Merge branch 'instance_python_promises' of...

Merge branch 'instance_python_promises' of https://lab.nexedi.com/xavier_thompson/slapos.core into wip/rc

slapos/grid/SlapObject.py

@@ -472,6 +472,8 @@ class Partition(object):
 
     self.instance_min_free_space = instance_min_free_space
 
+    self.instance_python = self.getInstalledPythonExecutable()
+
 
   def check_free_space(self):
     required = self.instance_min_free_space or 0
@@ -705,6 +707,7 @@ class Partition(object):
                          debug=self.buildout_debug)
     self.generateSupervisorConfigurationFile()
     self.createRetentionLockDelay()
+    self.instance_python = self.getInstalledPythonExecutable()
 
   def generateSupervisorConfiguration(self):
     """
@@ -1043,3 +1046,15 @@ class Partition(object):
         return float(date_file_path.read())
     else:
       return None
+
+  def getInstalledPythonExecutable(self):
+    """
+    Return the path of the python executable installed for the SR of this instance.
+    """
+    try:
+      with open(os.path.join(self.software_path, 'bin', 'buildout')) as f:
+        shebang = f.readline()
+    except OSError:
+      return
+    if shebang.startswith('#!'):
+      return shebang[2:].split(None, 1)[0]

slapos/grid/promise/__init__.py

@@ -44,7 +44,7 @@ import hashlib
 from datetime import datetime
 from multiprocessing import Process, Queue as MQueue
 from six.moves import queue, reload_module
-from slapos.util import str2bytes, mkdir_p, chownDirectory
+from slapos.util import str2bytes, mkdir_p, chownDirectory, listifdir
 from slapos.grid.utils import dropPrivileges, killProcessTree
 from slapos.grid.promise import interface
 from slapos.grid.promise.generic import (GenericPromise, PromiseQueueResult,
@@ -731,65 +731,63 @@ class PromiseLauncher(object):
     error = 0
     success = 0
     promise_name_list = []
-    if os.path.exists(self.promise_folder) and os.path.isdir(self.promise_folder):
-      for promise_name in os.listdir(self.promise_folder):
-        for suffix in ['.pyc', '.pyo']:
-          if promise_name.endswith(suffix):
-            promise_path = os.path.join(self.promise_folder, promise_name)
-            if not os.path.exists(promise_path[:-1]):
-              try:
-                os.unlink(promise_path)
-              except Exception as e:
-                self.logger.warning('Failed to remove %r because of %s', promise_path, e)
-              else:
-                self.logger.debug('Removed stale %r', promise_path)
-
-        if promise_name.startswith('__init__') or \
-            not promise_name.endswith('.py'):
-          continue
-
-        promise_name_list.append(promise_name)
-        if self.run_only_promise_list is not None and not \
-            promise_name in self.run_only_promise_list:
-          continue
-
+    for promise_name in listifdir(self.promise_folder):
+      if promise_name.endswith(('.pyc', '.pyo')):
         promise_path = os.path.join(self.promise_folder, promise_name)
-        config = {
-          'path': promise_path,
-          'name': promise_name
-        }
-        config.update(base_config)
-        promise_result = self._launchPromise(promise_name, promise_path, config)
-        if promise_result:
-          change_date = promise_result.date.strftime('%Y-%m-%dT%H:%M:%S+0000')
-          if promise_result.hasFailed():
-            promise_status = 'FAILED'
+        if not os.path.exists(promise_path[:-1]):
+          try:
+            os.unlink(promise_path)
+          except Exception as e:
+            self.logger.warning('Failed to remove %r because of %s', promise_path, e)
+          else:
+            self.logger.debug('Removed stale %r', promise_path)
+
+      if promise_name.startswith('__init__') or \
+          not promise_name.endswith('.py'):
+        continue
+
+      promise_name_list.append(promise_name)
+      if self.run_only_promise_list is not None and not \
+          promise_name in self.run_only_promise_list:
+        continue
+
+      promise_path = os.path.join(self.promise_folder, promise_name)
+      config = {
+        'path': promise_path,
+        'name': promise_name
+      }
+      config.update(base_config)
+      promise_result = self._launchPromise(promise_name, promise_path, config)
+      if promise_result:
+        change_date = promise_result.date.strftime('%Y-%m-%dT%H:%M:%S+0000')
+        if promise_result.hasFailed():
+          promise_status = 'FAILED'
+          error += 1
+        else:
+          promise_status = "OK"
+          success += 1
+        if promise_name in previous_state_dict:
+          status, previous_change_date, _ = previous_state_dict[promise_name]
+          if promise_status == status:
+            change_date = previous_change_date
+
+        message = promise_result.message if promise_result.message else ""
+        new_state_dict[promise_name] = [
+          promise_status,
+          change_date,
+          hashlib.md5(str2bytes(message)).hexdigest()]
+
+        if promise_result.hasFailed() and not failed_promise_name:
+          failed_promise_name = promise_name
+          failed_promise_output = promise_result.message
+      else:
+        # The promise was skip, so for statistic point of view we preserve
+        # the previous result
+        if promise_name in new_state_dict:
+          if new_state_dict[promise_name][0] == "FAILED":
             error += 1
           else:
-            promise_status = "OK"
             success += 1
-          if promise_name in previous_state_dict:
-            status, previous_change_date, _ = previous_state_dict[promise_name]
-            if promise_status == status:
-              change_date = previous_change_date
-
-          message = promise_result.message if promise_result.message else ""
-          new_state_dict[promise_name] = [
-            promise_status,
-            change_date,
-            hashlib.md5(str2bytes(message)).hexdigest()]
-      
-          if promise_result.hasFailed() and not failed_promise_name:
-            failed_promise_name = promise_name
-            failed_promise_output = promise_result.message
-        else:
-          # The promise was skip, so for statistic point of view we preserve
-          # the previous result
-          if promise_name in new_state_dict:
-            if new_state_dict[promise_name][0] == "FAILED":
-              error += 1
-            else:
-              success += 1
 
