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/20763
Reviewed-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/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()
 		}
 	}