Fix portability issue

The use of the "resource" module now prevents oslo.rootwrap from
being imported on Windows.

Although oslo.rootwrap is not effectively used on Windows, it's
important for it to at least be importable, since it's widely
used throughout OpenStack projects without having platform checks
in place.

This change checks if the "resource" module is avaialble before
attempting to use it.

Change-Id: I2391315f77718a3c9eb9fc8c03a6882237f33548
Closes-Bug: #1804639

oslo_rootwrap/cmd.py

@@ -33,7 +33,6 @@
 from __future__ import print_function
 
 import logging
-import resource
 import sys
 
 from oslo_rootwrap import subprocess
@@ -41,6 +40,12 @@ from oslo_rootwrap import wrapper
 
 from six import moves
 
+try:
+    # This isn't available on all platforms (e.g. Windows).
+    import resource
+except ImportError:
+    resource = None
+
 RC_UNAUTHORIZED = 99
 RC_NOCOMMAND = 98
 RC_BADCONFIG = 97
@@ -85,21 +90,23 @@ def main(run_daemon=False):
         _exit_error(execname, "Incorrect configuration file: %s" % configfile,
                     RC_BADCONFIG, log=False)
 
-    # When use close_fds=True on Python 2.x, we spend significant time
-    # in closing fds up to current soft ulimit, which could be large.
-    # Lower our ulimit to a reasonable value to regain performance.
-    fd_limits = resource.getrlimit(resource.RLIMIT_NOFILE)
-    sensible_fd_limit = min(config.rlimit_nofile, fd_limits[0])
-    if (fd_limits[0] > sensible_fd_limit):
-        # Unfortunately this inherits to our children, so allow them to
-        # re-raise by passing through the hard limit unmodified
-        resource.setrlimit(
-            resource.RLIMIT_NOFILE, (sensible_fd_limit, fd_limits[1]))
-        # This is set on import to the hard ulimit. if its defined we
-        # already have imported it, so we need to update it to the new limit
-        if (hasattr(subprocess, 'MAXFD') and
-                subprocess.MAXFD > sensible_fd_limit):
-            subprocess.MAXFD = sensible_fd_limit
+    if resource:
+        # When use close_fds=True on Python 2.x, we spend significant time
+        # in closing fds up to current soft ulimit, which could be large.
+        # Lower our ulimit to a reasonable value to regain performance.
+        fd_limits = resource.getrlimit(resource.RLIMIT_NOFILE)
+        sensible_fd_limit = min(config.rlimit_nofile, fd_limits[0])
+        if (fd_limits[0] > sensible_fd_limit):
+            # Unfortunately this inherits to our children, so allow them to
+            # re-raise by passing through the hard limit unmodified
+            resource.setrlimit(
+                resource.RLIMIT_NOFILE, (sensible_fd_limit, fd_limits[1]))
+            # This is set on import to the hard ulimit. if its defined we
+            # already have imported it, so we need to update it to the new
+            # limit.
+            if (hasattr(subprocess, 'MAXFD') and
+                    subprocess.MAXFD > sensible_fd_limit):
+                subprocess.MAXFD = sensible_fd_limit
 
     if config.use_syslog:
         wrapper.setup_syslog(execname,