[issue5630] Update CObject API so it is safe and regular

[Larry Hastings]

New submission from Larry Hastings <larry at hastings.org>:

The CObject API has two flaws.

First, there is no usable type safety mechanism.  You can store a void
*object, and a void *description.  There is no established schema for
the description; it could be an integer cast to a pointer, or it could
point to memory of any configuration, or it could be NULL.  Thus users
of the CObject API generally ignore it--thus working without any type
safety whatsoever.  A programmer could crash the interpreter from pure
Python by mixing and matching CObjects from different modules (e.g. give
"curses" a CObject from "_ctypes").

Second, the destructor callback is defined as taking *either* one *or*
two parameters, depending on whether the "descr" pointer is non-NULL. 
One can debate the finer points of what is and isn't defined behavior in
C, but at its heart this is a sloppy API.

MvL and I discussed this last night and decided to float a revision of
the API.  I wrote the patch, though, so don't blame Martin if you don't
like my specific approach.

The color of this particular bike shed is:
* The PyCObject is now a private data structure; you must use accessors.
 I added accessors for all the members.
* The constructors and the main accessor (PyCObject_AsVoidPtr) now all
*require* a "const char *type" parameter, which must be a non-NULL C
string of non-zero length.  If you call that accessor and the "type" is
invalid *or doesn't match,* it fails.
* The destructor now takes the PyObject *, not the PyCObject *.  You
must use accessors to get your hands on the data inside to free it.

Yes, you can easily skip around the "matching type" restriction by
calling PyCObject_AsVoidPtr(cobj, PyCObject_GetType(cobj)).  The point
of my API changes is to *encourage* correct use.


The attached patch was written py3k/trunk r70718.  It compiles with no
new warnings/errors and doesn't seem to cause any new failures in the
regression test.

Note: this patch is not complete; I fixed all the .c and .h files, but I
have yet to update the documentation.  I figure I don't want to put the
effort in until the dust settles.

----------
components: Interpreter Core
files: cobject.diff
keywords: patch
messages: 84864
nosy: lhastings
severity: normal
status: open
title: Update CObject API so it is safe and regular
versions: Python 3.1
Added file: http://bugs.python.org/file13521/cobject.diff

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