     if not self.run_only_promise_list and len(promise_name_list) > 0:
       # cleanup stale json files
@@ -816,10 +814,9 @@ class PromiseLauncher(object):
         if key not in promise_name_list:
           new_state_dict.pop(key, None)
 
-    if not self.run_only_promise_list and os.path.exists(self.legacy_promise_folder) \
-        and os.path.isdir(self.legacy_promise_folder):
+    if not self.run_only_promise_list:
       # run legacy promise styles
-      for promise_name in os.listdir(self.legacy_promise_folder):
+      for promise_name in listifdir(self.legacy_promise_folder):
         promise_path = os.path.join(self.legacy_promise_folder, promise_name)
         if not os.path.isfile(promise_path) or \
             not os.access(promise_path, os.X_OK):

slapos/grid/promise/runpromises.py

+from __future__ import print_function
+
+import argparse
+import ast
+import os
+import sys
+
+
+# Parse arguments
+
+parser = argparse.ArgumentParser()
+parser.add_argument('--promise-folder', required=True)
+parser.add_argument('--legacy-promise-folder', default=None)
+parser.add_argument('--promise-timeout', type=int, default=20)
+parser.add_argument('--partition-folder', default=None)
+parser.add_argument('--log-folder', default=None)
+parser.add_argument('--force', action='store_true')
+parser.add_argument('--check-anomaly', action='store_true')
+parser.add_argument('--debug', action='store_true')
+parser.add_argument('--master-url', default=None)
+parser.add_argument('--partition-cert', default=None)
+parser.add_argument('--partition-key', default=None)
+parser.add_argument('--partition-id', default=None)
+parser.add_argument('--computer-id', default=None)
+args = parser.parse_args()
+
+
+# Extract slapos.core path and all dependencies from first promise found
+# to import slapos.core
+
+promise_folder = args.promise_folder
+promise_file = next(
+  p for p in os.listdir(promise_folder)
+  if p.endswith('.py') and not p.startswith('__init__')
+)
+with open(os.path.join(promise_folder, promise_file)) as f:
+  promise_content = f.read()
+tree = ast.parse(promise_content, mode='exec')
+
+sys.path[0:0] = eval(compile(ast.Expression(tree.body[1].value), '', 'eval'))
+
+from slapos.grid.promise import PromiseLauncher, PromiseError
+from slapos.cli.entry import SlapOSApp
+
+
+# Configure promise launcher
+# with the same logger as standard slapos command
+
+app = SlapOSApp()
+app.options, _ = app.parser.parse_known_args([])
+app.configure_logging()
+
+config = {k.replace('_', '-') : v for k, v in vars(args).items()}
+promise_checker = PromiseLauncher(config=config, logger=app.log)
+
+
+# Run promises
+# Redirect stdout to stderr (logger only uses stderr already)
+# to reserve stdout for error reporting
+
+out = os.dup(1)
+os.dup2(2, 1)
+
+try:
+  promise_checker.run()
+except Exception as e:
+  os.write(out, str(e))
+  sys.exit(2 if isinstance(e, PromiseError) else 1)

slapos/grid/slapgrid.py

@@ -33,7 +33,6 @@ import pkg_resources
 import random
 import socket
 from io import BytesIO
-import subprocess
 import sys
 import tempfile
 import time
@@ -45,6 +44,11 @@ import shutil
 import six
 import errno
 
