When xmlrpclib fails to serialize objects into XML a generic "failed to
serialize output" error is returned to the client and no log messages
are produced to give insight into the true cause of the problem.  In
real-world scenarios where lots of data moves along the wire it can be
difficult to determine the cause of the serialization problem.

I propose the attached patch as an initial cut at improving xmlrpclib
under circumstances where serialization fails.

Christopher, are you willing to update your patch for the current trunk (2.7)?

Here is blunck2's patch updated against r77393. Once I have a better understanding of the issue I'll see if I can write tests (eg. I don't know the significance of 8002 on line 675, etc).

A few things about the patch:
1) the MarshallError class has a 'message' attribute that results in a warning on some Python versions, it should be renamed in something different (like 'msg');
2) I'm not sure why the <marshallerror></marshallerror> in the exception message is necessary, but I didn't look at the code in xmlrpclib.py so it might be ok;
3) the first 'pos' is not used in the for loop where it is created but only some lines later inside the except, this is not immediately clear, so a short comment should be added;
4) the str(e) is redundant -- there's already %s;
5) the "except Exception, e:" look suspicious, a more specific exception(s) should probably be specified;
6) 'as' should be used instead of the comma in the 'except' statements.

This patch should address all but #2. I'm not exactly sure of the use-case blunck2 had in mind so I'm not sure how the message would be used.

I was curious, so I googled.  You may have already found this as well: it's a 'fault code', and apparently there's no standard for such codes.  The choice of 8002 may be modeled on what twisted.web does with exceptions.  Staying consistent with them is not a bad idea if you decide this is a similar enough fault case, but whatever is done it will need to be documented.

On the other hand, I also happened on this document, but I have no idea what its status is in the xmlrpc community:

http://xmlrpc-epi.sourceforge.net/specs/rfc.fault_codes.php