util: agents: add udisks2 agent for remote mounting

While udisksctl exists as a client tool to talk to the udisks2 daemon,
it's not well-suited for programmatic use. We thus introduce an agent
that is copied to the remote system and that uses PyGObject to directly
talk DBus to the daemon to mount and unmount file systems.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: a3f

doc/configuration.rst

@@ -610,6 +610,14 @@ device.
      match:
        ID_PATH: 'pci-0000:06:00.0-usb-0:1.3.2:1.0-scsi-0:0:0:3'
 
+Writing images to disk requires installation of ``dd`` or optionally
+``bmaptool`` on the same system as the block device.
+
+For mounting the file system and writing into it,
+`PyGObject <https://pygobject.readthedocs.io/>`_ must be installed.
+For Debian, the necessary packages are `python3-gi` and `gir1.2-udisks-2.0`.
+This is not required for writing images to disks.
+
 Arguments:
   - match (dict): key and value pairs for a udev match, see `udev Matching`_
 

labgrid/util/agents/udisks2.py

@@ -0,0 +1,102 @@
+"""
+This module implements mounting file systems via communication with udisksd.
+"""
+import logging
+import time
+
+import gi
+gi.require_version('UDisks', '2.0')
+from gi.repository import GLib, UDisks
+
+class UDisks2Device:
+    UNMOUNT_MAX_RETRIES = 5
+    UNMOUNT_BUSY_WAIT = 3 # s
+
+    def __init__(self, devpath):
+        self._logger = logging.getLogger("Device: ")
+        self.devpath = devpath
+        client = UDisks.Client.new_sync(None)
+
+        manager = client.get_object_manager()
+        for obj in manager.get_objects():
+            block = obj.get_block()
+            if not block:
+                continue
+
+            device_path = block.get_cached_property("Device").get_bytestring().decode('utf-8')
+            if device_path == devpath:
+                self.fs = obj.get_filesystem()
+                if self.fs is None:
+                    raise ValueError(f"no filesystem found on {devpath}")
+
+                return
+
+        raise ValueError(f"No udisks2 device found for {devpath}")
+
+    def mount(self, readonly=False):
+        opts = GLib.Variant('a{sv}', {'options': GLib.Variant('s', 'ro' if readonly else 'rw')})
+
+        try:
+            mountpoint = self.fs.call_mount_sync(opts, None)
+        except GLib.GError as err:
+            if not err.matches(UDisks.error_quark(), UDisks.Error.ALREADY_MOUNTED):
+                raise err
+
+            self._logger.warning('Unmounting lazily and remounting %s...', self.devpath)
+            self._unmount_lazy()
+
+            mountpoint = self.fs.call_mount_sync(opts, None)
+
+        return mountpoint
+
+    def _unmount_lazy(self):
+        opts = GLib.Variant('a{sv}', {'force': GLib.Variant('b', True)})
+
+        try:
+            self.fs.call_unmount_sync(opts, None)
+        except GLib.GError as err:
+            if not err.matches(UDisks.error_quark(), UDisks.Error.NOT_MOUNTED):
+                raise err
+
+    def _unmount(self):
+        opts = GLib.Variant('a{sv}', {'force': GLib.Variant('b', False)})
+
+        for _ in range(self.UNMOUNT_MAX_RETRIES):
+            try:
+                self.fs.call_unmount_sync(opts, None)
+                return
+            except GLib.GError as err:
+                if not err.matches(UDisks.error_quark(), UDisks.Error.DEVICE_BUSY):
+                    raise err
+
+                self._logger.warning('waiting %s s for busy %s',
+                                     self.UNMOUNT_BUSY_WAIT, self.devpath)
+                time.sleep(self.UNMOUNT_BUSY_WAIT)
+
+        raise TimeoutError("Timeout waiting for device to become non-busy")
+
+    def unmount(self, lazy=False):
+        if lazy:
+            self._unmount_lazy()
+        else:
+            self._unmount()
+
+_devs = {}
+
+def _get_udisks2_dev(devpath):
+    if devpath not in _devs:
+        _devs[devpath] = UDisks2Device(devpath=devpath)
+    return _devs[devpath]
+
+def handle_mount(devpath):
+    dev = _get_udisks2_dev(devpath)
+    return dev.mount()
+
+def handle_unmount(devpath, lazy=False):
+    dev = _get_udisks2_dev(devpath)
+    return dev.unmount(lazy=lazy)
+
+methods = {
+    'mount': handle_mount,
+    'unmount': handle_unmount,
+}