Catching Exception during Silva export

[Benno Luthiger]

On our sites when trying a 'Silva Full-Media Export', we frequently experience a
failure (caused by an AttributeError). After analyzing the situation, I found
out that the problem was caused by changes in our custom metadata sets we
applied in the past.
Say we have two metadata sets 'my_container_set' and 'my_author_set'. During the
first period, the element 'force_https' was part of set 'my_container_set'.
After a while, however, we found out that this element needs to be member of the
set 'my_author_set'. Applying this change doesn't lead to any problems in the
productive site, i.e. the pages are displayed as expected.
However, such a change can lead to problems if such a site is exported using
'Silva Full-Media Export'. During the first period, objects are created and
stored to the ZODB making references to the metadata element 'force_https' in
the set 'my_container_set'. In the subsequent period ('force_https' member of
'my_author_set') when preparing the Silva export, a metadata item is retrieved
with the key 'force_https'. Then, the system tries to look up the metadata
field, calling Binding.getElement('my_container_set', 'force_https'). This
inevitably results in an AttributeError.

Therefore, my suggestion is to make this export more stable by catching the
AttributeError in Silva.silvaxml.xmlexport.metadata() (line 72ff) as follows:
---
            for key, value in items:
                try:
                    field = binding.getElement(set_id, key).field
                except AttributeError:
                    continue
                self.startElementNS(namespace, key)
                field.validator.serializeValue(field, value, self)
                self.endElementNS(namespace, key)
---

[Eric Casteleijn (thisfred)]

I'm sort of hesitant to introduce this sort of defensive coding, since it may
end up cloaking other more serious bugs. I'll have a look to see why the key is
looked up in the wrong metadata set in the first place.

If I understand correctly, the element was moved from set1 to set2, but on
export it is still looked up on set1, right?

[Flynt (flyntle)]

We (Benno and I) agree with eric's objection. So we can close this issue.