net/http: provide access to the listener address an HTTP request arrived on

This adds a context key named LocalAddrContextKey (for now, see #15229) to
let users access the net.Addr of the net.Listener that accepted the connection
that sent an HTTP request. This is similar to ServerContextKey which provides
access to the *Server. (A Server may have multiple Listeners)

Fixes #6732

Change-Id: I74296307b68aaaab8df7ad4a143e11b5227b5e62
Reviewed-on: https://go-review.googlesource.com/22672Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -4062,7 +4062,12 @@ func TestServerContext_ServerContextKey(t *testing.T) {
 		ctx := r.Context()
 		got := ctx.Value(ServerContextKey)
 		if _, ok := got.(*Server); !ok {
-			t.Errorf("context value = %T; want *http.Server")
+			t.Errorf("context value = %T; want *http.Server", got)
+		}
+
+		got = ctx.Value(LocalAddrContextKey)
+		if _, ok := got.(net.Addr); !ok {
+			t.Errorf("local addr value = %T; want net.Addr", got)
 		}
 	}))
 	defer ts.Close()

src/net/http/server.go

@@ -176,6 +176,12 @@ var (
 	// started the handler. The associated value will be of
 	// type *Server.
 	ServerContextKey = &contextKey{"http-server"}
+
+	// LocalAddrContextKey is a context key. It can be used in
+	// HTTP handlers with context.WithValue to access the address
+	// the local address the connection arrived on.
+	// The associated value will be of type net.Addr.
+	LocalAddrContextKey = &contextKey{"local-addr"}
 )
 
 // A conn represents the server side of an HTTP connection.
@@ -2189,6 +2195,7 @@ func (srv *Server) Serve(l net.Listener) error {
 	// use cases yet.
 	baseCtx := context.Background()
 	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
+	ctx = context.WithValue(ctx, LocalAddrContextKey, l.Addr())
 	for {
 		rw, e := l.Accept()
 		if e != nil {