increase the websocket write buffer size to 1MiB

See the comment on wsWriteBufferSize for context. This somewhat fixes
the regression in dc09186, as we went from a 25MiB message size limit
to a 4KiB one. 1MiB seems like a reasonable middle ground for the time
being, keeping the memory usage low.

Fixes #395.

chromedp_test.go

@@ -516,3 +516,17 @@ func TestListenCancel(t *testing.T) {
 		t.Fatalf("want %d target events; got %d", want, targetCount)
 	}
 }
+
+func TestLargeOutboundMessages(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := testAllocate(t, "")
+	defer cancel()
+
+	// ~50KiB of JS should fit just fine in our current buffer of 1MiB.
+	expr := fmt.Sprintf("//%s\n", strings.Repeat("x", 50 << 10))
+	res := new([]byte)
+	if err := Run(ctx, Evaluate(expr, res)); err != nil {
+		t.Fatal(err)
+	}
+}

conn.go

@@ -40,6 +40,19 @@ type Conn struct {
 	dbgf func(string, ...interface{})
 }
 
+// Chrome doesn't support fragmentation of incoming websocket messages. To
+// compensate this, they support single-fragment messages of up to 100MiB.
+//
+// If our write buffer size is too small, large messages will get fragmented,
+// and Chrome will silently crash. And if it's too large, chromedp will require
+// more memory for all users.
+//
+// For now, make this a middle ground. 1MiB is large enough for practically any
+// outgoing message, but small enough to not take too much meomry.
+//
+// See https://github.com/ChromeDevTools/devtools-protocol/issues/175.
+const wsWriteBufferSize = 1 << 20
+
 // DialContext dials the specified websocket URL using gobwas/ws.
 func DialContext(ctx context.Context, urlstr string, opts ...DialOption) (*Conn, error) {
 	// connect
@@ -53,8 +66,9 @@ func DialContext(ctx context.Context, urlstr string, opts ...DialOption) (*Conn,
 
 	// apply opts
 	c := &Conn{
-		conn:   conn,
-		writer: *wsutil.NewWriterBufferSize(conn, ws.StateClientSide, ws.OpText, 4096),
+		conn: conn,
+		writer: *wsutil.NewWriterBufferSize(conn,
+			ws.StateClientSide, ws.OpText, wsWriteBufferSize),
 	}
 	for _, o := range opts {
 		o(c)