This implements basic attachment checks like those present in other
MUAs (e.g. Thunderbird, IIRC). A hook watches for keywords, which are
implemented using a customizable regex, that indicate the user might
have wanted to include an attachement while writing the email, but has
forgotten.

We currently check for words in english and french and some care was
taken to avoid false positive, but those are bound to happen and we
embrace them with open arms: it's better to warn by mistake than
forget an attachment ever again. New languages can be added through
customization, which help string suggests contributing it back to the
community here. I am not sure this is the best way, but I'm unfamiliar
with Notmuch's translation system (if any).

This code is working: I use it daily and never worry about missing
attachments anymore (mostly because I'm always *thinking* about
putting the attachment because I want this script to trigger, so I
never forget attachments anymore). However there are no unit tests:
bremner suggested I add some in test/T310-emacs.sh and I looked there
briefly, but couldn't figure out where to add a blurb.

I did some summary tests using `re-builder' to see the effect the
regex has on random strings. I tested the possible false positive "a
joint attaché presse" which doesn't match and "here is an attachment,
attached in the pièce jointe or piece jointe" which does. I am not
sure how to implement this in a unit test: should we try to send an
email and check if it aborts like the "Sending a message via (fake)
SMTP" test? Or should I just check that `notmuch-message-check-attach'
works against a temporary buffer? Is there a hardness that allows
matching against messages like that already?

I'd also welcome comments on the approach in general. Another user
came up on IRC recently (impatkor) with the same need and used the
following snippet instead:

https://pastebin.com/N9ku3DBD

It is a very similar implementation although it checks for
`Content-Disposition: attachment` instead of `<#part>`: not sure which
one is actually accurate. It also adds the word `bifoga` as a pattern,
but I haven't verified that in swedish and would wait for feedback on
the multilingual approach before adding new words here.

Finally, note that an earlier version of this used
`save-mark-and-excursion` but I had to revert back to `save-excursion`
because the function was missing in some other version of Emacs I was
testing. That part does not work anyways: something else is moving the
mark around when sending, but I figured I would keep this hook
well-behaving even if others screw that up.

Oh, and one last thing: this commit log is longer than the patch,
which is probably wrong. Sorry about that. :p
---
 emacs/notmuch-message.el | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/emacs/notmuch-message.el b/emacs/notmuch-message.el
index 55e4cfee..c4e301cc 100644
--- a/emacs/notmuch-message.el
+++ b/emacs/notmuch-message.el
@@ -47,6 +47,43 @@ the \"inbox\" and \"todo\" tags, you would set:
 
 (add-hook 'message-send-hook 'notmuch-message-mark-replied)
 
+;; attachment checks
+;;
+;; should be sent upstream, but needs unit tests in test/T310-emacs.sh
+(defcustom notmuch-message-attach-regex
+  "\\b\\(attache\?ment\\|attached\\|attach\\|pi[èe]ce\s+jointe?\\)\\b"
+  "Pattern of text announcing there should be an attachment.
+
+This is used by `notmuch-message-check-attach' to check email
+bodies for words that might indicate the email should have an
+attachement. If the pattern matches and there is no attachment (a
+`<#part ...>' magic block), notmuch will show a confirmation
+prompt before sending the email.
+
+The default regular expression is deliberately liberal: we prefer
+false positive than forgotten attachments. This should be
+customized for non-english languages and notmuch welcomes
+additions to the pattern for your native language, unless it
+conflicts with common words in other languages."
+  :type '(regexp)
+  :group 'notmuch-send)
+
+(defun notmuch-message-check-attach ()
+  """Check for missing attachments.
+
+This is normally added to `message-send-hook' and is configured
+through `notmuch-message-attach-regex'."""
+  (save-excursion ;; XXX: this fails somehow: point is at the end of the 
buffer on error
+    (goto-char (point-min))
+    (if (re-search-forward notmuch-attach-regex nil t)
+        (progn
+          (goto-char (point-min))
+          (unless (re-search-forward "<#part [^>]*filename=[^>]*>" nil t)
+            (or (y-or-n-p "Email seem to refer to attachment, but nothing 
attached, send anyways?")
+                (error "No attachment found, aborting")))))))
+
+(add-hook 'message-send-hook 'notmuch-message-check-attach)
+
 (provide 'notmuch-message)
 
 ;;; notmuch-message.el ends here
-- 
2.11.0

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