filesystem: mount snapd/seed/systems only if it exists

The "$source"/var/lib/snapd/seed/systems directory only exists in certain
scenarios related to TPM-backed FDE. When the directory does not exist,
attempting to bind-mount it to /var/lib/snapd/seed/systems crashes the
install with a CalledProcessError.

We now make sure the directory exists before trying to mount it. For
dry-run test cases, we added a configuration item that simulates the
presence (or the non-presence) of the systems directory on the source.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

examples/answers-tpm.yaml

@@ -1,4 +1,5 @@
 #source-catalog: examples/tpm-sources.yaml
+#dr-config: examples/tpm-dr-config.yaml
 Source:
   source: src-prefer-encrypted
 Serial:

examples/tpm-dr-config.yaml

@@ -0,0 +1 @@
+systems_dir_exists: true

subiquity/server/controllers/filesystem.py

@@ -19,6 +19,7 @@ import glob
 import json
 import logging
 import os
+import pathlib
 import platform
 import select
 from typing import Dict, List, Optional
@@ -110,6 +111,10 @@ than GPT, which is not currently supported.
 """)
 
 
+class NoSnapdSystemsOnSource(Exception):
+    pass
+
+
 class FilesystemController(SubiquityController, FilesystemManipulator):
 
     endpoint = API.storage
@@ -171,6 +176,12 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
         source_path = self.app.controllers.Source.source_path
         cur_systems_dir = '/var/lib/snapd/seed/systems'
         source_systems_dir = os.path.join(source_path, cur_systems_dir[1:])
+        if self.app.opts.dry_run:
+            systems_dir_exists = self.app.dr_cfg.systems_dir_exists
+        else:
+            systems_dir_exists = pathlib.Path(source_systems_dir).is_dir()
+        if not systems_dir_exists:
+            raise NoSnapdSystemsOnSource
         self._system_mounter = Mounter(self.app)
         await self._system_mounter.bind_mount_tree(
             source_systems_dir, cur_systems_dir)
@@ -182,7 +193,10 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
 
     async def _get_system(self):
         await self._unmount_system()
+        try:
             await self._mount_system()
+        except NoSnapdSystemsOnSource:
+            return
         self._system = None
         label = self.app.base_model.source.current.snapd_system_label
         if label is not None:

subiquity/server/controllers/tests/test_filesystem.py

@@ -38,6 +38,7 @@ from subiquity.models.tests.test_filesystem import (
     )
 from subiquity.server import snapdapi
 from subiquity.server.controllers.filesystem import FilesystemController
+from subiquity.server.dryrun import DRConfig
 
 bootloaders = [(bl, ) for bl in list(Bootloader)]
 bootloaders_and_ptables = [(bl, pt)
@@ -472,6 +473,7 @@ class TestCoreBootInstallMethods(IsolatedAsyncioTestCase):
         self.app.prober = mock.Mock()
         self.app.snapdapi = snapdapi.make_api_client(
             AsyncSnapd(get_fake_connection()))
+        self.app.dr_cfg = DRConfig()
         self.fsc = FilesystemController(app=self.app)
         self.fsc._configured = True
         self.fsc.model = make_model(Bootloader.UEFI)
@@ -565,6 +567,9 @@ class TestCoreBootInstallMethods(IsolatedAsyncioTestCase):
         self.app.base_model.source.current.snapd_system_label = \
             'prefer-encrypted'
         self.app.controllers.Source.source_path = ''
+
+        self.app.dr_cfg.systems_dir_exists = True
+
         await self.fsc._get_system_task.start()
         await self.fsc._get_system_task.wait()
 

subiquity/server/dryrun.py

@@ -34,6 +34,9 @@ class DRConfig:
     All variables here should have default values ; to indicate the behavior we
     want by default in dry-run mode. """
 
+    # Tells whether "$source"/var/lib/snapd/seed/systems exists on the source.
+    systems_dir_exists: bool = False
+
     @classmethod
     def load(cls, stream):
         data = yaml.safe_load(stream)