Patch os.chdir to raise an exception, once twistd is done with it

Fixes #1007

master/buildbot/master.py

@@ -15,8 +15,8 @@
 from twisted.application.internet import TimerService
 
 import buildbot
-# sibling imports
 from buildbot.util import now, safeTranslate, eventual
+from buildbot.util import monkeypatches
 from buildbot.pbutil import NewCredPerspective
 from buildbot.process.builder import Builder, IDLE
 from buildbot.status.builder import Status, BuildSetStatus
@@ -571,6 +571,10 @@ def startService(self):
             b.builder_status.addPointEvent(["master", "started"])
             b.builder_status.saveYourself()
 
+        # now that we're running a master, don't use os.chdir.  Note that this
+        # will trip up twistd if we do it any earlier
+        monkeypatches.disable_os_chdir()
+
     def _handleSIGHUP(self, *args):
         reactor.callLater(0, self.loadTheConfigFile)
 

master/buildbot/util/monkeypatches.py

@@ -1,5 +1,11 @@
+import os
 from twisted.trial import unittest
 
+def disable_os_chdir():
+    def DoNotUseOsChDir(_):
+      raise ValueError('Do not use os.chdir() in your code.  You probably want to use path=foo or cwd=foo argument when calling out subprocesses instead')
+    os.chdir = DoNotUseOsChDir
+
 def add_debugging_monkeypatches():
     """
     DO NOT CALL THIS DIRECTLY