[gnome-shell] screenshot: Return an error when operation is blocked

[Florian Müllner <fmuellner src gnome org>]

commit 1fdd4ff31309506456da26897c76f3d852b557c0
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri May 29 00:35:27 2020 +0200

    screenshot: Return an error when operation is blocked
    
    Right now _createScreenshot() returns a tuple that indicates failure
    when a sender requests a screenshot operation before a previously
    started operation finished.
    
    However that doesn't work for the PickColor() method, as it uses a
    different return type than the other methods.
    
    Address this by returning an error instead, which works in any case;
    arguably trying to start multiple operations in parallel is an error
    by the caller more than it is a failed operation anyway.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1288

 js/ui/screenshot.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/js/ui/screenshot.js b/js/ui/screenshot.js
index 7be9ba051a..5b7e093872 100644
--- a/js/ui/screenshot.js
+++ b/js/ui/screenshot.js
@@ -37,7 +37,9 @@ var ScreenshotService = class {
 
         let sender = invocation.get_sender();
         if (this._screenShooter.has(sender) || lockedDown) {
-            invocation.return_value(GLib.Variant.new('(bs)', [false, '']));
+            invocation.return_error_literal(
+                Gio.IOErrorEnum, Gio.IOErrorEnum.BUSY,
+                'There is an ongoing operation for this sender');
             return null;
         }