net/http: add Server BaseContext & ConnContext fields to control early context

Fixes golang/go#30694

Change-Id: I12a0a870e4aee6576e879d88a4868666ef448298
Reviewed-on: https://go-review.googlesource.com/c/go/+/167681
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: JP Sugarbroad <jpsugar@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -6034,6 +6034,43 @@ func TestStripPortFromHost(t *testing.T) {
 	}
 }
 
+func TestServerContexts(t *testing.T) {
+	setParallel(t)
+	defer afterTest(t)
+	type baseKey struct{}
+	type connKey struct{}
+	ch := make(chan context.Context, 1)
+	ts := httptest.NewUnstartedServer(HandlerFunc(func(rw ResponseWriter, r *Request) {
+		ch <- r.Context()
+	}))
+	ts.Config.BaseContext = func(ln net.Listener) context.Context {
+		if strings.Contains(reflect.TypeOf(ln).String(), "onceClose") {
+			t.Errorf("unexpected onceClose listener type %T", ln)
+		}
+		return context.WithValue(context.Background(), baseKey{}, "base")
+	}
+	ts.Config.ConnContext = func(ctx context.Context, c net.Conn) context.Context {
+		if got, want := ctx.Value(baseKey{}), "base"; got != want {
+			t.Errorf("in ConnContext, base context key = %#v; want %q", got, want)
+		}
+		return context.WithValue(ctx, connKey{}, "conn")
+	}
+	ts.Start()
+	defer ts.Close()
+	res, err := ts.Client().Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	ctx := <-ch
+	if got, want := ctx.Value(baseKey{}), "base"; got != want {
+		t.Errorf("base context key = %#v; want %q", got, want)
+	}
+	if got, want := ctx.Value(connKey{}), "conn"; got != want {
+		t.Errorf("conn context key = %#v; want %q", got, want)
+	}
+}
+
 func BenchmarkResponseStatusLine(b *testing.B) {
 	b.ReportAllocs()
 	b.RunParallel(func(pb *testing.PB) {

src/net/http/server.go

@@ -2542,6 +2542,20 @@ type Server struct {
 	// If nil, logging is done via the log package's standard logger.
 	ErrorLog *log.Logger
 
+	// BaseContext optionally specifies a function that returns
+	// the base context for incoming requests on this server.
+	// The provided Listener is the specific Listener that's
+	// about to start accepting requests.
+	// If BaseContext is nil, the default is context.Background().
+	// If non-nil, it must return a non-nil context.
+	BaseContext func(net.Listener) context.Context
+
+	// ConnContext optionally specifies a function that modifies
+	// the context used for a newly connection c. The provided ctx
+	// is derived from the base context and has a ServerContextKey
+	// value.
+	ConnContext func(ctx context.Context, c net.Conn) context.Context
+
 	disableKeepAlives int32     // accessed atomically.
 	inShutdown        int32     // accessed atomically (non-zero means we're in Shutdown)
 	nextProtoOnce     sync.Once // guards setupHTTP2_* init
@@ -2838,6 +2852,7 @@ func (srv *Server) Serve(l net.Listener) error {
 		fn(srv, l) // call hook with unwrapped listener
 	}
 
+	origListener := l
 	l = &onceCloseListener{Listener: l}
 	defer l.Close()
 
@@ -2851,7 +2866,15 @@ func (srv *Server) Serve(l net.Listener) error {
 	defer srv.trackListener(&l, false)
 
 	var tempDelay time.Duration // how long to sleep on accept failure
-	baseCtx := context.Background() // base is always background, per Issue 16220
+
+	baseCtx := context.Background()
+	if srv.BaseContext != nil {
+		baseCtx = srv.BaseContext(origListener)
+		if baseCtx == nil {
+			panic("BaseContext returned a nil context")
+		}
+	}
+
 	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
 	for {
 		rw, e := l.Accept()
@@ -2876,6 +2899,12 @@ func (srv *Server) Serve(l net.Listener) error {
 			}
 			return e
 		}
+		if cc := srv.ConnContext; cc != nil {
+			ctx = cc(ctx, rw)
+			if ctx == nil {
+				panic("ConnContext returned nil")
+			}
+		}
 		tempDelay = 0
 		c := srv.newConn(rw)
 		c.setState(c.rwc, StateNew) // before Serve can return