+if six.PY3:
+  import subprocess
+else:
+  import subprocess32 as subprocess
+
 if sys.version_info < (2, 6):
   warnings.warn('Used python version (%s) is old and has problems with'
       ' IPv6 connections' % sys.version.split('\n')[0])
@@ -55,14 +59,18 @@ from slapos import manager as slapmanager
 from slapos.slap.slap import NotFoundError
 from slapos.slap.slap import ServerError
 from slapos.slap.slap import COMPUTER_PARTITION_REQUEST_LIST_TEMPLATE_FILENAME
-from slapos.util import mkdir_p, chownDirectory, string_to_boolean
+from slapos.util import mkdir_p, chownDirectory, string_to_boolean, listifdir
 from slapos.grid.exception import BuildoutFailedError
 from slapos.grid.SlapObject import Software, Partition
 from slapos.grid.svcbackend import (launchSupervisord,
                                     createSupervisordConfiguration,
                                     _getSupervisordConfigurationDirectory,
                                     _getSupervisordSocketPath)
-from slapos.grid.utils import (md5digest, dropPrivileges, SlapPopen, updateFile)
+from slapos.grid.utils import (md5digest,
+                              dropPrivileges,
+                              killProcessTree,
+                              SlapPopen,
+                              updateFile)
 from slapos.grid.promise import PromiseLauncher, PromiseError
 from slapos.grid.promise.generic import PROMISE_LOG_FOLDER_NAME
 from slapos.human import human2bytes
@@ -302,10 +310,12 @@ def check_required_only_partitions(existing, required):
   """
   Verify the existence of partitions specified by the --only parameter
   """
-  missing = set(required) - set(existing)
+  required = set(required)
+  missing = required.difference(existing)
   if missing:
     plural = ['s', ''][len(missing) == 1]
     raise ValueError('Unknown partition%s: %s' % (plural, ', '.join(sorted(missing))))
+  return required
 
 
 class Slapgrid(object):
@@ -549,6 +559,18 @@ stderr_logfile_backups=1
       self.logger.fatal(exc)
       raise
 
+  def getRequiredComputerPartitionList(self):
+    """Return the computer partitions that should be processed.
+    """
+    cp_list = self.getComputerPartitionList()
+    cp_id_list = [cp.getId() for cp in cp_list]
+    required_cp_id_set = check_required_only_partitions(
+      cp_id_list, self.computer_partition_filter_list)
+    busy_cp_list = self.FilterComputerPartitionList(cp_list)
+    if required_cp_id_set:
+      return [cp for cp in busy_cp_list if cp.getId() in required_cp_id_set]
+    return busy_cp_list
+
   def processSoftwareReleaseList(self):
     """Will process each Software Release.
     """
@@ -655,18 +677,17 @@ stderr_logfile_backups=1
     return SLAPGRID_SUCCESS
 
   def _checkPromiseList(self, partition, force=True, check_anomaly=False):
+    self.logger.info("Checking %s promises...",  partition.partition_id)
+
     instance_path = os.path.join(self.instance_root, partition.partition_id)
     promise_log_path = os.path.join(instance_path, PROMISE_LOG_FOLDER_NAME)
+    promise_dir = os.path.join(instance_path, 'etc', 'plugin')
+    legacy_promise_dir = os.path.join(instance_path, 'etc', 'promise')
 
-    self.logger.info("Checking %s promises..." % partition.partition_id)
-    uid, gid = None, None
     stat_info = os.stat(instance_path)
-
-    #stat sys call to get statistics informations
     uid = stat_info.st_uid
     gid = stat_info.st_gid
-    promise_dir = os.path.join(instance_path, 'etc', 'plugin')
-    legacy_promise_dir = os.path.join(instance_path, 'etc', 'promise')
+
     promise_config = {
       'promise-folder': promise_dir,
       'legacy-promise-folder': legacy_promise_dir,
@@ -684,8 +705,51 @@ stderr_logfile_backups=1
       'computer-id': self.computer_id,
     }
 
