mu4e: implement mu4e-quit-hook

As seen on r/emacs.

NEWS.org

@@ -309,6 +309,9 @@
       and connect to the SCM REPL with ~M-x mu4e-mu-scm-repl~ after restart (see
       their docstrings for details)
 
+    - 1.12.14: add ~mu4e-quit-hook~, a hook is just before quitting mu4e. See its
+      docstring for details.
+
 *** scm
 
     - 1.12.12: add new guile/scheme binding in ~scm/~. These are to replace the

mu4e/mu4e.el

@@ -45,12 +45,22 @@
 (require 'mu4e-notification)
 (require 'mu4e-server)     ;; communication with backend
 
-
 (when mu4e-speedbar-support
   (require 'mu4e-speedbar)) ;; support for speedbar
 (when mu4e-org-support
   (require 'mu4e-org))      ;; support for org-mode links
 
+(defcustom mu4e-quit-hook nil
+  "Hook run just before quitting mu4e.
+
+This hook runs just before mu4e performs its own cleanup.
+
+Note that it only fires when mu4e actually terminates; i.e.,
+`mu4e-quit' _without_ the \"bury-only\" parameter."
+  :type 'hook
+  :package-version '(mu4e . "1.12.14")
+  :group 'mu4e)
+
 ;; We can't properly use compose buffers that are revived using
 ;; desktop-save-mode; so let's turn that off.
 (with-eval-after-load 'desktop
@@ -171,6 +181,7 @@ invoke FUNC (if available) afterwards."
 
 (defun mu4e--stop ()
   "Stop mu4e."
+  (run-hooks 'mu4e-quit-hook)
   (when mu4e--update-timer
     (cancel-timer mu4e--update-timer)
     (setq mu4e--update-timer nil))

mu4e/mu4e.texi

@@ -909,8 +909,7 @@ visible only if you have actually set up mail-queuing. @ref{Queuing
 mail}
 @item @t{[A]bout mu4e} provides general information about the program
 @item @t{[H]elp} shows help information for this view
-@item Finally, @t{[q]uit mu4e} quits your @code{mu4e}-session@footnote{@t{mu4e-quit}; or with a @t{C-u}
-prefix argument, it merely buries the buffer}
+@item Finally, @t{[q]uit mu4e} quits your @code{mu4e}-session@footnote{@t{mu4e-quit}; or with a @t{C-u} prefix argument, it merely buries the buffer. A hook @code{mu4e-quit-hook} is available as well}
 @end itemize
 
 @node Headers view
@@ -4359,7 +4358,7 @@ usually because one of the following reasons:
 changes in @code{mu4e} are not immediately visible in the command-line @command{mu},
 since it can take a while before changes are committed to disk (for performance
 reasons). Note that quitting @code{mu4e} or invoking @t{mu4e-update-mail-and-index}
-and @t{mu4e-update-index} trigger committing to disk as a side-effect.
+and @t{mu4e-update-index} have the side-effect of committing changed data to disk.
 @item different options:
 @code{mu4e} defaults to having @t{mu4e-headers-include-related}, and
 @t{mu4e-headers-results-limit} set to 500. However, the command-line @command{mu