Linux has an additional function to setrlimit and getrlimit which supports to query and/or modify resource limits of another process.

http://www.kernel.org/doc/man-pages/online/pages/man2/getrlimit.2.html

The patch implements resource.prlimit(pid, resource[, limits]) on all platforms that have a prlimit() function. I haven't included the regenerated configure script. You have to run autoreconf in order to test the new function.

prlimit() needs glibc 2.13+ (thanks Arfrever).

Updated patch, now raises PermissionError on EPERM.

Does anybody want to review the code before I commit it?

From my original post which unfortunately was ignored :( :  
http://mail.python.org/pipermail/python-ideas/2012-June/015323.html
...for completeness perhaps it also makes sense to expose Linux-specific RLIMIT constants:

RLIMIT_MSGQUEUE
RLIMIT_NICE
RLIMIT_RTPRIO
RLIMIT_RTTIME
RLIMIT_SIGPENDING

Fedora 18, this is the result after applying your patch and execute your unit test:

[sky@localhost cpython]$ ./python Lib/test/test_resource.py 
test_args (__main__.ResourceTest) ... ok
test_fsize_enforced (__main__.ResourceTest) ... ok
test_fsize_ismax (__main__.ResourceTest) ... ok
test_fsize_toobig (__main__.ResourceTest) ... ok
test_getrusage (__main__.ResourceTest) ... ok
test_prlimit (__main__.ResourceTest) ... ERROR
test_setrusage_refcount (__main__.ResourceTest) ... ok

======================================================================
ERROR: test_prlimit (__main__.ResourceTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 131, in test_prlimit
    1, resource.RLIMIT_AS)
  File "/home/sky/Code/python/programming_language/cpython/Lib/unittest/case.py", line 614, in assertRaises
    return context.handle('assertRaises', callableObj, args, kwargs)
  File "/home/sky/Code/python/programming_language/cpython/Lib/unittest/case.py", line 150, in handle
    callable_obj(*args, **kwargs)
PermissionError: [Errno 1] Operation not permitted

----------------------------------------------------------------------
Ran 7 tests in 0.990s

FAILED (errors=1)
Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 143, in <module>
    test_main()
  File "Lib/test/test_resource.py", line 140, in test_main
    support.run_unittest(ResourceTest)
  File "/home/sky/Code/python/programming_language/cpython/Lib/test/support.py", line 1615, in run_unittest
    _run_suite(suite)
  File "/home/sky/Code/python/programming_language/cpython/Lib/test/support.py", line 1590, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 131, in test_prlimit
    1, resource.RLIMIT_AS)
  File "/home/sky/Code/python/programming_language/cpython/Lib/unittest/case.py", line 614, in assertRaises
    return context.handle('assertRaises', callableObj, args, kwargs)
  File "/home/sky/Code/python/programming_language/cpython/Lib/unittest/case.py", line 150, in handle
    callable_obj(*args, **kwargs)
PermissionError: [Errno 1] Operation not permitted

With sudo, I got this:

[sky@localhost cpython]$ sudo ./python Lib/test/test_resource.py 
[sudo] password for sky: 
test_args (__main__.ResourceTest) ... ok
test_fsize_enforced (__main__.ResourceTest) ... ok
test_fsize_ismax (__main__.ResourceTest) ... ok
test_fsize_toobig (__main__.ResourceTest) ... ok
test_getrusage (__main__.ResourceTest) ... ok
test_prlimit (__main__.ResourceTest) ... FAIL
test_setrusage_refcount (__main__.ResourceTest) ... ok

======================================================================
FAIL: test_prlimit (__main__.ResourceTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 131, in test_prlimit
    1, resource.RLIMIT_AS)
AssertionError: ValueError not raised by prlimit

----------------------------------------------------------------------
Ran 7 tests in 1.002s

FAILED (failures=1)
Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 143, in <module>
    test_main()
  File "Lib/test/test_resource.py", line 140, in test_main
    support.run_unittest(ResourceTest)
  File "/home/sky/Code/python/programming_language/cpython/Lib/test/support.py", line 1615, in run_unittest
    _run_suite(suite)
  File "/home/sky/Code/python/programming_language/cpython/Lib/test/support.py", line 1590, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_resource.py", line 131, in test_prlimit
    1, resource.RLIMIT_AS)
AssertionError: ValueError not raised by prlimit

Thanks for your tests. I may have to rework my test scenario a bit.

New changeset 796c21e27a92 by Christian Heimes in branch 'default':
Issue #16595: Add prlimit() to resource module
http://hg.python.org/cpython/rev/796c21e27a92

Let's see how the buildbots respond. I may have to disable some tests for ancient Kernel versions.

+        self.assertRaises(PermissionError, resource.prlimit,
+                          1, resource.RLIMIT_AS)

Please skip this when run as root.

+        self.assertEqual(resource.prlimit(0, resource.RLIMIT_AS), (-1, -1))

What if it's not that value by default? Please fix the test to make it
more robust.

Roundup bot hangs. I have addressed your concerns in http://hg.python.org/cpython/rev/8c77117f41a9

The test is failing with ENOSYS on one of the buildbots:
"""
======================================================================
ERROR: test_prlimit (test.test_resource.ResourceTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/test/test_resource.py", line 148, in test_prlimit
    1, resource.RLIMIT_AS)
  File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/unittest/case.py", line 689, in assertRaises
    return context.handle('assertRaises', callableObj, args, kwargs)
  File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/unittest/case.py", line 158, in handle
    callable_obj(*args, **kwargs)
OSError: [Errno 38] Function not implemented
"""

I wonder if we could add a @support.ignore_enosys decorator...

New changeset 87d41a5a9077 by Christian Heimes in branch 'default':
Issue #16595: prlimit() needs Linux kernel 2.6.36+
http://hg.python.org/cpython/rev/87d41a5a9077

The buildbot is Linux-2.6.35-vs2.3.0.36.32-gentoo-i686-Intel-R-_Core-TM-2_CPU_6600_@_2.40GHz-with-gentoo-2.1 but prlimit() requires 2.6.36+. I didn't expect to see a combination of glibc with prlimit() and Kernel without prlimit(). According to man prlimit it's not suppose to fail with ENOSYS, too.

Or we should extend with supress(OSerror, errno=errno.ENOSYS): ... :-)

(Just kidding, ignored tests must be marked as skipped.)

> I didn't expect to see a combination of glibc with prlimit() and Kernel without prlimit(). According to man prlimit it's not suppose to fail with ENOSYS, too.

Yeah, we've seen this several times on some buildbots.
Note that actually, any syscall can fail with ENOSYS: the configure
check just checks that the libc exposes the wrapper library, but when
the libc wrapper makes the actual syscall, the kernel can return
-ENOSYS if it doesn't implement the syscall.

Tests are passing on all buildbots for quite some time now.

Just received this report on psutil bug tracker:
https://code.google.com/p/psutil/issues/detail?id=455
It seems os.prlimit() is affected by the same problem:

>>> import resource
>>> resource.RLIM_INFINITY
-1
>>>

s/os.prlimit/resource.prlimit

How is that a problem? In any case, this shouldn't have anything to do with prlimit(), please open another issue.