-    promise_checker = PromiseLauncher(config=promise_config, logger=self.logger)
-    return promise_checker.run()
+    plugins = sum(
+      1 for p in listifdir(promise_dir)
+      if p.endswith('.py') and not p.startswith('__init__')
+    )
+    if partition.instance_python is not None and plugins:
+      self.logger.info("Switching to %s's python", partition.partition_id)
+      self.logger.info("Located at %s", partition.instance_python)
+      runpromise_script = os.path.join(
+        os.path.dirname(__file__), 'promise', 'runpromises.py')
+      command = [partition.instance_python, runpromise_script]
+      for option, value in promise_config.items():
+        if option in ('uid', 'gid'):
+          continue
+        if isinstance(value, bool):
+          if value:
+            command.append('--' + option)
+        else:
+          command.append('--' + option)
+          command.append(str(value))
+      process = subprocess.Popen(
+        command,
+        preexec_fn=lambda: dropPrivileges(uid, gid, logger=self.logger),
+        cwd=instance_path,
+        stdout=subprocess.PIPE)
+      promises = plugins + len(listifdir(legacy_promise_dir))
+      # Add a timeout margin to let the process kill the promises and cleanup
+      timeout = promises * self.promise_timeout + 10
+      try:
+        # The logger logs everything to stderr, so runpromise redirects
+        # stdout to stderr in case a promise prints to stdout
+        # and reserves stdout to progagate exception messages.
+        out, _ = process.communicate(timeout=timeout)
+        if process.returncode == 2:
+          raise PromiseError(out)
+        elif process.returncode:
+          raise Exception(out)
+        elif out:
+          self.logger.warn('Promise runner unexpected output:\n%s', out)
+      except subprocess.TimeoutExpired:
+        killProcessTree(process.pid, self.logger)
+        # The timeout margin was exceeded but this should be infrequent
+        raise Exception('Promise runner timed out')
+
+    else:
+      return PromiseLauncher(config=promise_config, logger=self.logger).run()
 
   def _endInstallationTransaction(self, computer_partition):
     partition_id = computer_partition.getId()
@@ -972,12 +1036,6 @@ stderr_logfile_backups=1
     if not computer_partition_id:
       raise ValueError('Computer Partition id is empty.')
 
-    # Check if we defined explicit list of partitions to process.
-    # If so, if current partition not in this list, skip.
-    if len(self.computer_partition_filter_list) > 0 and \
-         (computer_partition_id not in self.computer_partition_filter_list):
-      return
-
     instance_path = os.path.join(self.instance_root, computer_partition_id)
     os.environ['SLAPGRID_INSTANCE_ROOT'] = self.instance_root
     try:
@@ -1038,12 +1096,6 @@ stderr_logfile_backups=1
     if not computer_partition_id:
       raise ValueError('Computer Partition id is empty.')
 
-    # Check if we defined explicit list of partitions to process.
-    # If so, if current partition not in this list, skip.
-    if len(self.computer_partition_filter_list) > 0 and \
-         (computer_partition_id not in self.computer_partition_filter_list):
-      return
-
     self.logger.debug('Check if %s requires processing...' % computer_partition_id)
 
     instance_path = os.path.join(self.instance_root, computer_partition_id)
@@ -1357,12 +1409,7 @@ stderr_logfile_backups=1
     # Boolean to know if every promises correctly passed
     clean_run_promise = True
 
-    check_required_only_partitions([cp.getId() for cp in self.getComputerPartitionList()],
-                                   self.computer_partition_filter_list)
-
-    # Filter all dummy / empty partitions
-    computer_partition_list = self.FilterComputerPartitionList(
-        self.getComputerPartitionList())
+    computer_partition_list = self.getRequiredComputerPartitionList()
 
     process_error_partition_list = []
     promise_error_partition_list = []
@@ -1438,12 +1485,8 @@ stderr_logfile_backups=1
     self.logger.info('Processing promises...')
     # Return success value
     clean_run_promise = True
-    check_required_only_partitions([cp.getId() for cp in self.getComputerPartitionList()],
-                                   self.computer_partition_filter_list)
 
