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I found this problem when adding "print" statements to multi-threaded
code. When applying the attached diff to a py3k installation, the output
on screen always contains some garbage.
The following code is an extract of fileio_write (in Modules/_fileio.c),
but the same behavior appears everywhere:
	if (!PyArg_ParseTuple(args, "s#", &ptr, &n))
		return NULL;
	Py_BEGIN_ALLOW_THREADS
	errno = 0;
	n = write(self->fd, ptr, n);
	Py_END_ALLOW_THREADS
io.BufferedWriter calls this function with a bytearray.
In this case, the GIL is released when holding a pointer to the
bytearray data.
But another thread may mutate the bytearray in between, the pointer
becomes stale and can lead to segfaults or funny results.