feature request: caching message arrival time

[Daniel Kahn Gillmor]

Hi Notmuch folks--

I'm working on Autocrypt integration for notmuch right now, and it
occurs to me that it might be useful to know the time that any given
message was first seen by notmuch.

I'm trying to not get distracted by implementing such a feature, but I
wanted to log this as a feature request, along with a few thoughts about
it.

My idea is that the first time notmuch indexes a message, it would add a
property to the message like firstseen=2019-05-31T23:15:24Z.

Some nuances spring to mind:

 * This should *not* be cleared and reset on reindexing, so it doesn't
   belong in the index.* property namespace.

 * What happens when you delete a message?  Maybe we should keep that
   value around for "ghosts" too -- can ghost documents have properties?
   Or is it bad to remember that we've seen the message if someone
   deletes it?

 * When even the ghost goes away (e.g. full thread deletion), presumably
   this property would go away.  So If you deleted the message from your
   message store, notmuch would forget about it, and then the next time
   you ingest it it would get a later "firstseen=" property.  I'm ok
   with this.

 * i don't think we have a way to search properties by range (e.g. the
   way that we can search date ranges).  i don't need that feature for
   my use case, but maybe someone will come up with a use case that
   wants it?  is there a way to store the datestamp in a way that it can
   be scanned the way that "date" can?  or do we already have this and
   i'm just unaware?

 * What is the cost in terms of database size?  It doesn't look like it
   would be expensive to me, but i haven't profiled it.

 * if we make such a change, how should we deal with already-indexed
   messages?

Anyone have any thoughts, suggestions, or objections to this?  I'm happy
to explain more about my use case if people are interested too.

       --dkg
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