udev/resource: fix mux device race condition leading to outdated control/disk paths

When a udev event is retrieved from the UdevManager's queue, it is
matched against each resource. If a match is found, the resource's
device is updated (on "add"/"change"/"move" events) or removed (on
"unbind"/"remove" events).

Normally the `avail` attribute is set afterwards, but not for the
USBSDMuxDevice and USBSDWireDevice: those devices prevent their
USBResource super class from setting the `avail` attribute. Instead, the
devices are available depending on the availability of their disk and
control paths. Then the device's `update()` method is called, which is a
no-op for those devices.

On `poll()`, their disk and control paths is set to None if their device
is None. Otherwise, these paths are set if the device is not already
available.

In case of a USB reset (or fast replug) of a big USB tree, loads of udev
events need to be processed. Cases were observed where `poll()` did not
run between processing of a "remove" and "add" event of the same device.
This leads to a race condition, following the description from above:
The USBSDMuxDevice's or USBSDWireDevice's underlying device is set to
None on "remove" and then set to the new device again, on "add". The
`avail` attribute is not updated as intended. After that, the `poll()`
method runs: the device is set and the `avail` attribute evaluates to
True because the disk and control paths are still set from the time
before the reset replug. If this situation happens, these paths stay
invalid until the next reset/replug or restart of the labgrid exporter.

These invalid paths might point to non-existent /dev/sg* and dev/sd*
devices, rendering interaction with the Mux and its SD card impossible.
Or, even worse, the paths point to valid /dev/sg* and dev/sd* devices of
another Mux connected as observed here (note different USB devnums):

  $ labgrid-client -vv resources rlab/usb-2-p3/NetworkUSBSDMuxDevice
  Exporter 'rlab':
    Group 'usb-2-p3' (rlab/usb-2-p3/*):
      Resource 'USBSDMuxDevice' (rlab/usb-2-p3/NetworkUSBSDMuxDevice[/USBSDMuxDevice]):
        {'acquired': 'board1',
         'avail': True,
         'cls': 'NetworkUSBSDMuxDevice',
         'params': {'busnum': 2,
                    'control_path': '/dev/sg6',
                    'devnum': 42,
                    'extra': {'proxy': '[...]',
                              'proxy_required': False},
                    'host': 'rlab',
                    'model_id': 16449,
                    'path': '/dev/sdg',
                    'vendor_id': 1060}}
  $ labgrid-client -vv r rlab/usb-1-p4/NetworkUSBSDMuxDevice
  Exporter 'rlab':
    Group 'usb-1-p4' (rlabC-srv/c-usb-1-p4/*):
      Resource 'USBSDMuxDevice' (rlab/usb-1-p4/NetworkUSBSDMuxDevice[/USBSDMuxDevice]):
        {'acquired': 'board2',
         'avail': True,
         'cls': 'NetworkUSBSDMuxDevice',
         'params': {'busnum': 2,
                    'control_path': '/dev/sg6',
                    'devnum': 43,
                    'extra': {'proxy': '[...]',
                              'proxy_required': False},
                    'host': 'rlab',
                    'model_id': 16449,
                    'path': '/dev/sdg',
                    'vendor_id': 1060}}

Fix this race condition by implementing the disk and control paths reset
in the USBSDMuxDevice's or USBSDWireDevice's update() method, in case the
underlying device is gone. The update() method is called during the udev
event processing, so it does no longer matter when the poll() method is
called.

Fixes: 27ff02f ("udev: correct availability for USBSDMuxDevice")
Fixes: 9324d32 ("Add sdwire driver")
Signed-off-by: Bastian Krause <bst@pengutronix.de>
[bst: cherry-picked from commit 029e60b]

Author: Bastian-Krause

labgrid/resource/udev.py

@@ -467,15 +467,17 @@ def avail(self, prop):
     # paths are available.
     def poll(self):
         super().poll()
+        if self.device is not None and not self.avail:
+            for child in self.device.parent.children:
+                if child.subsystem == 'block' and child.device_type == 'disk':
+                    self.disk_path = child.device_node
+            self.control_serial = self.device.properties.get('ID_SERIAL_SHORT')
+
+    def update(self):
+        super().update()
         if self.device is None:
             self.disk_path = None
             self.control_serial = None
-        else:
-            if not self.avail:
-                for child in self.device.parent.children:
-                    if child.subsystem == 'block' and child.device_type == 'disk':
-                        self.disk_path = child.device_node
-                self.control_serial = self.device.properties.get('ID_SERIAL_SHORT')
 
     @property
     def path(self):
@@ -510,16 +512,18 @@ def avail(self, prop):
     # paths are available.
     def poll(self):
         super().poll()
+        if self.device is not None and not self.avail:
+            for child in self.device.children:
+                if child.subsystem == 'block' and child.device_type == 'disk':
+                    self.disk_path = child.device_node
+                elif child.subsystem == 'scsi_generic':
+                    self.control_path = child.device_node
+
+    def update(self):
+        super().update()
         if self.device is None:
             self.control_path = None
             self.disk_path = None
-        else:
-            if not self.avail:
-                for child in self.device.children:
-                    if child.subsystem == 'block' and child.device_type == 'disk':
-                        self.disk_path = child.device_node
-                    elif child.subsystem == 'scsi_generic':
-                        self.control_path = child.device_node
 
     @property
     def path(self):