net/http: panic when a nil handler is passed to (*ServeMux)HandleFunc

Fixes #24297

Change-Id: I759e88655632fda97dced240b3f13392b2785d0a
Reviewed-on: https://go-review.googlesource.com/99575Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -379,6 +379,18 @@ func TestServeMuxHandler(t *testing.T) {
 	}
 }
 
+// Issue 24297
+func TestServeMuxHandleFuncWithNilHandler(t *testing.T) {
+	setParallel(t)
+	defer func() {
+		if err := recover(); err == nil {
+			t.Error("expected call to mux.HandleFunc to panic")
+		}
+	}()
+	mux := NewServeMux()
+	mux.HandleFunc("/", nil)
+}
+
 var serveMuxTests2 = []struct {
 	method  string
 	host    string

src/net/http/server.go

@@ -2358,6 +2358,9 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 
 // HandleFunc registers the handler function for the given pattern.
 func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
+	if handler == nil {
+		panic("http: nil handler")
+	}
 	mux.Handle(pattern, HandlerFunc(handler))
 }