Adds a fallback using gnome-screenshot for screen capture.

helper/helper/qr.py

@@ -2,19 +2,42 @@ import mss
 import zxingcpp
 import base64
 import io
+import os
+import sys
+import subprocess
+import tempfile
+from mss.exception import ScreenShotError
 from PIL import Image
 
 
+def _capture_screen():
+    try:
+        with mss.mss() as sct:
+            monitor = sct.monitors[0]  # 0 is the special "all monitors" value.
+            sct_img = sct.grab(monitor)  # mss format
+        return Image.frombytes("RGB", sct_img.size, sct_img.bgra, "raw", "BGRX")
+    except ScreenShotError:
+        # One common error is that mss doesn't work with Wayland
+        if sys.platform.startswith("linux"):
+            # Try gnome-screenshot fallback
+            fd, fname = tempfile.mkstemp(suffix=".png")
+            try:
+                rc = subprocess.call(["gnome-screenshot", "-f", fname])  # nosec
+                if rc == 0:
+                    return Image.open(fname)
+            finally:
+                os.unlink(fname)
+    raise ValueError("Unable to capture screenshot")
+
+
 def scan_qr(image_data=None):
     if image_data:
         msg = base64.b64decode(image_data)
         buf = io.BytesIO(msg)
         img = Image.open(buf)
     else:
-        with mss.mss() as sct:
-            monitor = sct.monitors[0]  # 0 is the special "all monitors" value.
-            sct_img = sct.grab(monitor)  # mss format
-        img = Image.frombytes("RGB", sct_img.size, sct_img.bgra, "raw", "BGRX")
+        img = _capture_screen()
+
     result = zxingcpp.read_barcode(img)
     if result.valid:
         return result.text