Merge pull request #85 from willmtemple/mount-overlay-backend

OverlayFS DockerMount backend for atomic mount command.

Atomic/mount.py

@@ -242,12 +242,21 @@ class DockerMount(Mount):
                          ''.format(identifier))
 
     @staticmethod
-    def _no_gd_api(cid):
+    def _no_gd_api_dm(cid):
         # TODO: Deprecated
         desc_file = os.path.join('/var/lib/docker/devicemapper/metadata', cid)
         desc = json.loads(open(desc_file).read())
         return desc['device_id'], desc['size']
 
+    @staticmethod
+    def _no_gd_api_overlay(cid):
+        # TODO: Deprecated
+        prefix = os.path.join('/var/lib/docker/overlay/', cid)
+        ld_metafile = open(os.path.join(prefix, 'lower-id'))
+        ld_loc = os.path.join('/var/lib/docker/overlay/', ld_metafile.read())
+        return (os.path.join(ld_loc, 'root'), os.path.join(prefix, 'upper'),
+                os.path.join(prefix, 'work'))
+
     def mount(self, identifier, options=[]):
         """
         Mounts a container or image referred to by identifier to
@@ -305,13 +314,13 @@ class DockerMount(Mount):
         dm_dev_name, dm_dev_id, dm_dev_size = '', '', ''
         dm_pool = info['DriverStatus'][0][1]
 
-        if 'GraphDriver' in cinfo:
+        try:
             dm_dev_name = cinfo['GraphDriver']['Data']['DeviceName']
             dm_dev_id = cinfo['GraphDriver']['Data']['DeviceId']
             dm_dev_size = cinfo['GraphDriver']['Data']['DeviceSize']
-        else:
+        except:
             # TODO: deprecated when GraphDriver patch makes it upstream
-            dm_dev_id, dm_dev_size = DockerMount._no_gd_api(cid)
+            dm_dev_id, dm_dev_size = DockerMount._no_gd_api_dm(cid)
             dm_dev_name = dm_pool.replace('pool', cid)
 
         dm_dev_path = os.path.join('/dev/mapper', dm_dev_name)
@@ -338,6 +347,42 @@ class DockerMount(Mount):
             self._cleanup_container(cinfo)
             raise de
 
+    def _mount_overlay(self, identifier, options):
+        """
+        OverlayFS mount backend.
+        """
+        if os.geteuid() != 0:
+            raise MountError('Insufficient privileges to mount device.')
+
+        if self.live:
+            raise MountError('The OverlayFS backend does not support live '
+                             'mounts.')
+        elif 'rw' in options:
+            raise MountError('The OverlayFS backend does not support '
+                             'writeable mounts.')
+
+        cid = self._identifier_as_cid(identifier)
+        cinfo = self.client.inspect_container(cid)
+
+        ld, ud, wd = '', '', ''
+        try:
+            ld = cinfo['GraphDriver']['Data']['lowerDir']
+            ud = cinfo['GraphDriver']['Data']['upperDir']
+            wd = cinfo['GraphDriver']['Data']['workDir']
+        except:
+            ld, ud, wd = DockerMount._no_gd_api_overlay(cid)
+
+        options += ['ro', 'lowerdir=' + ld, 'upperdir=' + ud, 'workdir=' + wd]
+        optstring = ','.join(options)
+        cmd = ['mount', '-t', 'overlay', '-o', optstring, 'overlay',
+               self.mountpoint]
+        status = util.subp(cmd)
+
+        if status.return_code != 0:
+            self._cleanup_container(cinfo)
+            raise MountError('Failed to mount OverlayFS device.\n' +
+                             status.stderr)
+
     def _cleanup_container(self, cinfo):
         """
         Remove a container and clean up its image if necessary.
@@ -398,3 +443,30 @@ class DockerMount(Mount):
 
         Mount._remove_thin_device(dev_name)
         self._cleanup_container(cinfo)
+
+    def _get_overlay_mount_cid(self):
+        """
+        Returns the cid of the container mounted at mountpoint.
+        """
+        cmd = ['findmnt', '-o', 'OPTIONS', '-n', self.mountpoint]
+        r = util.subp(cmd)
+        if r.return_code != 0:
+            raise MountError('No devices mounted at that location.')
+        optstring = r.stdout.strip().split('\n')[-1]
+        upperdir = [o.replace('upperdir=', '') for o in optstring.split(',')
+                    if o.startswith('upperdir=')][0]
+        cdir = upperdir.rsplit('/', 1)[0]
+        if not cdir.startswith('/var/lib/docker/overlay/'):
+            raise MountError('The device mounted at that location is not a '
+                             'docker container.')
+        return cdir.replace('/var/lib/docker/overlay/', '')
+
+    def _unmount_overlay(self):
+        """
+        OverlayFS unmount backend.
+        """
+        if Mount.get_dev_at_mountpoint(self.mountpoint) != 'overlay':
+            raise MountError('Device mounted at {} is not an atomic mount.')
+        cid = self._get_overlay_mount_cid()
+        Mount.unmount_path(self.mountpoint)
+        self._cleanup_container(self.client.inspect_container(cid))

docs/atomic-mount.1.md

@@ -19,23 +19,27 @@ If the given UUID or NAME is a container, and **--live** is not set, then
 temporary image and spawning a temporary container from that image. If UUID or
 REPO refers to an image, then *atomic mount* will simply create a temporary
 container from the given image. All temporary artifacts are cleaned upon
-*atomic unmount*.
+*atomic unmount*. Atomic mount is *only* supported on the devicemapper and
+overlayfs docker storage backends.
 
 # OPTIONS
 **-o|--options** *OPTIONS*
 Specify options to be passed to *mount*. All options accepted by the 'mount'
-command are valid. The default mount options (if the **--live** flag is unset)
-are: 'ro,nodev,nosuid'. If the **-o** flag is specified, then no default
-options are assumed. Use of the 'rw' flag is discouraged, as writes into the
-atomic temporary containers are never preserved. Use of this option conflicts
-with **--live**, as live containers have predetermined, immutable mount
-options.
+command are valid. The default mount options for the devicemapper backend (if
+the **--live** flag is unset) are: 'ro,nodev,nosuid'. If the **-o** flag is
+specified, then no default options are assumed. Use of the 'rw' flag is
+discouraged, as writes into the atomic temporary containers are never
+preserved. Use of this option conflicts with **--live**, as live containers
+have predetermined, immutable mount options. The OverlayFS driver has, by
+default, only the 'ro' option set, and the 'rw' option is illegal and will
+cause the program to terminate.
 
 **--live**
 Mount a container live, writable, and synchronized. This option allows the user
 to modify the container's contents as it runs or update the container's
 software without rebuilding the container. If live mode is used, no mount
-options may be provided.
+options may be provided. Live mode is *not* supported on the OverlayFS docker
+storage driver.
 
 # HISTORY
 June 2015, Originally compiled by William Temple (wtemple at redhat dot com)