cmd/godoc: inform users that the playground doesn't work via local godoc

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5843065

doc/play/playground.js

@@ -166,10 +166,13 @@ function playground(opts) {
 				}
 				pre.text(out);
 			},
-			error: function() {
-				output.addClass("error").text(
-					"Error communicating with remote server."
-				);
+			error: function(xhr) {
+				var text = "Error communicating with remote server.";
+				console.log(xhr.status);
+				if (xhr.status == 501) {
+					text = xhr.responseText;
+				}
+				output.addClass("error").text(text);
 			}
 		});
 	}
@@ -190,6 +193,10 @@ function playground(opts) {
 				type: "POST",
 				complete: function(xhr) {
 					sharing = false;
+					if (xhr.status == 501) {
+						alert(xhr.responseText);
+						return;
+					}
 					if (xhr.status != 200) {
 						alert("Server error; try again.");
 						return;

src/cmd/godoc/main.go

@@ -274,6 +274,10 @@ func main() {
 
 		registerPublicHandlers(http.DefaultServeMux)
 
+		// Playground handlers are not available in local godoc.
+		http.HandleFunc("/compile", disabledHandler)
+		http.HandleFunc("/share", disabledHandler)
+
 		// Initialize default directory tree with corresponding timestamp.
 		// (Do it in a goroutine so that launch is quick.)
 		go initFSTree()
@@ -450,3 +454,9 @@ type httpWriter struct {
 
 func (w *httpWriter) Header() http.Header  { return w.h }
 func (w *httpWriter) WriteHeader(code int) { w.code = code }
+
+// disabledHandler serves a 501 "Not Implemented" response.
+func disabledHandler(w http.ResponseWriter, r *http.Request) {
+	w.WriteHeader(http.StatusNotImplemented)
+	fmt.Fprint(w, "This functionality is not available via local godoc.")
+}