POP3 stuff, particularly a streaming downloader and a simple command line which runs it.

Latest release 20220918:

• Emit an error instead of stack trace for messages which cannot be saved (and do not delete).
• POP3Command.cmd_dl: new -n (no action) option.

I spend some time on a geostationary satellite connection, where round trip ping times are over 600ms when things are good.

My mail setup involves fetching messages from my inbox for local storage in my laptop, usually using POP3. The common standalone tools for this are fetchmail and getmail. However, both are very subject to the link latency, in that they request a message, collect it, issue a delete, then repeat. On a satellite link that incurs a cost of over a second per message, making catch up after a period offline a many minutes long exercise in tedium.

This module does something I've been meaning to do for literally years: a bulk fetch. It issues RETRieves for every message up front as fast as possible. A separate thread collects the messages as they are delivered and issues DELEtes for the saved messages as soon as each is saved.

This results in a fetch process which is orders of magnitude faster. Even on a low latency link the throughput is much faster; on the satellite it is gobsmackingly faster.

Class ConnectionSpec(ConnectionSpec, builtins.tuple)

A specification for a POP3 connection.

Function main(argv=None)

The pop3 command line mode.

Class NetrcEntry(NetrcEntry, builtins.tuple)

A namedtuple representation of a netrc entry.

Class POP3(cs.resources.MultiOpenMixin, cs.context.ContextManagerMixin)

Simple POP3 class with support for streaming use.

Class POP3Command(cs.cmdutils.BaseCommand)

Command line implementation for POP3 operations.

Credentials are obtained via the .netrc file presently.

Connection specifications consist of an optional leading mode prefix followed by a netrc(5) account name or an explicit connection specification from which to derive:

• user: the user name to log in as
• tcp_host: the hostname to which to establish a TCP connection
• port: the TCP port to connect on, default 995 for TLS/SSL or 110 for cleartext
• sni_host: the TLS/SSL SNI server name, which may be different from the tcp_host

The optional mode prefix is one of:

• ssl:: use TLS/SSL - this is the default
• tcp:: use cleartext - this is useful for ssh port forwards to some not-publicly-exposed clear text POP service; in particular streaming performs better this way, I think because the Python SSL layer does not buffer writes

Example connection specifications:

• username@mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in as username
• mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in with the same login as the local effective user
• tcp:username@localhost:1110: use cleartext to connect to localhost:1110, typically an ssh port forward to a remote private cleartext POP service, logging in as username
• username@localhost!mail.example.com:1995: use TLS/SSL to connect to localhost:1995, usually an ssh port forward to a remote private TLS/SSL POP service, logging in as username and passing mail.exampl.com as the TLS/SSL server name indication (which allows certificate verification to proceed correctly)

Note that the specification may also be a netrc account name. If the specification matches such an account name then values are derived from the netrc entry. The entry's machine name becomes the TCP connection specification, the entry's login provides a default for the username, the entry's account host part provides the sni_host.

Example netrc entry:

machine username@localhost:1110
  account username@mail.example.com
  password ************

Such an entry allows you to use the specification tcp:username@mail.example.com and obtain the remaining detail via the netrc entry.

Command line usage:

Usage: pop3 subcommand [...]
  Subcommands:
    dl [-n] [{ssl,tcp}:]{netrc_account|[user@]host[!sni_name][:port]} maildir
    help [-l] [subcommand-names...]
      Print the full help for the named subcommands,
      or for all subcommands if no names are specified.
      -l  Long help even if no subcommand-names provided.

Release Log

Release 20220918:

• Emit an error instead of stack trace for messages which cannot be saved (and do not delete).
• POP3Command.cmd_dl: new -n (no action) option.

Release 20220606: Minor updates.

Release 20220312: Make POP3Command.cmd_dl an instance method (static methods broke with the latest cs.cmdutils release).

Release 20211208:

• POP3.startup: do not start the worker queue until authenticated.
• POP3.get_response: return (None,None,None) at EOF.
• POP3.shutdown: catch exceptions from client QUIT.

Release 20210407.2: Provide "pop3" console_script.

Release 20210407.1: Bump for cs.cmdutils minor bugfix, also fix a few docstring typos.

Release 20210407: Initial PyPI release.