Experimental IMAP branch

[Eric Gillespie]

Hi all,

Here's what I've been up to.  Quoting from docs/imap/DESIGN:

The world is now mobile-first, and many of us are mobile-only, or nearly so.
When my smart phone took over my life, two things happened to my nmh usage:

- At work, I was frustrated constantly by messages read under nmh being unread
  on my phone and vice versa.
- In my personal life, that same frustration drove me away from nmh completely.
  Whenever I need to reply to email from my computer, I find my self
  incorporating months of messages.

We can use IMAP to make nmh a first-class citizen in the modern mail world.

* Goals

- Treat the IMAP server as the source of truth.
- Minimize startup penalty to nmh commands.
- Minimize the impact to the nmh code-base.
- Minimize round-trips to the server.
- Enhance =inc= to incorporate mail from a hierarchy of IMAP mailboxes.
- Enhance =inc= to delete mail locally that has been deleted on the server.
- Enhance all commands interpreting the user's =Unseen-Sequence= (i.e. all those
  taking message specifications (=mh-sequence(5)=), e.g. =mark=, =pick=, =scan=,
  =show=) to interpret references to the =Unseen-Sequence= as all messages not
  flagged as =\Seen= on the server.
- Enhance =scan= output to reflect live =\Seen= status when it is asked to
  display whether messages appear in the user's =Unseen-Sequence=.
- Enhance =show= to set the =\Seen= flag on messages shown.
- Enhance =mark= to set or clear the =\Seen= flag for messages removed from or
  added to the user's =Unseen-Sequence=, respectively.
- Enhance =rmm= to delete messages from the server.
- Enhance =refile= to move messages on the server.
- Enhance =comp= and =repl= to allow usage of a server-side draft folder.

** Language

I've programmed in C and C++ for most of my career and I've just about had it
with unsafe languages.  The thought of implementing IMAP synchronization in C
discouraged me from even starting for years.

In my last position at Google, my fellow tech lead and I would often joke
about dividing the team in two, with one team continuing to chase down a
memory corruption error we'd failed for months to find, and the other
rewriting the system in Rust.  It was a joke, but the idea stuck with me.

Since Rust allows transparent calls into C, we can seamlessly embed Rust into
nmh programs.  Why not give it a shot?

I know it wasn't all that long ago I proposed using Perl from the
test suite and that was rejected for the perfectly valid reason
that we want the test suite to keep working without Perl.

So how can I show up and propose Rust, which is not even
supported on many of our supported platforms?

Well, unlike Perl in the test suite, this is fully optional and
fully self-contained.  Users without access to Rust can use use
mh just the same way they could for the last 4 decades.

I am already using this as my daily driver, but a lot of work
remains before I'd be comfortable proposing merging this to
master, and taking the "experimental" tag off it is even
farther out.

I have plenty of things to do here in the short term (testing and
handling so many edge and error cases, cleaning up the big mess
I've made in sbr are top of the list), but if others have any
ideas or suggestions, I'd be happy to hear them.  I'll be working
on the branch in public now; no more big code drops.

Also, if anyone wants to try it out but has trouble getting it
working, let me know.  It's quite rough around the edges now.
Aside from daily usage on FreeBSD, I periodically test it on
Oracle Linux.

Here's what I have in .mh_profile:

Managed-Folders: g gmail
#: epg@pretzelnet.org at Gandi
g-manager: /home/epg/work/nmh/.o/clang13/target/debug/imap-folder-manager
g-db: /home/epg/Mail/.epggandi.db
g-socket: /home/epg/Mail/.epggandi.db/socket
g-log-trace-to: /home/epg/Mail/.epggandi.db/log
g-log-level: debug
g-max-fetch-messages-in-flight: 100
g-host: mail.gandi.net
g-port: imaps
g-tls: implicit
g-auth: SASL
g-user: epg@pretzelnet.org
#: GMail IMAP
gmail-manager: /home/epg/work/nmh/.o/clang13/target/debug/imap-folder-manager
gmail-db: /home/epg/Mail/.gmail.db
gmail-socket: /home/epg/Mail/.gmail.db/socket
gmail-log-trace-to: /home/epg/Mail/.gmail.db/log
gmail-log-level: debug
gmail-max-fetch-messages-in-flight: 100
gmail-host: imap.gmail.com
gmail-port: imaps
gmail-tls: implicit
gmail-auth: SASL
gmail-saslmech: xoauth2
gmail-user: eric.gillespie@gmail.com
gmail-authservice: gmail
gmail-mailbox-root: [Gmail]/
gmail-mailbox-exclude: .
gmail-mailbox-include: All Mail

Thanks, and happy hacking!