crypto/tls: help linker remove code when only Client or Server is used

This saves 166 KiB for a tls.Dial hello world program (5382441 to
5212356 to bytes), by permitting the linker to remove TLS server code.

Change-Id: I16610b836bb0802b7d84995ff881d79ec03b6a84
Reviewed-on: https://go-review.googlesource.com/c/go/+/228111
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/tls/conn.go

@@ -24,8 +24,9 @@ import (
 // It implements the net.Conn interface.
 type Conn struct {
 	// constant
-	conn     net.Conn
-	isClient bool
+	conn        net.Conn
+	isClient    bool
+	handshakeFn func() error // (*Conn).clientHandshake or serverHandshake
 
 	// handshakeStatus is 1 if the connection is currently transferring
 	// application data (i.e. is not currently processing a handshake).
@@ -1349,11 +1350,7 @@ func (c *Conn) Handshake() error {
 	c.in.Lock()
 	defer c.in.Unlock()
 
-	if c.isClient {
-		c.handshakeErr = c.clientHandshake()
-	} else {
-		c.handshakeErr = c.serverHandshake()
-	}
+	c.handshakeErr = c.handshakeFn()
 	if c.handshakeErr == nil {
 		c.handshakes++
 	} else {

src/crypto/tls/link_test.go

@@ -0,0 +1,121 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"bytes"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// Tests that the linker is able to remove references to the Client or Server if unused.
+func TestLinkerGC(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	t.Parallel()
+	goBin := testenv.GoToolPath(t)
+	testenv.MustHaveGoBuild(t)
+
+	tests := []struct {
+		name    string
+		program string
+		want    []string
+		bad     []string
+	}{
+		{
+			name: "empty_import",
+			program: `package main
+import _ "crypto/tls"
+func main() {}
+`,
+			bad: []string{
+				"tls.(*Conn)",
+				"type.crypto/tls.clientHandshakeState",
+				"type.crypto/tls.serverHandshakeState",
+			},
+		},
+		{
+			name: "only_conn",
+			program: `package main
+import "crypto/tls"
+var c = new(tls.Conn)
+func main() {}
+`,
+			want: []string{"tls.(*Conn)"},
+			bad: []string{
+				"type.crypto/tls.clientHandshakeState",
+				"type.crypto/tls.serverHandshakeState",
+			},
+		},
+		{
+			name: "client_and_server",
+			program: `package main
+import "crypto/tls"
+func main() {
+  tls.Dial("", "", nil)
+  tls.Server(nil, nil)
+}
+`,
+			want: []string{
+				"crypto/tls.(*Conn).clientHandshake",
+				"crypto/tls.(*Conn).serverHandshake",
+			},
+		},
+		{
+			name: "only_client",
+			program: `package main
+import "crypto/tls"
+func main() { tls.Dial("", "", nil) }
+`,
+			want: []string{
+				"crypto/tls.(*Conn).clientHandshake",
+			},
+			bad: []string{
+				"crypto/tls.(*Conn).serverHandshake",
+			},
+		},
+		// TODO: add only_server like func main() { tls.Server(nil, nil) }
+		// That currently brings in the client via Conn.handleRenegotiation.
+
+	}
+	tmpDir := t.TempDir()
+	goFile := filepath.Join(tmpDir, "x.go")
+	exeFile := filepath.Join(tmpDir, "x.exe")
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := ioutil.WriteFile(goFile, []byte(tt.program), 0644); err != nil {
+				t.Fatal(err)
+			}
+			os.Remove(exeFile)
+			cmd := exec.Command(goBin, "build", "-o", "x.exe", "x.go")
+			cmd.Dir = tmpDir
+			if out, err := cmd.CombinedOutput(); err != nil {
+				t.Fatalf("compile: %v, %s", err, out)
+			}
+
+			cmd = exec.Command(goBin, "tool", "nm", "x.exe")
+			cmd.Dir = tmpDir
+			nm, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Fatalf("nm: %v, %s", err, nm)
+			}
+			for _, sym := range tt.want {
+				if !bytes.Contains(nm, []byte(sym)) {
+					t.Errorf("expected symbol %q not found", sym)
+				}
+			}
+			for _, sym := range tt.bad {
+				if bytes.Contains(nm, []byte(sym)) {
+					t.Errorf("unexpected symbol %q found", sym)
+				}
+			}
+		})
+	}
+}

src/crypto/tls/tls.go

@@ -32,7 +32,12 @@ import (
 // The configuration config must be non-nil and must include
 // at least one certificate or else set GetCertificate.
 func Server(conn net.Conn, config *Config) *Conn {
-	return &Conn{conn: conn, config: config}
+	c := &Conn{
+		conn:   conn,
+		config: config,
+	}
+	c.handshakeFn = c.serverHandshake
+	return c
 }
 
 // Client returns a new TLS client side connection
@@ -40,7 +45,13 @@ func Server(conn net.Conn, config *Config) *Conn {
 // The config cannot be nil: users must set either ServerName or
 // InsecureSkipVerify in the config.
 func Client(conn net.Conn, config *Config) *Conn {
-	return &Conn{conn: conn, config: config, isClient: true}
+	c := &Conn{
+		conn:     conn,
+		config:   config,
+		isClient: true,
+	}
+	c.handshakeFn = c.clientHandshake
+	return c
 }
 
 // A listener implements a network listener (net.Listener) for TLS connections.