net/http: use dynamic type assertion to remove HTTP server code from cmd/go

I was wondering why cmd/go includes the HTTP server implementations.

Dumping the linker's deadcode dependency graph into a file and doing
some graph analysis, I found that the only reason cmd/go included an
HTTP server was because the maxBytesReader type (used by both the HTTP
transport & HTTP server) did a static type assertion to an HTTP server
type.

Changing it to a interface type assertion reduces the size of cmd/go
by 533KB (5.2%)

On linux/amd64, cmd/go goes from 10549200 to 10002624 bytes.

Add a test too so this doesn't regress. The test uses cmd/go as the
binary to test (a binary which needs the HTTP client but not the HTTP
server), but this change and test are equally applicable to any such
program.

Change-Id: I93865f43ec03b06d09241fbd9ea381817c2909c5
Reviewed-on: https://go-review.googlesource.com/20763Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/http_test.go

@@ -2,12 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Tests of internal functions with no better homes.
+// Tests of internal functions and things with no better homes.
 
 package http
 
 import (
+	"bytes"
+	"internal/testenv"
+	"os/exec"
+	"path/filepath"
 	"reflect"
+	"runtime"
 	"testing"
 )
 
@@ -56,3 +61,39 @@ func TestCleanHost(t *testing.T) {
 		}
 	}
 }
+
+// Test that cmd/go doesn't link in the HTTP server.
+//
+// This catches accidental dependencies between the HTTP transport and
+// server code.
+func TestCmdGoNoHTTPServer(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	var exeSuffix string
+	if runtime.GOOS == "windows" {
+		exeSuffix = ".exe"
+	}
+
+	goBin := filepath.Join(runtime.GOROOT(), "bin", "go"+exeSuffix)
+	out, err := exec.Command("go", "tool", "nm", goBin).Output()
+	if err != nil {
+		t.Fatalf("go tool nm: %v", err)
+	}
+	wantSym := map[string]bool{
+		// Verify these exist: (sanity checking this test)
+		"net/http.(*Client).Get":          true,
+		"net/http.(*Transport).RoundTrip": true,
+
+		// Verify these don't exist:
+		"net/http.http2Server":     false,
+		"net/http.(*Server).Serve": false,
+	}
+	for sym, want := range wantSym {
+		got := bytes.Contains(out, []byte(sym))
+		if !want && got {
+			t.Errorf("cmd/go unexpectedly links in HTTP server code; found symbol %q in cmd/go", sym)
+		}
+		if want && !got {
+			t.Errorf("expected to find symbol %q in cmd/go; not found", sym)
+		}
+	}
+}

src/net/http/request.go

@@ -820,7 +820,18 @@ type maxBytesReader struct {
 func (l *maxBytesReader) tooLarge() (n int, err error) {
 	if !l.stopped {
 		l.stopped = true
-		if res, ok := l.w.(*response); ok {
+
+		// The server code and client code both use
+		// maxBytesReader. This "requestTooLarge" check is
+		// only used by the server code. To prevent binaries
+		// which only using the HTTP Client code (such as
+		// cmd/go) from also linking in the HTTP server, don't
+		// use a static type assertion to the server
+		// "*response" type. Check this interface instead:
+		type requestTooLarger interface {
+			requestTooLarge()
+		}
+		if res, ok := l.w.(requestTooLarger); ok {
 			res.requestTooLarge()
 		}
 	}