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The attached script hits some "This should never happen" code in the subprocess module.
These lines here:
https://github.com/python/cpython/blob/2.7/Lib/subprocess.py#L1036-L1038
The root cause is a lack of checking WIFSTOPPED and WSTOPSIG in the handler.
When a process elects into being ptraced via PTRACE_TRACEME, it is stopped on the SIGSEGV instead of terminating, allowing the user to attach a debugger before the kernel destroys the process.
This bug makes it impossible to wait on any process which crashes, which is set up to wait for a debugger.

To further clarify the report:
When the attached proof-of-concept is executed, a RuntimeException is raised, which has a comment "Should never happen".
The issue isn't due to SIGCHLD, but rather following a waitpid() call. The code attempts to suss the exit code / reason for waitpid() returning, but does not check for WIFSTOPPED in its handler.

The attached patch should fix it.
I want to incorporate a bug.py like regression test into test_subprocess.py.

test added.

New changeset 269296b2a047 by Gregory P. Smith in branch '3.5':
Issue #29335: Fix subprocess.Popen.wait() when the child process has
https://hg.python.org/cpython/rev/269296b2a047
New changeset ed5255a61648 by Gregory P. Smith in branch '3.6':
Issue #29335: Fix subprocess.Popen.wait() when the child process has
https://hg.python.org/cpython/rev/ed5255a61648
New changeset 4f5e7d018195 by Gregory P. Smith in branch 'default':
Issue #29335: Fix subprocess.Popen.wait() when the child process has
https://hg.python.org/cpython/rev/4f5e7d018195 

Among other buildbot failures:
http://buildbot.python.org/all/builders/x86%20Tiger%203.6/builds/142/steps/test/logs/stdio
======================================================================
ERROR: test_child_terminated_in_stopped_state (test.test_subprocess.POSIXProcessTestCase)
Test wait() behavior when waitpid returns WIFSTOPPED; issue29335.
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/db3l/buildarea/3.6.bolen-tiger/build/Lib/test/test_subprocess.py", line 2514, in test_child_terminated_in_stopped_state
 libc = ctypes.CDLL(libc_name)
 File "/Users/db3l/buildarea/3.6.bolen-tiger/build/Lib/ctypes/__init__.py", line 348, in __init__
 self._handle = _dlopen(self._name, mode)
OSError: dlopen(libc..dylib, 6): image not found
----------------------------------------------------------------------
Ran 260 tests in 102.297s
Also, http://buildbot.python.org/all/builders/x86%20Ubuntu%20Shared%203.x/builds/240/steps/test/logs/stdio 

New changeset 8e3d412f8e89 by Gregory P. Smith in branch '2.7':
Issue #29335: Fix subprocess.Popen.wait() when the child process has
https://hg.python.org/cpython/rev/8e3d412f8e89 

thanks Ned, I was awaiting interesting buildbot results. :)
fixed in 2.7 and 3.5 onwards. thanks for the report Zach.
not closing until I also apply the fix to the subprocess32 backport.

Of note, there's no need to actually cause a SIGSEGV to generate the signal.
The tests might be more clear to replace:
 libc.printf(ctypes.c_char_p(0xdeadbeef))
with
 os.kill(os.getpid(), signal.SIGSEGV)

If you want crashes, look at the portable faulthandler._sigsegv() :-)

Neat, though that's not in the standard library.
The current logic for getting a handle to libc could also be simplified via ctypes.util.find_library (https://docs.python.org/3/library/ctypes.html#finding-shared-libraries).
Darwin:
 >>> import ctypes.util
 >>> ctypes.util.find_library('c')
 '/usr/lib/libc.dylib'
 
Linux:
 >>> import ctypes.util
 >>> ctypes.util.find_library('c')
 'libc.so.6'

New changeset 56bc3b768c3cc3817031b56d5e7a279aa1296bc9 by Gregory P. Smith in branch 'master':
bpo-29335 - apply suggested test_subprocess simplifications from haypo and Zach: (#1757)
https://github.com/python/cpython/commit/56bc3b768c3cc3817031b56d5e7a279aa1296bc9

Fixed applied to subprocess32 in https://github.com/google/python-subprocess32/commit/0f1958e982bf44db569470def7281bcafa2a8b0e 

New changeset cdee3f14f7f4c995e7eedb0bf6a67e260c739f7d by Victor Stinner in branch 'master':
bpo-30764: test_subprocess uses SuppressCrashReport (#2405)
https://github.com/python/cpython/commit/cdee3f14f7f4c995e7eedb0bf6a67e260c739f7d

New changeset 849b062a82ca2f09e33259d34067faba196c9e23 by Victor Stinner in branch '3.5':
bpo-30764: test_subprocess uses SuppressCrashReport (#2405) (#2411)
https://github.com/python/cpython/commit/849b062a82ca2f09e33259d34067faba196c9e23

New changeset 9ad50d94599aed0c37beaf78948ec271c8aa3881 by Victor Stinner in branch '3.6':
bpo-30764: test_subprocess uses SuppressCrashReport (#2405) (#2410)
https://github.com/python/cpython/commit/9ad50d94599aed0c37beaf78948ec271c8aa3881

New changeset 2097b9e0ef32ab7a0d745edc0f707c615780c006 by Victor Stinner in branch '2.7':
[2.7] bpo-30764: test_subprocess uses SuppressCrashReport (#2405) (#2412)
https://github.com/python/cpython/commit/2097b9e0ef32ab7a0d745edc0f707c615780c006