net/http: add ServerContextKey to let a handler access its Server

Fixes #12438
Updates #15229 (to decide context key variable naming convention)

Change-Id: I3ba423e91b689e232143247d044495a12c97a7d2
Reviewed-on: https://go-review.googlesource.com/21829Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/http.go

@@ -10,3 +10,11 @@ const maxInt64 = 1<<63 - 1
 
 // TODO(bradfitz): move common stuff here. The other files have accumulated
 // generic http stuff in random places.
+
+// contextKey is a value for use with context.WithValue. It's used as
+// a pointer so it fits in an interface{} without allocation.
+type contextKey struct {
+	name string
+}
+
+func (k *contextKey) String() string { return "net/http context value " + k.name }

src/net/http/serve_test.go

@@ -4056,6 +4056,23 @@ func TestServerRequestContextCancel_ConnClose(t *testing.T) {
 	}
 }
 
+func TestServerContext_ServerContextKey(t *testing.T) {
+	defer afterTest(t)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		ctx := r.Context()
+		got := ctx.Value(ServerContextKey)
+		if _, ok := got.(*Server); !ok {
+			t.Errorf("context value = %T; want *http.Server")
+		}
+	}))
+	defer ts.Close()
+	res, err := Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+}
+
 func BenchmarkClientServer(b *testing.B) {
 	b.ReportAllocs()
 	b.StopTimer()

src/net/http/server.go

@@ -147,6 +147,14 @@ type CloseNotifier interface {
 	CloseNotify() <-chan bool
 }
 
+var (
+	// ServerContextKey is a context key. It can be used in HTTP
+	// handlers with context.WithValue to access the server that
+	// started the handler. The associated value will be of
+	// type *Server.
+	ServerContextKey = &contextKey{"http-server"}
+)
+
 // A conn represents the server side of an HTTP connection.
 type conn struct {
 	// server is the server on which the connection arrived.
@@ -1402,7 +1410,7 @@ type badRequestError string
 func (e badRequestError) Error() string { return "Bad Request: " + string(e) }
 
 // Serve a new connection.
-func (c *conn) serve() {
+func (c *conn) serve(ctx context.Context) {
 	c.remoteAddr = c.rwc.RemoteAddr().String()
 	defer func() {
 		if err := recover(); err != nil {
@@ -1445,10 +1453,7 @@ func (c *conn) serve() {
 	c.bufr = newBufioReader(c.r)
 	c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
 
-	// TODO: allow changing base context? can't imagine concrete
-	// use cases yet.
-	baseCtx := context.Background()
-	ctx, cancelCtx := context.WithCancel(baseCtx)
+	ctx, cancelCtx := context.WithCancel(ctx)
 	defer cancelCtx()
 
 	for {
@@ -2151,6 +2156,10 @@ func (srv *Server) Serve(l net.Listener) error {
 	if err := srv.setupHTTP2(); err != nil {
 		return err
 	}
+	// TODO: allow changing base context? can't imagine concrete
+	// use cases yet.
+	baseCtx := context.Background()
+	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
 	for {
 		rw, e := l.Accept()
 		if e != nil {
@@ -2172,7 +2181,7 @@ func (srv *Server) Serve(l net.Listener) error {
 		tempDelay = 0
 		c := srv.newConn(rw)
 		c.setState(c.rwc, StateNew) // before Serve can return
-		go c.serve()
+		go c.serve(ctx)
 	}
 }