TODO items for public-inbox

(Not in any particular order, and
performance, ease of setup, installation, maintainability, etc.
all need to be considered for everything we introduce.)

* general performance improvements, but without relying on
  XS or pre-built modules any more than we currently do.
  (Optional Inline::C and user-compiled re2c acceptable)

* mailmap support (same as git) for remapping expired email addresses

* support remapping of expired URLs similar to mailmap
  (coordinate with git.git with this?)

* HTTP, IMAP, NNTP, POP3 proxy support.  Allow us to be a frontend for
  firewalled off (or Tor-exclusive) instances.  The use case is
  for offering a publicly accessible IP with a cheap VPS,
  yet storing large amounts of data on computers without a
  public IP behind a home Internet connection.

* support HTTP(S) CONNECT proxying to IMAP/NNTP/POP3 for users with
  firewall problems

* DHT (distributed hash table) for mapping Message-IDs to various
  archive locations to avoid SPOF.

* optional Cache::FastMmap support so production deployments won't
  need Varnish (Varnish doesn't protect NNTP or IMAP, either)

* dogfood and take advantage of new kernel APIs (while maintaining
  portability to older Linux, free BSDs and maybe Hurd).

* dogfood latest Xapian, Perl5, SQLite, git and various modules to
  ensure things continue working as they should (or better)
  while retaining compatibility with old versions.

* Support more of RFC 3977 (NNTP)
  Is there anything left for read-only support?

* Configurable linkification for per-inbox shorthands:
  "$gmane/123456" could be configured to expand to the
  appropriate link pointing to the gmane.io list archives,
  likewise "[Bug #123456]" could be configured to expand to
  point to some project's bug tracker at http://example.com/bug/123456

* configurable synonym and spelling support in Xapian

* Support optional "HTTPS Everywhere" for mapping old HTTP to HTTPS
  links if (and only if) the user wants to use HTTPS.  We may also
  be able to configure redirects for expired URLs.

  Note: message bodies rendered as HTML themselves must NOT change,
  the links should point to an anchor tag within the same page,
  instead; giving the user options.

* configurable constants (index limits, search results)

* handle messages with multiple Message-IDs (done for v2, doable for v1)

* handle broken double-bracketed References properly (maybe)
  and totally broken Message-IDs

  cf.  https://public-inbox.org/git/20160814012706.GA18784@starla/

* improve documentation

* linkify thread skeletons better
  https://public-inbox.org/git/6E3699DEA672430CAEA6DEFEDE6918F4@PhilipOakley/

* Further lower mail parser memory usage.  We still slurp entire
  message bodies into memory and incur 2-3x overhead on
  multipart messages.  Inline::C (and maybe gmime) could work.

* use REQUEST_URI properly for CGI / mod_perl2 compatibility
  with Message-IDs which include '%' (done?)

* handle unencoded slashes from user-generated URLs properly
  https://public-inbox.org/meta/20230217085255.xcsaoozloz2yuxil@pengutronix.de/

* better test cases, make faster by reusing more setup
  code across tests

* large mbox/Maildir/MH/NNTP spool import (in lei, but not
  for public-facing inboxes)

* MH import support (read-only, at least)

* Read-only WebDAV interface to the git repo so it can be mounted
  via davfs2 or fusedav to avoid full clones.
  davfs2 needs Range: request support for this to be feasible:
    https://savannah.nongnu.org/bugs/?33259
    https://savannah.nongnu.org/support/?107649

* Contribute something like IMAP IDLE for "git fetch".
  Inboxes (and any git repos) can be kept up-to-date without
  relying on polling.

* Improve bundle support in git to make it cheaper to host/clone
  with dumb HTTP(S) servers.

* Expose targeted reindexing of individual messages.
  Sometimes an indexing bug only affects a handful of messages,
  so it's not worth the trouble of doing a full reindex.

* code repository integration (cgit: done, TODO: gitweb, etc...)

* migration path to v2 (making it transparent for "git fetch"
  may not be possible, but "public-inbox-fetch" will handle it)

* imperfect scraper importers for obfuscated list archives
  (e.g. obfuscated Mailman stuff, Google Groups, etc...)

* improve performance and avoid head-of-line blocking on slow storage
  (done for most git blob retrievals, Xapian needs work)

* allow optional use of separate Xapian worker process to implement
  timeouts and avoid head-of-line blocking problems.  Consider
  just-ahead-of-time builds to take advantage of custom date parsers
  (approxidate) and other features not available to Perl bindings.

* integrate git approxidate parsing into Xapian w/o spawning git

* HTTP(S) search API (likely JMAP, but GraphQL could be an option)
  It should support git-specific prefixes (dfpre:, dfpost:, dfn:, etc)
  as extensions.  If JMAP, it should have HTTP(S) analogues to
  various IMAP extensions.

* scalability to tens/hundreds of thousands of inboxes

  - inotify-based manifest.js.gz updates

  ...

* lei - see %CMD in lib/PublicInbox/LEI.pm
  (there's a truckload here..)

* make "git cat-file --batch" detect unlinked packfiles so we don't
  have to restart processes (very long-term)

* linter to check validity of config file

* linter option and WWW endpoint to graph relationships and flows
  between inboxes, addresses, Maildirs, coderepos, newsgroups,
  IMAP mailboxes, etc...

* pygments support - via Python script similar to `git cat-file --batch'
  to avoid startup penalty.  pygments.rb (Ruby) can be inspiration, too.

* highlighting + linkification for "git format-patch --interdiff" output

* highlighting for "git format-patch --range-diff" output
  (linkification is too expensive, as it requires mirroring)

* support UUCP addresses for legacy archives

* support pipelining as an IMAP/NNTP client for -watch + lei

* expose lei contents via read/write IMAP/JMAP server for personal use

* git SHA-256 migration/coexistence path

* decode RFC 3676 format=flowed + DelSp properly (see mflow (mblaze), mutt, ...)