Many of the external links found in the notmuch source can be resolved
using https instead of http. This changeset addresses as many as i
could find, without touching the e-mail corpus or expected outputs
found in tests.
When shell executes background process using '&' the scheduling of
that new process is arbitrary. It could be that smtp-dummy doesn't
get execution time to listen() it's server socket until some other
process attempts to connect() to it. The --background option in
smtp-dummy makes it to go background *after* it started to listen
its server socket.
When --background option is used, the line "smtp_dummy_pid='<pid>'"
is printed to stdout from where shell can eval it.
The main() function should be written as just another function with a
return value. This allows for more reliable code reuse. Imagine that
main() grows too large and needs to be factored into multiple
functions. At that point, exit() is probably the wrong thing, yet can
also be hard to notice as it's in less-frequently-tested exceptional
cases.
Rather than *reall* sending mail here, we instead have a new test
program, smtp-dummy which implements (a small piece of) the
server-side SMTP protocol and saves a mail message to the filename
provided. This gives us reasonable test coverage of a large chunk of
the notmuch+emacs code base (down to talking to an SMTP server with
the final mail contents).
