Merge pull request #4 from murraybd/debian/sid

Handle the situation where you may not be able to decrease process niceness e.g. on an lxc container.

unattended-upgrade

@@ -41,6 +41,7 @@ import sys
 
 
 from email.message import Message
+from errno import EACCESS, EPERM
 from gettext import gettext as _
 from io import StringIO
 from optparse import (
@@ -1103,7 +1104,16 @@ def main(options, rootdir=""):
 
     # FIXME: make this into a ContextManager
     # be nice when calculating the upgrade as its pretty CPU intensive
-    os.nice(19)
+    old_priority = os.nice(0)
+    try:
+        # Check that we will be able to restore the priority
+        os.nice(-1)
+        os.nice(20)
+    except OSError as e:
+        if e.errno in (EPERM, EACCESS):
+            pass
+        else:
+            raise
 
     # speed things up with latest apt
     actiongroup = apt_pkg.ActionGroup(cache._depcache)
@@ -1121,7 +1131,7 @@ def main(options, rootdir=""):
 
     # FIXME: make this into a ContextManager
     # stop being nice
-    os.nice(-19)
+    os.nice(old_priority - os.nice(0))
 
     # download what looks good
     if options.debug: