mu4e: rework html2text conversion

Rework the conversion from html to text; the functions doing that now
expect to receive one parameter, the message, and return the converted
message.

In the old way, the function got invoked in a buffer with html text, and
were expected to modify it to text. This old way is still supported for
backward compatibility.

mu4e/mu4e-message.el

@@ -194,17 +194,18 @@ unless PREFER-HTML is non-nil."
   (setq mu4e~message-body-html (mu4e~message-use-html-p msg prefer-html))
   (let ((body
 	  (if mu4e~message-body-html
-	    ;; use an HTML body
-	    (cond
-	      ((stringp mu4e-html2text-command)
-		(mu4e-html2text-shell msg mu4e-html2text-command))
-	      ((functionp mu4e-html2text-command)
-		(if (help-function-arglist mu4e-html2text-command)
-		  (funcall mu4e-html2text-command msg)
-		  (mu4e~html2text-wrapper mu4e-html2text-command msg)))
-	      (t (mu4e-error "Invalid `mu4e-html2text-command'")))
-	    (setq mu4e~message-body-html t)
-	    (buffer-string)
+	    (progn
+	      ;; use an HTML body
+	      (cond
+		((stringp mu4e-html2text-command)
+		  (mu4e-html2text-shell msg mu4e-html2text-command))
+		((functionp mu4e-html2text-command)
+		  (if (help-function-arglist mu4e-html2text-command)
+		    (funcall mu4e-html2text-command msg)
+		    ;; oldskool parameterless mu4e-html2text-command
+		    (mu4e~html2text-wrapper mu4e-html2text-command msg)))
+		(t (mu4e-error "Invalid `mu4e-html2text-command'"))
+		(setq mu4e~message-body-html t)))
 	    ;; use a text body
 	    (or (mu4e-message-field msg :body-txt) ""))))
     ;; and finally, remove some crap from the remaining string; it seems
@@ -219,7 +220,7 @@ unless PREFER-HTML is non-nil."
 	     (replace-match
 	       (cond
 		 ((string= (match-string 0) "<EFBFBD>") "'")
-		 (t		                       ""))))
+		 (t		                       ""))))
 	   (buffer-string))))
 
 (defun mu4e-message-contact-field-matches (msg cfield rx)
@@ -275,12 +276,22 @@ point in eiter the headers buffer or the view buffer."
 point in eiter the headers buffer or the view buffer."
   (plist-get (mu4e-message-at-point) field))
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-(defun mu4e-shr2text ()
-  "Html to text using the shr engine; this can be used in
-`mu4e-html2text-command' in a new enough emacs. Based on code by
-Titus von der Malsburg."
-  (interactive)
+
+(defun mu4e~html2text-wrapper (func msg)
+  "Fill a temporary buffer with html from MSG, then call
+FUNC. Return the buffer contents."
+  (with-temp-buffer
+    (insert (or (mu4e-message-field msg :body-html) ""))
+    (funcall func)
+    (message "buffer string...")
+    (or (buffer-string) "")))
+
+(defun mu4e-shr2text (msg)
+  "Convert html in MSG to text using the shr engine; this can be
+used in `mu4e-html2text-command' in a new enough emacs. Based on
+code by Titus von der Malsburg."
+  (mu4e~html2text-wrapper
+    (lambda ()
 	(let (
 	 ;; When HTML emails contain references to remote images,
 	 ;; retrieving these images leaks information. For example,
@@ -289,8 +300,7 @@ Titus von der Malsburg."
 	 ;; to not retrieve images.
 	 ;; See this discussion on mu-discuss:
 	 ;; https://groups.google.com/forum/#!topic/mu-discuss/gr1cwNNZnXo
-	 (shr-inhibit-images t))
-    (shr-render-region (point-min) (point-max))
+	 (shr-inhibit-images t))
 	  (shr-render-region (point-min) (point-max)))) msg))
 
 (defun mu4e~html2text-shell (msg cmd)
@@ -302,12 +312,4 @@ Titus von der Malsburg."
 	(erase-buffer)
 	(call-process-shell-command mu4e-html2text-command tmp-file t t)
 	(delete-file tmp-file))) msg))
-
-(defun mu4e~html2text-wrapper (func msg)
-  "Fill a temporary buffer with html from MSG, then call
-FUNC. Return the buffer contents."
-  (with-temp-buffer
-    (insert (mu4e-message-field msg :body-html))
-    (funcall func)
-    (buffer-string)))
 

mu4e/mu4e.texi

@@ -1381,9 +1381,26 @@ alternative:
 @subsection Html2text functions
 @anchor{Html2text functions}
 
-If @code{mu4e-html2text-command} refers to an elisp function, it is
-expected to take the current buffer in html as input, and transform it
-into text (just like the @code{html2text} function).
+If @code{mu4e-html2text-command} refers to an elisp function, the
+function is expected to take a message plist as its input, and returns
+the transformed data.
+
+
+You can easily create your own function, for instance:
+
+@lisp
+(defun my-mu4e-html2text (msg)
+  "My html2text function; shows short message inline, show
+longer functions in browser."
+  (let ((html (or (mu4e-message-field msg :body-html) "")))
+    (if (> (length html) 1000)
+      (progn
+	(mu4e-action-view-in-browser msg)
+	"[Viewing message in external browser]")
+      (mu4e-shr2text msg))))
+
+(setq mu4e-html2text-command 'my-mu4e-html2text)
+@end lisp
 
 @subsection Privacy aspects
 @anchor{Privacy aspects}