component/python-2.7: Lib/subprocess: Speedup close_fds=True

@romain reports that Popen(close_fds=True) is slow on py2. Let's semantically
backport from py3 how to close only actually opened file descriptors instead of
whole 3..`ulimit -n` range.

Attached test benchmark shows the following results with `ulimit -n`=65K:

Before this patch:

    $ ./bin/python2.7 ~/x.py
    close_fds=False:
    0.001251 s/call
    0.001337 s/call
    0.001486 s/call

    close_fds=True:
    0.017973 s/call
    0.018152 s/call
    0.018204 s/call

After the patch:

    $ ./bin/python2.7 ~/x.py
    close_fds=False:
    0.001391 s/call
    0.001416 s/call
    0.001570 s/call

    close_fds=True:
    0.001469 s/call
    0.001479 s/call
    0.001491 s/call

i.e. ~12x speedup.

References on this subject are in the patch itself.

The test benchmark is below:

---- 8< ----
import timeit
from subprocess import check_call

def f():
    check_call(['true'], close_fds=False)
def g():
    check_call(['true'], close_fds=True)

N=3
n=100
print 'close_fds=False:'
for i in range(N):
    print '%.6f s/call' % (timeit.timeit(f, number=n) / n)
print
print 'close_fds=True:'
for i in range(N):
    print '%.6f s/call' % (timeit.timeit(g, number=n) / n)

/helped-by @jm

component/python-2.7/buildout.cfg

@@ -42,6 +42,7 @@ patches =
   ${:_profile_base_location_}/pytracemalloc_pep445.patch#9f3145817afa2b7fad801fde8447e396
   ${:_profile_base_location_}/disabled_module_list.patch#e038a8016475574c810cbaaf0e42f4ac
   ${:_profile_base_location_}/asyncore_poll_insteadof_select.patch#ab6991c0ee6e25aeb8951e71f280a2f1
+  ${:_profile_base_location_}/py27-subproc-closefds-fast.patch#e495e44491694a8972da11739206f2e6
 url =
   http://www.python.org/ftp/python/${:package_version}/Python-${:package_version}${:package_version_suffix}.tar.xz
 configure-options =

component/python-2.7/py27-subproc-closefds-fast.patch

+From 9a79fa64c6298998d04a5ae6d699222a93cd0cb8 Mon Sep 17 00:00:00 2001
+From: Kirill Smelkov <kirr@nexedi.com>
+Date: Fri, 24 Dec 2021 13:55:08 +0300
+Subject: [PATCH] [py27] subprocess: Use /proc/self/fd/ to know opened file
+ descriptors
+
+... to avoid O(MAXFD) slowdown on closefds=True.
+
+This is semantic backport of what Python3 does.
+---
+ Lib/subprocess.py | 23 ++++++++++++++++++++++-
+ 1 file changed, 22 insertions(+), 1 deletion(-)
+
+diff --git a/Lib/subprocess.py b/Lib/subprocess.py
+index 1f2da0ffbe8..300557220ac 100644
+--- a/Lib/subprocess.py
++++ b/Lib/subprocess.py
+@@ -869,7 +869,28 @@ def pipe_cloexec(self):
+ 
+ 
+         def _close_fds(self, but):
+-            if hasattr(os, 'closerange'):
++            # try to retrieve list of opened file descriptors to avoid being slow on large MAXFD
++            # https://www.erp5.com/project_section/vifib/forum/python2--subprocess.Popen-is-slow-when-close_fds-True-and-nofile-ulimit-is-high-PO01sREVXn
++            # https://bugs.python.org/issue8052
++            # https://github.com/python/cpython/commit/8facece99a59
++            fdv = None
++            try:
++                fdv = os.listdir('/proc/self/fd')
++            except OSError as e:
++                if e.errno != errno.ENOENT:
++                    raise
++            if fdv is not None:
++                for fd_ in fdv:
++                    fd = int(fd_)
++                    if (0 <= fd <= 2) or fd == but:
++                        continue
++                    try:
++                        os.close(fd)
++                    except Exception: # e.g. not KeyboardInterrupt nor SystemExit
++                        pass
++
++            # fallback to closing all file descriptors in 3..MAXFD range
++            elif hasattr(os, 'closerange'):
+                 os.closerange(3, but)
+                 os.closerange(but + 1, MAXFD)
+             else:
+-- 
+2.30.2
+