I can't see what this code is supposed to accomplish (see patch):

  while (collend < end) {
      if ((0 < *collend && *collend < 256) ||
          !Py_UNICODE_ISSPACE(*collend) ||
          Py_UNICODE_TODECIMAL(*collend))
          break;
  }


Since 'collend' and 'end' don't change in the loop, it would be
infinite if the 'if' condition evaluated to false. But the 'if'
condition is always true.

The code is also present in Python 2.7 and 3.2.

PyUnicode_EncodeDecimal() was used in Python 2 by int, long and complex constructors. In Python 3, the function is no more used: it has been replaced by _PyUnicode_TransformDecimalAndSpaceToASCII().

Looking at it again, the intention was probably to increment
collend so that it points to the first non-garbage character
(or '\0'). If that's the case, the loop should be something
like this:

  while (collend < end) {
      if ((0 < *collend && *collend < 256) ||
          Py_UNICODE_ISSPACE(*collend) ||
          Py_UNICODE_TODECIMAL(*collend) >= 0)
          break;
      collend++;
  }


I can understand if no one wants to touch this given that
the function is deprecated, provided that you agree that
the existing code is redundant but harmless; i.e. it cannot
enter an infinite loop.

AFAICT, this code was introduced in 0337dad8403e, implementing PEP 293 (see #432401). The intention clearly was what Stefan figured out: compute the list of unencodable characters, to pass the longest run of unencodable characters to the error handler.

Fixing this in this respect is fine with me for all branches.

New changeset f29d7d597fae by Victor Stinner in branch '3.2':
Issue #13093: Fix error handling on PyUnicode_EncodeDecimal()
http://hg.python.org/cpython/rev/f29d7d597fae

New changeset bc53c11804ab by Victor Stinner in branch 'default':
(Merge 3.2) Issue #13093: Fix error handling on PyUnicode_EncodeDecimal()
http://hg.python.org/cpython/rev/bc53c11804ab

New changeset 0cd197f13400 by Victor Stinner in branch '2.7':
Issue #13093: Fix error handling on PyUnicode_EncodeDecimal()
http://hg.python.org/cpython/rev/0cd197f13400

I fixed the issue in Python 2.7, 3.2 and 3.3.

See also the changeset 849e9277906a (Python 3.3).

New changeset 52fecdc1c5d8 by Charles-François Natali in branch 'default':
Issue #13093: Perform a real merge.
http://hg.python.org/cpython/rev/52fecdc1c5d8

A 2.7 OS X buildbot segfaults in test_unicode since 0cd197f13400 :

http://www.python.org/dev/buildbot/all/builders/AMD64 Snow Leopard 2 2.7/builds/409/steps/test/logs/stdio

"""
test_unicode
make: *** [buildbottest] Segmentation fault
program finished with exit code 2
"""

> A 2.7 OS X buildbot segfaults in test_unicode since 0cd197f13400
> http://www.python.org/dev/buildbot/all/builders/AMD64 Snow Leopard 2 2.7/builds/409/steps/test/logs/stdio

Hum, I'm unable to reproduce the crash on Linux or Mac OS X Tiger.

I am able to reproduce the problem on 2.7 OS X 10.7 64-bit. unicode._encodedecimal is gobbling up memory.  Looks like length is incorrect.

Breakpoint 1, unicode_encodedecimal (self=0x0, args=0x1007ce0d0) at /Users/nad/Projects/PyDev/active/temp/u27-clang/Modules/_testcapimodule.c:1122
1122	    decimal = PyBytes_FromStringAndSize(NULL, decimal_length);
(gdb) p length
$1 = 4294967299
Current language:  auto; currently minimal
(gdb) p unicode
$2 = (Py_UNICODE *) 0x101827ab8
(gdb) p errors
$3 = 0x0

New changeset 3ecddf168f1f by Victor Stinner in branch '2.7':
Issue #13093: Fix _testcapi.unicode_encodedecimal()
http://hg.python.org/cpython/rev/3ecddf168f1f

> Looks like length is incorrect.

Oh ok, _testcapimodule.c is "sssize_t" safe in Python 3, not in Python 2. Can you please try with the last tip?

That fixes it.