[PATCH v4] emacs: Call "notmuch tag" once when applying tag changes to a thread.

[David Edmondson <dme@dme.org>]

Optimize thread tagging by combining all the tagging operations to a
single "notmuch tag" call.

For threads in the order of tens or a hundred inbox tagged messages,
this gives a noticeable speedup. On two different machines, archiving
a thread of about 50 inbox tagged messages goes down from 10+ seconds
to about 0.5 seconds.

The bottleneck is not within emacs; the same behaviour can be observed
in the CLI. This approach has the added benefit of being more
reliable: any of the individual tagging operations might face a locked
database, leading to partial results.

This introduces a limitation to the number of messages that can be
archived at the same time (through ARG_MAX limiting the command
line). While at least on Linux this seems more like a theoretical
limitation than a real one, it could be avoided by archiving at most a
few hundred messages at a time.

Based on code from Jani Nikula <jani@nikula.org>.
---

v4:
- Rebased after Dmitry's tagging changes (yay!).

 emacs/notmuch-show.el |   18 +++++++++++-------
 1 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index faa9f9b..9294bc8 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -1599,14 +1599,18 @@ argument, hide all of the messages."
   (backward-button 1))
 
 (defun notmuch-show-tag-thread-internal (tag &optional remove)
-  "Add tag to the current set of messages.
+  "Add TAG to the current set of messages.
 
-If the remove switch is given, tags will be removed instead of
-added."
-  (goto-char (point-min))
-  (let ((op (if remove "-" "+")))
-    (loop do (notmuch-show-tag-message (concat op tag))
-	  until (not (notmuch-show-goto-message-next)))))
+If REMOVE is non-nil, TAG will be removed rather than added."
+  (save-excursion
+    (goto-char (point-min))
+    (let ((tag-op (concat (if remove "-" "+") tag))
+	  (message-ids
+	   (loop collect (notmuch-show-get-message-id)
+		 until (not (notmuch-show-goto-message-next)))))
+      (if message-ids
+	  (notmuch-tag (mapconcat #'identity message-ids " OR ")
+		       tag-op)))))
 
 (defun notmuch-show-add-tag-thread (tag)
   "Add tag to all messages in the current thread."
-- 
1.7.8.3