twisted.mail.smtp.ESMTPClient tries to STARTTLS multiple times

[Jean-Paul Calderone]

If ESMTPClient is used to connect to an SMTP server that offers the STARTTLS feature, it will try to use it. If the SMTP server still offers the STARTTLS after the command has been used once to start TLS, ESMTPClient will try to use it again. This is useless at best, and in practice probably always triggers an error from the server and prevents mail from being sent.

This is a regression introduced by #3989.

[Jean-Paul Calderone]

Here's a transcript demonstrating the consequences of this bug:

<<< 220 email-smtp.amazonaws.com ESMTP SimpleEmailService-376766033
>>> EHLO top
<<< 250-email-smtp.amazonaws.com
<<< 250-8BITMIME
<<< 250-SIZE 10485760
<<< 250-STARTTLS
<<< 250-AUTH PLAIN LOGIN
<<< 250 Ok
>>> STARTTLS
<<< 220 Ready to start TLS
>>> EHLO top
<<< 250-email-smtp.amazonaws.com
<<< 250-8BITMIME
<<< 250-SIZE 10485760
<<< 250-STARTTLS
<<< 250-AUTH PLAIN LOGIN
<<< 250 Ok
>>> STARTTLS
<<< 454 TLS not available due to temporary reason: TLS already active
>>> QUIT

And attached is a script that reproduces it.

[Jean-Paul Calderone]

It does not work.

[Jean-Paul Calderone]

(In [38818]) Branching to 'judicious-smtp-starttls-6524'

[Jean-Paul Calderone]

Relatively straightforward; only wished I had schematic tables once during the entire development process (arguably the minimum number of times it is possible to wish for schematic tables when writing a piece of software).

[radix]

Schematic tables would be nice. Maybe we should sprint on them some time.

[1] You have a "verify that" in a docstring in a test module, but I guess that's ok because it's not a test_ method itself, just a utility assertion method?

No flakes, tests pass, +1, please merge.

[Jean-Paul Calderone]

(In [38833]) Merge judicious-smtp-starttls-6524

Author: exarkun Reviewer: radix Fixes: #6524

Avoid having twisted.mail.smtp.ESMTPClient try to use the STARTTLS command if the server offers it but the connection is already using TLS.