Always close all passed in fds beyond sensible_fd_limit on launch

Change Idd98c183eca3e2df8648fc0f37d27fe9cc6d0563 stopped closing
filedescriptors beyond the sensible_fd_limit. With this patch
the previous behavior is restored. We can close the fds more efficiently
by just iterating over /proc/self/fd on Linux and only change
the rlimit/MAXFD setting when successful.

Change-Id: I3b481ddd14ae2b948270d715aad157cf3996def7

doc/source/user/usage.rst

@@ -114,12 +114,14 @@ syslog_log_level
     ``syslog_log_level=ERROR``
 
 rlimit_nofile
-    Which rlimit for number of open file descriptors should be set for rootwrap
-    and its children processes by default. This is useful in case there is a
+    Specify rlimit for number of open file descriptors used by oslo rootwrap
+    and its child processes by default. This is useful in case there is a
     excessively large ulimit configured for the calling process that shouldn't
     inherit to oslo.rootwrap and its called processes. Will not attempt to raise
     the limit. Defaults to 1024.
 
+    Ignored on platforms that do not provide "/proc/self/fd" (e.g. non-Linux).
+
 
 .filters files
 ==============

etc/rootwrap.conf.sample

@@ -29,5 +29,5 @@ syslog_log_level=ERROR
 # Rootwrap daemon exits after this seconds of inactivity
 daemon_timeout=600
 
-# Rootwrap daemon limits itself to that many file descriptors
+# Rootwrap daemon limits itself to that many file descriptors (Linux only)
 rlimit_nofile=1024

oslo_rootwrap/cmd.py

@@ -33,6 +33,7 @@
 from __future__ import print_function
 
 import logging
+import os
 import sys
 
 from oslo_rootwrap import subprocess
@@ -91,12 +92,24 @@ def main(run_daemon=False):
                     RC_BADCONFIG, log=False)
 
     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.
+        # When use close_fds=True on Python 2.x, calling subprocess with
+        # close_fds=True (which we do by default) can be inefficient when
+        # the current fd ulimits are large, because it blindly closes
+        # all fds in the range(1, $verylargenumber)
+
         # 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):
+            # Close any fd beyond sensible_fd_limit prior adjusting our
+            # rlimit to ensure all fds are closed
+            for fd_entry in os.listdir('/proc/self/fd'):
+                # NOTE(dmllr): In a previous patch revision non-numeric
+                # dir entries were silently ignored which reviewers
+                # didn't like. Readd exception handling when it occurs.
+                fd = int(fd_entry)
+                if fd >= sensible_fd_limit:
+                    os.close(fd)
             # Unfortunately this inherits to our children, so allow them to
             # re-raise by passing through the hard limit unmodified
             resource.setrlimit(