Findings so far:
NOTE: An "empty" multi polygon is one that doesn't have all of it's elements. A "full" one has all of the nodes and ways

• Converting an "empty" multi polygon:
  • Without any tags streams the input to the output without calling the translation script. Therefore, no error tag
  • Converting to shapefile also doesn't call the translation script. It creates the shapefile directory and no files
• Converting a "full" multi polygon:
  • Without any tags streams the input to the output without calling the translation script. Therefore, no error tag
  • With a "good" tag it translates correctly
  • With a "bad" tag it translates and sends back the error tag

Since the translator isn't run when there are no tags to translate - I.e. only having the "type" tag - I think we could modify the core to add an error tag if the translator isn't run on a feature. I don't know what this would impact.

I tried using an osm file with two multi polygons, one with a tag (building=yes) and one without. Converting them resulted in:

• Exporting to OGR (shape etc) basically drops the feature that doesn't have any tags.
  • Two features in, One feature out
• Converting to XML just passes the feature without tags through
  • Two features in, two features out.

Trying the same file but changing building=yes to be papa=smurf was interesting.

• The shapefile had a feature in the O2S_A layer - which is expected - and the other feature was dropped
• The XML had no tags at all! It dropped the invalid tag even though the debug output said that it translated it to o2s_a

Getting curious, I created another OSM file that has:

1. two ways with valid tags and a relation with valid tags joining them
2. two ways, one with valid tags, one with invalid tags and a relation with valid tags joining them
3. two ways without tags and a relation with valid tags joining them
4. two ways without tags and a relation with invalid tags joining them
5. two ways without tags and a relation without tags joining them
6. two ways with valid tags and a relation with invalid tags joining them

Results:

1. Both roads exported with the tags from the relation
2. Both roads exported with the tags from the relation. The road with the invalid tags is also in the o2s_l layer
3. Both roads exported with the tags from the relation
4. Did not export - I expected it to go to o2s_l but with no tags the translator didn't run
5. Did not export - as above
6. Both roads exported with their tags.

Looking at this, it seems that tags on a relation stomp on tags on members.