net/http: update bundled http2 for ErrAbortHandler support, document it more

Updates http2 to x/net/http2 git rev 0e2717d for:

   http2: conditionally log stacks from panics in Server Handlers like net/http
   https://golang.org/cl/33102

Fixes #17790

Change-Id: Idd3f0c65540398d41b412a33f1d80de3f7f31409
Reviewed-on: https://go-review.googlesource.com/33103Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/net/http/clientserver_test.go

@@ -1189,9 +1189,6 @@ func testInterruptWithPanic(t *testing.T, h2 bool, panicValue interface{}) {
 		if gotLog == "" {
 			return
 		}
-		if h2 {
-			t.Skip("TODO: make http2.Server respect ErrAbortHandler")
-		}
 		t.Fatalf("want no log output; got: %s", gotLog)
 	}
 	if gotLog == "" {

src/net/http/h2_bundle.go

@@ -2184,6 +2184,10 @@ func http2configureServer18(h1 *Server, h2 *http2Server) error {
 	return nil
 }
 
+func http2shouldLogPanic(panicValue interface{}) bool {
+	return panicValue != nil && panicValue != ErrAbortHandler
+}
+
 var http2DebugGoroutines = os.Getenv("DEBUG_HTTP2_GOROUTINES") == "1"
 
 type http2goroutineLock uint64
@@ -4534,15 +4538,17 @@ func (sc *http2serverConn) runHandler(rw *http2responseWriter, req *Request, han
 		rw.rws.stream.cancelCtx()
 		if didPanic {
 			e := recover()
-			// Same as net/http:
-			const size = 64 << 10
-			buf := make([]byte, size)
-			buf = buf[:runtime.Stack(buf, false)]
 			sc.writeFrameFromHandler(http2FrameWriteRequest{
 				write:  http2handlerPanicRST{rw.rws.stream.id},
 				stream: rw.rws.stream,
 			})
-			sc.logf("http2: panic serving %v: %v\n%s", sc.conn.RemoteAddr(), e, buf)
+
+			if http2shouldLogPanic(e) {
+				const size = 64 << 10
+				buf := make([]byte, size)
+				buf = buf[:runtime.Stack(buf, false)]
+				sc.logf("http2: panic serving %v: %v\n%s", sc.conn.RemoteAddr(), e, buf)
+			}
 			return
 		}
 		rw.handlerDone()

src/net/http/server.go

@@ -75,7 +75,9 @@ var (
 // If ServeHTTP panics, the server (the caller of ServeHTTP) assumes
 // that the effect of the panic was isolated to the active request.
 // It recovers the panic, logs a stack trace to the server error log,
-// and hangs up the connection.
+// and hangs up the connection. To abort a handler so the client sees
+// an interrupted response but the server doesn't log an error, panic
+// with the value ErrAbortHandler.
 type Handler interface {
 	ServeHTTP(ResponseWriter, *Request)
 }