main: warn once about USB permissions on scan

When the user press the scan button and we get a Permissions Denied
error from the USB core then we know that the Crazyradio dongle is
present, but we cannot access it.

We will warn, once, about this with a message box that gives a link
to instructions on how to install the udev files.

Closes bitcraze#444

src/cfclient/ui/main.py

@@ -28,6 +28,7 @@
 """
 import logging
 import sys
+import usb
 
 import cfclient
 from cfclient.ui.pose_logger import PoseLogger
@@ -54,6 +55,7 @@
 from PyQt5.QtCore import QDir
 from PyQt5.QtCore import QThread
 from PyQt5.QtCore import QUrl
+from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import QAction
 from PyQt5.QtWidgets import QActionGroup
 from PyQt5.QtWidgets import QShortcut
@@ -185,7 +187,7 @@ def __init__(self, *args):
                 scan_button=self.scanButton))
 
         self._connectivity_manager.connect_button_clicked.connect(self._connect)
-        self._connectivity_manager.scan_button_clicked.connect(self._scan)
+        self._connectivity_manager.scan_button_clicked.connect(self._scan_from_button)
 
         self._disable_input = False
 
@@ -359,6 +361,9 @@ def __init__(self, *args):
             self._theme_group.addAction(node)
             self.menuThemes.addAction(node)
 
+        # We only want to warn about USB permission once
+        self._permission_warned = False
+
     def _set_address(self):
         address = 0xE7E7E7E7E7
         try:
@@ -598,6 +603,30 @@ def _scan(self, address):
         self._update_ui_state()
         self.scanner.scanSignal.emit(address)
 
+    def _scan_from_button(self, address):
+        #
+        # Below we check if we can open the Crazyradio device.
+        # If it is there, but we have no permissions we inform the user, once,
+        # about how to install the udev rules.
+        #
+        if not self._permission_warned:
+            try:
+                radio = cflib.crtp.radiodriver.RadioManager.open(0)
+                radio.close()
+            except usb.core.USBError as e:
+                if e.errno == 13:  # Permission denied
+                    link = "<a href='https://www.bitcraze.io/documentation/repository/crazyflie-lib-python/master/installation/usb_permissions/'>Install USB Permissions</a>" # noqa
+                    msg = QMessageBox()
+                    msg.setIcon(QMessageBox.Information)
+                    msg.setTextFormat(Qt.RichText)
+                    msg.setText("Could not access Crazyradio")
+                    msg.setInformativeText(link)
+                    msg.setWindowTitle("Crazyradio permissions")
+                    msg.exec()
+                    self._permission_warned = True
+
+        self._scan(address)
+
     def _display_input_device_error(self, error):
         self.cf.close_link()
         QMessageBox.critical(self, "Input device error", error)