Jameson Graef Rollins <jrollins@finestructure.net> writes:
> Just a thought: what if messages with a given tag (e.g. "new-thread")
> were always treated as the source of a new thread?

It's a good start. And an approach like that would have the advantage
that one could undo a thread-split by just removing the tag. (That's not
an explicit thread-join feature, but I don't think anyone has ever asked
for that.)

> A message with the given tag could just be (re)indexed with any
> In-Reply-To/References headers stripped before indexing.

It would require a little more than that. Imagine this thread:

  A: Subject: An original thread
  └─B: Subject: Thread hijacking is fun (tag:new-thread)
    └─C: Subject: Re: Thread hijacking is fun

In this case, message C is likely to have a References header that
mentions both A and B. So the thread stitching logic in notmuch will
want to merge threads A and B when indexing C. So special care will have
to be taken here as well, (not just when indexing B).

And that special care may not be cheap if it requires additional
database lookups for each unique thread ID encountered among references
of a message.

Though, I don't mean to dissuade anyone from thinking this through and
coding it up. The relevant code for the pieces I'm referring to starts
in _notmuch_database_link_message in lib/database.cc.

-Carl

-- 
carl.d.worth@intel.com