canceled {
+		t.Errorf("Unexpected client handshake error: %v", err)
+	}
+	if err := <-serverErr; err != nil {
+		t.Errorf("Unexpected server error: %v", err)
+	}
+	if runtime.GOARCH == "wasm" {
+		t.Skip("conn.Close does not error as expected when called multiple times on WASM")
+	}
+	err = cli.Close()
+	if err == nil {
+		t.Error("Client connection was not closed when the context was canceled")
+	}
+}