[ldns-users] adding FILE* support for Python3

[Karel Slany]

Hi Wouter,

many thanks for applying the previous patches. Here is another one. As
promised it contains a workaround to enable SWIG to generate FILE*
wrapper code for Python3. The Python2 wrapper code remains unaffected.

Here is a brief explanation why this patch does what it does:
 The I/O in Python2 relies on C standard library -- it uses FILE* and
the Python2/C API contains a straightforward solution how to access
these structures from C.
 Python3 uses the io module containing layers of buffers over the low
level I/O which the OS provides. The Python3/C file API was reduced --
no more functions for direct FILE* access. Also the io module C API does
not contain support for direct FILE* access.
 The Python3/C file API provides a way how to obtain a file descriptor.
But since the Python3 has their own buffers the documentation warns
against mixing access between Python3 and OS-level file descriptors.

What this patch does:
 In order to get a valid FILE* pointer from a Python object it wraps a
ldns C function which takes a FILE* argument with C code that:
* ensures itself whether the object is a file object
* flushes the python buffers using a python method call
* gets the file descriptor and flags and creates a copy of the file
descriptor
* opens a FILE* for the descriptor copy
* passes the opened FILE* to the wrapped function
* after the execution of the function flushes the FILE* and closes it

The buffer flushes in Python and C ensure the correct order of written
data but also may slow-down the code. I have not tested the performance
impacts very thoroughly. The goal was to make FILE* work in Python3. I
also do not know what the code will be behaving like when executed in a
multi-threaded application.

In order to get rid of the sometimes unnecessary buffer flushes the
Python3 wrapper can be extended with function opening a file for ldns
only. Such an approach, however, introduces an inconvenience of
closing/reopening/closing the file when accessing the files from Python
(but giving the user control over unnecessary buffer flushing).

The patch contains new files to the SVN repository so please do not
forget to make the Python3 example scripts executable and to 'svn add'
all the new files:

chmod +x trunk/contrib/python/examples/python3/ldns-*.py
svn add trunk/contrib/python/file_py3.i
svn add trunk/contrib/python/examples/python3

I have tested the new code under the GNU/Linux userland using Python
3.1. I intent to test it also under FreeBSD -- don't know whether it
would compile under Windows.

Best regards,
K.
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