prune: tolerate all forms of broken slapos.cfg in instance

When we recurse in instance to find nested slapos that might be using
parts from ourselves, we check all files named slapos.cfg but we may
encounter some slapos.cfg that are not actual slapos config files, for
example in testnodes we have a working copy of slapos repository which
contains a stack/slapos.cfg which is completly different.

This was breaking prune with an error like this:

    2020-04-29 20:07:28 slapos[2097] DEBUG Reading config at /srv/slapgrid/slappart9/srv/testnode/czp/slapos-repository/stack/slapos.cfg
    2020-04-29 20:07:28 slapos[2097] ERROR No section: 'slapos'

slapos/cli/prune.py

@@ -138,14 +138,38 @@ def getUsageSignaturesFromSubInstance(logger, instance_root):
   signatures = {}
   for slapos_cfg in getInstanceSlaposCfgList(logger, instance_root):
     cfg = readSlaposCfg(logger, slapos_cfg)
+    if not cfg:
+      return {}
     shared_root = None
     if cfg['shared_part_list']:
       shared_root = cfg['shared_part_list'][-1]
-    signatures.update(
-        getUsageSignatureFromSoftwareAndSharedPart(
-            logger, cfg['software_root'], shared_root))
-    signatures.update(
-        getUsageSignaturesFromSubInstance(logger, cfg['instance_root']))
+      if not os.path.exists(shared_root):
+        logger.debug(
+            "Ignoring non existant shared root %s from %s",
+            shared_root,
+            slapos_cfg,
+        )
+        shared_root = None
+
+    if not os.path.exists(cfg['software_root']):
+      logger.debug(
+          "Ignoring non existant software root %s from %s",
+          cfg['software_root'],
+          slapos_cfg,
+      )
+    else:
+      signatures.update(
+          getUsageSignatureFromSoftwareAndSharedPart(
+              logger, cfg['software_root'], shared_root))
+    if not os.path.exists(cfg['instance_root']):
+      logger.debug(
+          "Ignoring non existant instance root %s from %s",
+          cfg['instance_root'],
+          slapos_cfg,
+      )
+    else:
+      signatures.update(
+          getUsageSignaturesFromSubInstance(logger, cfg['instance_root']))
   return signatures
 
 
@@ -163,12 +187,16 @@ def readSlaposCfg(logger, path):
   """
   logger.debug('Reading config at %s', path)
   parser = configparser.ConfigParser({'shared_part_list': ''})
-  parser.read([path])
-  cfg = {
+  try:
+    parser.read([path])
+    cfg = {
       'software_root': parser.get('slapos', 'software_root'),
       'instance_root': parser.get('slapos', 'instance_root'),
       'shared_part_list': parser.get('slapos', 'shared_part_list').splitlines()
-  }
+    }
+  except configparser.Error:
+    logger.debug('Ignored config at %s because of error', path, exc_info=True)
+    return None
   logger.debug('Read config: %s', cfg)
   return cfg
 

slapos/tests/test_prune.py

@@ -197,3 +197,52 @@ shared_part_list =
 
     self.logger.warning.assert_called_with(
         'Unusued shared parts at %s%s', not_used, ' ... removed')
+
+  def test_recursive_instance_broken_slapos_cfg(self):
+    instance = os.path.join(self.instance_root, 'slappart0')
+    instance_etc = os.path.join(instance, 'etc')
+    os.makedirs(instance_etc)
+    instance_slapos_cfg = os.path.join(instance_etc, 'slapos.cfg')
+
+    # slapos.cfg might be not valid .ini file
+    with open(instance_slapos_cfg, 'w') as f:
+      f.write('this is not an actual slapos.cfg')
+    do_prune(self.logger, self.config, False)
+    self.logger.debug.assert_any_call(
+        'Ignored config at %s because of error', instance_slapos_cfg, exc_info=True)
+
+    # ... or an ini file without [slapos] section
+    with open(instance_slapos_cfg, 'w') as f:
+      f.write('[something]')
+    do_prune(self.logger, self.config, False)
+    self.logger.debug.assert_any_call(
+        'Ignored config at %s because of error', instance_slapos_cfg, exc_info=True)
+
+    # ... or referencing non existant paths
+    with open(instance_slapos_cfg, 'w') as f:
+      f.write('''
+[slapos]
+software_root = NOT EXIST SOFTWARE
+instance_root = NOT EXIST INSTSTANCE
+shared_part_list = NOT EXIST SHARED PART
+''')
+    do_prune(self.logger, self.config, False)
+    self.assertIn(
+        mock.call(
+            'Ignoring non existant software root %s from %s',
+            'NOT EXIST SOFTWARE',
+            instance_slapos_cfg),
+        self.logger.debug.mock_calls)
+    self.assertIn(
+        mock.call(
+            'Ignoring non existant instance root %s from %s',
+            'NOT EXIST INSTSTANCE',
+            instance_slapos_cfg),
+        self.logger.debug.mock_calls)
+
+    self.assertIn(
+        mock.call(
+            'Ignoring non existant shared root %s from %s',
+            'NOT EXIST SHARED PART',
+            instance_slapos_cfg),
+        self.logger.debug.mock_calls)
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