[PATCH v3] emacs: letf enriched-decode-display-prop for text/encriched display

[Tomi Ollila]

Dynamically bind enriched-decode-display-prop when inserting
text/enriched part. This complements commit 9b0582383833 for
emacs versions before 24.4 which do not have advice-add
functionality.

Since emacs 25.3 this particular bug is fixed.
---

V3 since id:20170920052544.2893-1-tomi.ollila at iki.fi

* added version< check to apply this fix for emacsen before 25.3

* changed cl-letf to letf for emacs 23.x support

* added comments why (require 'enriched) was needed
  (asked by dme)

db asked for test -- after a new moments of brief thinking I
could not come up with good robust way to test this. I now
tested this using debian 7.11 container (emacs 23.4),
centos 7.0 container (emacs 24.3) and then self-compiled
emacs 25.3 for fedora 26. the change worked as expected and
I don't see a way how this could cause a regression.

 emacs/notmuch-show.el | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 993902770095..7acdfd7542e5 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -773,14 +773,16 @@ (defun notmuch-show-insert-part-text/calendar (msg part content-type nth depth b
 (defun notmuch-show-insert-part-text/x-vcalendar (msg part content-type nth depth button)
   (notmuch-show-insert-part-text/calendar msg part content-type nth depth button))
 
-;; https://bugs.gnu.org/28350
-(defun notmuch-show--enriched-decode-display-prop (start end &optional param)
-  (list start end))
-
-(defun notmuch-show-insert-part-text/enriched (msg part content-type nth depth button)
-  (advice-add 'enriched-decode-display-prop :override
-	      #'notmuch-show--enriched-decode-display-prop)
-  nil)
+(if (version< emacs-version "25.3")
+    ;; https://bugs.gnu.org/28350
+    (defun notmuch-show-insert-part-text/enriched (msg part content-type nth depth button)
+      ;; By requiring enriched below, we ensure that the function enriched-decode-display-prop
+      ;; is defined before it will be shadowed by the letf below. Otherwise the version
+      ;; in enriched.el may be loaded a bit later and used instead (for the first time).
+      (require 'enriched)
+      (letf (((symbol-function 'enriched-decode-display-prop)
+		 (lambda (start end &optional param) (list start end))))
+	(notmuch-show-insert-part-*/* msg part content-type nth depth button))))
 
 (defun notmuch-show-get-mime-type-of-application/octet-stream (part)
   ;; If we can deduce a MIME type from the filename of the attachment,
-- 
2.13.3