-    # Filter all dummy / empty partitions
-    computer_partition_list = self.FilterComputerPartitionList(
-        self.getComputerPartitionList())
+    computer_partition_list = self.getRequiredComputerPartitionList()
 
     promise_error_partition_list = []
     for computer_partition in computer_partition_list:
@@ -1599,37 +1642,24 @@ stderr_logfile_backups=1
       try:
         computer_partition_id = computer_partition.getId()
 
-        # We want to execute all the script in the report folder
-        instance_path = os.path.join(self.instance_root,
-            computer_partition.getId())
-        report_path = os.path.join(instance_path, 'etc', 'report')
-        if os.path.isdir(report_path):
-          script_list_to_run = os.listdir(report_path)
-        else:
-          script_list_to_run = []
+        instance_path = os.path.join(self.instance_root, computer_partition_id)
 
-        # We now generate the pseudorandom name for the xml file
-        # and we add it in the invocation_list
-        f = tempfile.NamedTemporaryFile()
-        name_xml = '%s.%s' % ('slapreport', os.path.basename(f.name))
-        path_to_slapreport = os.path.join(instance_path, 'var', 'xml_report',
-            name_xml)
+        # We now generate a pseudorandom name for the report xml file
+        # that will be passed to the invocation list
+        slapreport_path = tempfile.mktemp(
+          prefix='slapreport.',
+          dir=os.path.join(instance_path, 'var', 'xml_report'))
 
+        # We want to execute all the script in the report folder
         failed_script_list = []
-        for script in script_list_to_run:
+        report_dir = os.path.join(instance_path, 'etc', 'report')
+        for script in listifdir(report_dir):
           invocation_list = []
-          invocation_list.append(os.path.join(instance_path, 'etc', 'report',
-            script))
-          # We add the xml_file name to the invocation_list
-          #f = tempfile.NamedTemporaryFile()
-          #name_xml = '%s.%s' % ('slapreport', os.path.basename(f.name))
-          #path_to_slapreport = os.path.join(instance_path, 'var', name_xml)
-
-          invocation_list.append(path_to_slapreport)
+          invocation_list.append(os.path.join(report_dir, script))
+          invocation_list.append(slapreport_path)
+
           # Dropping privileges
-          uid, gid = None, None
           stat_info = os.stat(instance_path)
-          #stat sys call to get statistics informations
           uid = stat_info.st_uid
           gid = stat_info.st_gid
           process_handler = SlapPopen(invocation_list,

slapos/tests/test_slapgrid.py

@@ -45,6 +45,7 @@ import json
 import re
 import grp
 import hashlib
+import errno
 
 import mock
 from mock import patch
@@ -65,6 +66,11 @@ import slapos.grid.SlapObject
 from slapos import manager as slapmanager
 from slapos.util import dumps
 
+from slapos import __path__ as slapos_path
+from zope import __path__ as zope_path
+
+PROMISE_PATHS = sorted(set(map(os.path.dirname, slapos_path + zope_path)))
+
 import httmock
 
 
@@ -113,6 +119,9 @@ touch worked
 """
 
 PROMISE_CONTENT_TEMPLATE = """
+import sys
+sys.path[0:0] = %(paths)r
+
 from zope.interface import implementer
 from slapos.grid.promise import interface
 from slapos.grid.promise import GenericPromise
@@ -122,7 +131,7 @@ class RunPromise(GenericPromise):
 
   def __init__(self, config):
     super(RunPromise, self).__init__(config)
-    self.setPeriodicity(minute=%(periodicity)s)
+    self.setPeriodicity(minute=%(periodicity)r)
  
   def sense(self):
     %(content)s
@@ -133,11 +142,11 @@ class RunPromise(GenericPromise):
       self.logger.info("success")
 
   def anomaly(self):
-    return self._anomaly(result_count=2, failure_amount=%(failure_amount)s)
+    return self._anomaly(result_count=2, failure_amount=%(failure_amount)r)
 
   def test(self):
-    return self._test(result_count=1, failure_amount=%(failure_amount)s)
-""" 
+    return self._test(result_count=1, failure_amount=%(failure_amount)r)
+"""
 
