Re: RFC: Make iterator printers fail more gracefully

[David Malcolm <dmalcolm at redhat dot com>]

On Thu, 2016-12-15 at 17:39 +0000, Jonathan Wakely wrote:
> This patch tries to improve the user experience when debugging
> container iterators, for cases where some of the typedefs used by the
> printers are not in the debuginfo, so gdb.lookup_type() calls fail.
> That happens if the iterator's operator*() and operator->() haven't
> been instantiated, or if they've been inlined.
> 
> Currently this results in an exception:
> 
> $1 = Python Exception <class 'ValueError'> Cannot find type
> std::_List_iterator<int>::_Node: 
> 
> If the iterator being printed is part of some other object the whole
> thing fails due to the exception.
> 
> With this patch the iterator instead prints:
> 
> $1 = <insufficient debuginfo for std::list iterator>
> 
> and if it's a subobject the rest of the object is printed, with that
> as the value of the iterator.
> 
> 	* python/libstdcxx/v6/printers.py
> (StdListIteratorPrinter.to_string):
> 	Handle exception from failed type lookup and return user
> -friendly
> 	string.
> 	(StdRbtreeIteratorPrinter.__init__): Handle exception from
> failed
> 	type lookup.
> 	(StdRbtreeIteratorPrinter.to_string): Return user-friendly
> string.
> 
> Seem reasonable?
> 
> I consider this a stop-gap until we have Xmethods for all our
> iterator
> types, then we'll be able to "print *iter" even without debuginfo for
> all the iterator's members, and we can disable these printers.

BTW, is it always a ValueError exception?

(I'm a little wary of naked "except:" in Python, as it can catch
*anything*, including syntax errors in the try/except-guarded code).