platforms/evdev: Clean up mir::Device handle on device remove (#3150)

Not all `ConsoleProvider`s will send a `removed` event on the `DeviceObserver`
(in fact, only logind will), so we can't rely on that to clean up the handle
in `device_watchers`.

Furthermore, libinput itself will notice a device going away (by monitoring
the fd) and generate `LIBINPUT_EVENT_DEVICE_REMOVED`, which we might process
*before* the udev `REMOVED` event, and in *that* case, `device_removed` will
have already deleted the `LibInputDevice` and so the udev event handler
will *also* not clean up `device_watchers`.

Resolve this by cleaning up `device_watchers` *only* in `device_removed`,
and rely on the `DeviceObserver::removed` codepath triggering a libinput
removal event.

Fixes: #3149

src/platforms/evdev/platform.cpp

@@ -212,9 +212,7 @@ class InputDeviceObserver : public mir::Device::Observer
         action_queue->enqueue(
             [
                 &devices = devices,
-                syspath = syspath,
-                &device_watchers = device_watchers,
-                devnum = devnum
+                syspath = syspath
             ]()
             {
                 for (auto const& input_device : devices)
@@ -229,7 +227,6 @@ class InputDeviceObserver : public mir::Device::Observer
                         }
                     }
                 }
-                device_watchers.erase(devnum);
             });
     }
 
@@ -238,9 +235,7 @@ class InputDeviceObserver : public mir::Device::Observer
         action_queue->enqueue(
             [
                 &devices = devices,
-                syspath = syspath,
-                &device_watchers = device_watchers,
-                devnum = devnum
+                syspath = syspath
             ]()
             {
                 for (auto const& input_device : devices)
@@ -255,7 +250,6 @@ class InputDeviceObserver : public mir::Device::Observer
                         }
                     }
                 }
-                device_watchers.erase(devnum);
             });
     }
 
@@ -352,7 +346,6 @@ void mie::Platform::start()
                             if (strcmp(device.syspath(), udev_device_get_syspath(device_udev)) == 0)
                             {
                                 libinput_path_remove_device(input_device->device());
-                                device_watchers.erase(device.devnum());
                             }
                         }
                     }
@@ -453,8 +446,18 @@ void mie::Platform::device_removed(libinput_device* dev)
     if (known_device_pos == end(devices))
         return;
 
+    auto const udev_device = libinput_device_get_udev_device(dev);
     input_device_registry->remove_device(*known_device_pos);
     devices.erase(known_device_pos);
+    if (udev_device)
+    {
+        /* libinput documents that input devices might not have udev devices
+         * In practise all our devices *will*, because we only add devices from udev events,
+         * but let's not crash if not!
+         */
+        device_watchers.erase(udev_device_get_devnum(udev_device));
+        udev_device_unref(udev_device);
+    }
 
     log_info("Removed %s", describe(dev).c_str());
 }