require service_id,date uniqueness in calendar_dates.txt

[Tom Brown]

http://code.google.com/transit/spec/transit_feed_specification.html#calendar_dates_txt___Field_Definitions
does not prohibit multiple rows in calendar_dates.txt with the same
service_id and date. If two such rows have different values for
exception_type the meaning is ambiguous. I think we should modify the
spec to say that each service_id must have no more than one row for
each date. This change is not backwards compatible but the only time
we've seen it in data fetched at Google is in files with many other
problems.

It is possible that someone will propose an extension of GTFS needing
multiple rows with the same service_id and date. Such a proposal will
need to provide a way to avoid allowing ambiguous interpretations of
valid data.

Have you seen data with duplicate service_id,date pairs? Have I
overlooked any complications with tightening the specification?

If there are no issues we can put it in the spec and I'll fix
http://code.google.com/p/googletransitdatafeed/issues/detail?id=82

[Tom Brown]

Any comments on this proposal before I request it to be added to the official spec?

In the mean time Jiri already added a validation warning. In general

we should try to make sure feedvalidator only checks for things that

are officially in the spec, though I haven't been very strict with

warnings for issues that are causing real problems.