Add support for juju resources

Simplestreams package is now installed via snap. By having the juju
resources for the snap package, it can benefit in offline environment.

Closes-Bug: 1921832
Change-Id: I5f21ca9faff35427281076e142942c3458e71091

README.md

@@ -70,6 +70,16 @@ Juju [actions][juju-docs-actions] allow specific operations to be performed on
 a per-unit basis. This charm supports the single action `sync-images`, which
 allows for a one-time image sync from the currently configured mirror list.
 
+## Juju resources
+
+The charm support juju resources, which is handy in offline deployments. Prefetch the snaps:
+
+    snap download --channel=stable simplestreams
+
+Provide downloaded snaps as resources to the application:
+
+    juju deploy cs:glance-simplestreams-sync --resource simplestreams=simplestreams_27.snap
+
 # Bugs
 
 Please report bugs on [Launchpad][lp-bugs-charm-glance-simplestreams-sync].

hooks/hooks.py

@@ -310,8 +310,14 @@ def install():
 
     apt_install(_packages)
 
-    snap_install('simplestreams',
-                 *['--channel={}'.format(hookenv.config('snap-channel'))])
+    snap = 'simplestreams'
+    res_path = _resource_get(snap)
+    if res_path is None:
+        snap_install(snap,
+                     *['--channel={}'.format(hookenv.config('snap-channel'))])
+    else:
+        snap_install(res_path,
+                     *['--dangerous'])
 
     install_gss_wrappers()
 
@@ -411,6 +417,19 @@ def certs_changed(relation_id=None, unit=None):
     identity_service_changed()
 
 
+def _resource_get(snapname):
+    """Used to fetch the resource path of the given name.
+
+    This wrapper obtains a resource path and adds an additional
+    check to return False if the resource is zero length.
+    """
+    res_path = hookenv.resource_get(snapname)
+    if res_path and os.stat(res_path).st_size != 0:
+        return res_path
+
+    return None
+
+
 if __name__ == '__main__':
     try:
         hooks.execute(sys.argv)

metadata.yaml

@@ -29,3 +29,8 @@ requires:
     interface: keystone
   certificates:
     interface: tls-certificates
+resources:
+  simplestreams:
+    type: file
+    filename: simplestreams.snap
+    description: simplestreams snap

unit_tests/test_hooks.py

@@ -105,3 +105,14 @@ class TestConfigChanged(CharmTestCase):
                                             hooks.CRON_JOB_FILENAME))
         remove.assert_any_call(hooks.CRON_POLL_FILEPATH)
         update_nrpe_config.assert_called()
+
+    @mock.patch("charmhelpers.core.hookenv.resource_get")
+    @mock.patch("os.stat")
+    def test_resource_get_simplestreams(self, mock_os_stat, mock_resource_get):
+        mock_path = "/tmp/file"
+        mock_resource_get.return_value = mock_path
+        mock_os_stat.return_value.st_size = 100
+        self.assertEqual(hooks._resource_get('simplestreams'), mock_path)
+
+        mock_os_stat.return_value.st_size = 0
+        self.assertEqual(hooks._resource_get('simplestreams'), None)