 class BasicMixin(object):
   def setUp(self):
@@ -150,6 +159,14 @@ class BasicMixin(object):
       del os.environ['SLAPGRID_INSTANCE_ROOT']
     logging.basicConfig(level=logging.DEBUG)
     self.setSlapgrid()
+    self.setMock()
+
+  def setMock(self):
+    cls = SlapObject.Partition
+    attr = 'getInstalledPythonExecutable'
+    orig = getattr(cls, attr)
+    setattr(cls, attr, lambda self: None)
+    self.addCleanup(setattr, cls, attr, orig)
 
   def setSlapgrid(self, develop=False, force_stop=False):
     if getattr(self, 'master_url', None) is None:
@@ -576,7 +593,8 @@ class InstanceForTest(object):
          {'success': success, 
           'content': promise_content, 
           'failure_amount': failure_count,
-          'periodicity': periodicity}
+          'periodicity': periodicity,
+          'paths': PROMISE_PATHS}
  
     with open(os.path.join(promise_dir, promise_name), 'w') as f:
       f.write(_promise_content)
@@ -599,7 +617,7 @@ class InstanceForTest(object):
 class SoftwareForTest(object):
   """
   Class to prepare and simulate software.
-  each instance has a sotfware attributed
+  each instance has a software attributed
   """
   def __init__(self, software_root, name=''):
     """
@@ -2297,6 +2315,7 @@ exit 1
       self.assertFalse(os.path.exists(promise_file))
       self.assertTrue(instance.error)
 
+
 class TestSlapgridDestructionLock(MasterMixin, unittest.TestCase):
   def test_retention_lock(self):
     """
@@ -3999,9 +4018,6 @@ class TestSlapgridPromiseWithMaster(MasterMixin, unittest.TestCase):
 
       self.assertEqual('success', result["result"]["message"])
 
-
-
-
   def test_one_succeeding_one_timing_out_promises(self):
     computer = ComputerForTest(self.software_root, self.instance_root)
     with httmock.HTTMock(computer.request_handler):
@@ -4095,6 +4111,58 @@ class TestSlapgridPromiseWithMaster(MasterMixin, unittest.TestCase):
                 ".slapgrid/promise/result/fail.status.json")))
 
 
+class TestSlapgridPluginPromiseWithInstancePython(TestSlapgridPromiseWithMaster):
+  expect_plugin = False
+
+  def setPython(self):
+    self.python_called = os.path.join(self.software_root, 'called')
+    wrapper = """#!/bin/sh
+    touch %s
+    exec %s "$@"
+    """ % (self.python_called, sys.executable)
+    path = os.path.join(self.software_root, 'python')
+    with open(path, 'w') as f:
+      f.write(wrapper)
+    os.chmod(path, 0o755)
+    return path
+
+  def patchBuildoutSetter(self):
+    cls = SoftwareForTest
+    attr = 'setBuildout'
+    orig = getattr(cls, attr)
+    def setBuildout(soft):
+      buildout = "#!" + self.setPython()
+      orig(soft, buildout)
+    setattr(cls, attr, setBuildout)
+    self.addCleanup(setattr, cls, attr, orig)
+
+  def patchPluginSetter(self):
+    cls = InstanceForTest
+    attr = 'setPluginPromise'
+    orig = getattr(cls, attr)
+    def setPluginPromise(inst, *args, **kwargs):
+      self.expect_plugin = inst.requested_state == 'started'
+      return orig(inst, *args, **kwargs)
+    setattr(cls, attr, setPluginPromise)
+    self.addCleanup(setattr, cls, attr, orig)
+
+  def setMock(self):
+    self.patchBuildoutSetter()
+    self.patchPluginSetter()
+
+  def tearDown(self):
+    try:
+      os.remove(self.python_called)
+      called = True
+    except OSError as e:
+      if e.errno != errno.ENOENT:
+        raise
+      called = False
+    finally:
+      super(TestSlapgridPluginPromiseWithInstancePython, self).tearDown()
+    self.assertEqual(self.expect_plugin, called)
+
+
 class TestSVCBackend(unittest.TestCase):
   """Tests for supervisor backend.
   """

slapos/util.py

@@ -108,6 +108,18 @@ def mkdir_p(path, mode=0o700):
             raise
 
 
+def listifdir(path):
+  """
+  Like listdir, but returns an empty tuple if the path is not a directory.
+  """
+  try:
+    return os.listdir(path)
+  except OSError as e:
+    if e.errno != errno.ENOENT:
+      raise
+    return ()
+
+
 def chownDirectory(path, uid, gid):
   if os.getuid() != 0:
     # we are probably inside of a webrunner