http://www.python.org/dev/buildbot/all/builders/AMD64%20debian%20bigmem%203.x/builds/58/steps/test/logs/stdio

I tried to reproduce this crash on my desktop system.
AMD64, 8 GB RAM (only) and on Debian unstable from today.
Testing the exact same Python version (hg update d2cf8a34ddf90fb1bc8938de0f736694e61f73fa) the test passes just fine here...

I've also been unable to reproduce it on my own machine (AMD64; 8GB RAM).

I guess I'll have to do some trial-and-error debugging using the custom
builder to figure this out.

The same test found a bug in Mac OS X kernel: issue #11277.

I'm unable to reproduce the crash on Fedora 16 (with 12 GB of RAM). It may depend on zlib version or the kernel version. I'm running Linux 3.2.6-3.fc16.x86_64 with zlib 1.2.5-6.fc16.

A recent crash:

[241/364/1] test_zlib
Fatal Python error: Bus error

Current thread 0x00002b8f2240d260:
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/test_zlib.py", line 96 in test_big_buffer
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/case.py", line 385 in _executeTestPart
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/case.py", line 440 in run
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/case.py", line 492 in __call__
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/suite.py", line 105 in run
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/suite.py", line 67 in __call__
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/suite.py", line 105 in run
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/suite.py", line 67 in __call__
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/unittest/runner.py", line 168 in run
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/support.py", line 1369 in _run_suite
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/support.py", line 1403 in run_unittest
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/test_zlib.py", line 666 in test_main
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/regrtest.py", line 1221 in runtest_inner
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/regrtest.py", line 907 in runtest
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/regrtest.py", line 710 in main
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/__main__.py", line 13 in <module>
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/runpy.py", line 73 in _run_code
  File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/runpy.py", line 160 in _run_module_as_main
make: *** [buildbottest] Bus error

http://www.python.org/dev/buildbot/all/builders/AMD64%20debian%20bigmem%203.x/builds/136/steps/test/logs/stdio

"""
File "/var/tmpfs/martin.vonloewis/3.x.loewis-parallel2/build/Lib/test/test_zlib.py", line 96 in test_big_buffer
"""

The SIGBUS could be due to the buildbot running out of tmpfs.

New changeset f3f3bb45205b by Nadeem Vawda in branch 'default':
Issue #13873: Fix crash in test_zlib on bigmem buildbot.
http://hg.python.org/cpython/rev/f3f3bb45205b

> The SIGBUS could be due to the buildbot running out of tmpfs.

I haven't been able to reproduce the crash by running the test on a tmpfs
on my own machine (Ubuntu AMD64; 8GB RAM; Linux 3.0.0-15-generic; zlib
1:1.2.3.4.dfsg-3ubuntu3), but maybe it's due to something specific about
the configuration of the buildbot machine?

I've temporarily changed the test to use a regular chunk of memory
instead of the mmap hack). If possible, I'd like to change back to the
old technique in the long run (since it allows the test to run on
machines with less RAM), but until we can figure out the problem, I'd
rather not have the test failing needlessly.

> but maybe it's due to something specific about  the configuration of the buildbot
> machine?

Maybe you didn't try with a large enough file. Here's a trial on my box:
"""
$ df -h /tmp/
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           253M   68K  253M   1% /tmp
$ cat /tmp/test.py
import mmap
import zlib

f = open('/tmp/foo', 'wb+')
f.seek(512 * 1024 * 1024)
f.write(b'x')
f.flush()
m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
zlib.crc32(m)
m.close()
$ python /tmp/test.py
Bus error (core dumped)
"""

> I've temporarily changed the test to use a regular chunk of memory
>instead of the mmap hack). If possible, I'd like to change back to the
>old technique in the long run (since it allows the test to run on
>machines with less RAM)

Yes, but this kind of test is only supposed to be run on machines
which have enough memory.
Also, if the filesystem doesn't support sparse files, this writes a
lot to the disk (and if it crashes, you end up with a huge file).
I'd say it's fine this way...

Well, it turns out that when I tested it on my own machine, I actually
wasn't using a tmpfs - I misread the output of df and used /tmp¹ instead
of /run. Doing the test in /run does in fact give a bus error. Mea culpa.

¹ Apparently on my system /tmp isn't a tmpfs. Go figure.


> Also, if the filesystem doesn't support sparse files, this writes a
> lot to the disk (and if it crashes, you end up with a huge file).

You may be right; I hadn't thought about that possibility. My concern was
that the test suite isn't run with -M very often, so these sorts of tests
could often be broken for a while before someone found out. In the past,
none of the buildbots ran bigmem tests, so there was a real danger that
we wouldn't notice breakages. However, with the addition of the debian
bigmem buildbot, that is no longer the case, so this isn't such an issue
any more.

I'm okay with leaving the tests as they are in 3.3. Any objections?
If not, I'll also backport the change to 3.2 and 2.7.

> I'm okay with leaving the tests as they are in 3.3. Any objections?

Nope, it's fine.

New changeset fc43b051ae1c by Nadeem Vawda in branch '3.2':
Issue #13873: Fix crash in test_zlib when running on a small (<4GB) tmpfs.
http://hg.python.org/cpython/rev/fc43b051ae1c