TAP 1.0/1.1 inconsistency: error document format

[Mark Taylor]

Hi DAL.

Following a taplint query by Stelios, I've just noticed the following
inconsistency between TAP 1.0 and TAP 1.1 as regards the format of
UWS error documents resulting from failed async TAP queries:

   TAP 1.0 Sec 2.7.3:
      "Error documents for TAP errors must be VOTable documents;
       any result-format specified in the request is ignored."

   TAP 1.1 Sec 3.3:
      "Error documents should be in a format that matches the requested
      format where possible; see DALI for details."

   DALI 1.1 Sec 4.2:
      "An error document describing errors in use of the DAL service
       protocol may be a VOTable document (...) or a plain text document."

For an expected VOTable result, this is effectively a downgrade of a
MUST to a SHOULD for VOTable as the format of the error document.

So a TAP 1.0 client will expect a VOTable error document from a failed
async TAP query, but may get tripped up by receiving instead a plain 
text error document.  This change is not explicitly noted in
Appendix A of TAP 1.1 "Changes from TAP-1.0 to TAP-1.1",
though A.1 says, somewhat misleadingly, "Removed text that duplicates
material from DALI.".

This looks to me like an oversight since it is (from the client's 
point of view) a backwardly incompatible behaviour change from TAP 1.0
to TAP 1.1.

Has anybody come across this before, or remember whether it was
deliberate?

Mark

--
Mark Taylor  Astronomical Programmer  Physics, Bristol University, UK
m.b.taylor at bristol.ac.uk          https://www.star.bristol.ac.uk/mbt/