AVFoundation: fix authorization request not being shown

modules/videoio/src/cap_avfoundation_mac.mm

@@ -328,10 +328,28 @@ int CvCaptureCAM::startCaptureDevice(int cameraNum) {
     }
     else if (status != AVAuthorizationStatusAuthorized)
     {
-        fprintf(stderr, "OpenCV: not authorized to capture video (status %ld), requesting...\n", status);
-        // TODO: doesn't work via ssh
-        [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL) { /* we don't care */}];
-        // we do not wait for completion
+        if (!cv::utils::getConfigurationParameterBool("OPENCV_AVFOUNDATION_SKIP_AUTH", false))
+        {
+            fprintf(stderr, "OpenCV: not authorized to capture video (status %ld), requesting...\n", status);
+            [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL) { /* we don't care */}];
+            if ([NSThread isMainThread])
+            {
+                // we run the main loop for 0.1 sec to show the message
+                [[NSRunLoop mainRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
+            }
+            else
+            {
+                fprintf(stderr, "OpenCV: can not spin main run loop from other thread, set "
+                                "OPENCV_AVFOUNDATION_SKIP_AUTH=1 to disable authorization request "
+                                "and perform it in your application.\n");
+            }
+        }
+        else
+        {
+            fprintf(stderr, "OpenCV: not authorized to capture video (status %ld), set "
+                            "OPENCV_AVFOUNDATION_SKIP_AUTH=0 to enable authorization request or "
+                            "perform it in your application.\n", status);
+        }
         [localpool drain];
         return 0;
     }