Should next_ds be set to execution_date for manually triggered runs?

[Dan Davydov <dd...@twitter.com.INVALID>]

next_ds is useful when you need cron-style scheduling, e.g. a task that
runs for date "X" uses that date for its logic, e.g. send an email to users
saying the run that was supposed to run for date "X" has completed. The
problem is it doesn't behave as expected when it comes to manually
triggered runs as illustrated by the diagrams below.

Using execution_date in a task
*Scheduled Run (works as expected)*
execution_date1           start_date1
\/                                  \/
 *|-----------------------------|*
/\                                  /\
 \_________________/
   scheduling_interval

*Manual Run** (works as expected)*
triggered_date + execution_date + start_date
\/
*|*

Using next_ds in a Task
*Scheduled Run (works as expected)*
next_ds1 + start_date1           next_ds2 + start_date2
\/                                                         \/
 *|------------------------------------------------|*
/\                                                         /\
 \____________________________/
             scheduling_interval

*Manual Run* *(next_ds1 is expected to match triggered_date as in the case
for the manually triggered run that uses the regular execution_date above)*
triggered_date                                    next_ds1 + start_date
\/                                                         \/
*|-------------------------------------------------|*
/\                                                         /\
 \____________________________/
             0 to scheduling_interval (depending on when the next execution
date is)

Proposal
Have next_ds always set to execution_date for manually triggered runs
instead of the next schedule-interval aligned execution date.

This *might* break backwards compatibility for some users but it can be
argued that the current functionality is a bug. If it's really desired we
can create new aliases that behave logically although I am against this.

prev_ds should probably also be made consistent with this logic.

Thoughts?
- Dan