emacs: Use notmuch tag --batch for large tag queries

(Unfortunately, it's difficult to first demonstrate this problem with
a known-broken test because modern Linux kernels have argument length
limits in the megabytes, which makes Emacs really slow!)

emacs/notmuch-lib.el

@@ -355,6 +355,14 @@ user-friendly queries."
   "Return a query that matches the message with id ID."
   (concat "id:" (notmuch-escape-boolean-term id)))
 
+(defun notmuch-hex-encode (str)
+  "Hex-encode STR (e.g., as used by batch tagging).
+
+This replaces spaces, percents, and double quotes in STR with
+%NN where NN is the hexadecimal value of the character."
+  (replace-regexp-in-string
+   "[ %\"]" (lambda (match) (format "%%%02x" (aref match 0))) str))
+
 ;;
 
 (defun notmuch-common-do-stash (text)

emacs/notmuch-tag.el

@@ -261,6 +261,12 @@ from TAGS if present."
 	   (error "Changed tag must be of the form `+this_tag' or `-that_tag'")))))
     (sort result-tags 'string<)))
 
+(defconst notmuch-tag-argument-limit 1000
+  "Use batch tagging if the tagging query is longer than this.
+
+This limits the length of arguments passed to the notmuch CLI to
+avoid system argument length limits and performance problems.")
+
 (defun notmuch-tag (query tag-changes)
   "Add/remove tags in TAG-CHANGES to messages matching QUERY.
 
@@ -279,8 +285,13 @@ notmuch-after-tag-hook will be run."
 	tag-changes)
   (unless (null tag-changes)
     (run-hooks 'notmuch-before-tag-hook)
+    (if (<= (length query) notmuch-tag-argument-limit)
 	(apply 'notmuch-call-notmuch-process "tag"
 	       (append tag-changes (list "--" query)))
+      ;; Use batch tag mode to avoid argument length limitations
+      (let ((batch-op (concat (mapconcat #'notmuch-hex-encode tag-changes " ")
+			      " -- " query)))
+	(notmuch-call-notmuch-process :stdin-string batch-op "tag" "--batch")))
     (run-hooks 'notmuch-after-tag-hook)))
 
 (defun notmuch-tag-change-list (tags &optional reverse)

test/emacs

@@ -122,6 +122,14 @@ test_emacs "(notmuch-search \"$os_x_darwin_thread\")
 output=$(notmuch search $os_x_darwin_thread | notmuch_search_sanitize)
 test_expect_equal "$output" "thread:XXX   2009-11-18 [4/4] Jjgod Jiang, Alexander Botero-Lowry; [notmuch] Mac OS X/Darwin compatibility issues (inbox unread)"
 
+test_begin_subtest "Add tag (large query)"
+# We use a long query to force us into batch mode and use a funny tag
+# that requires escaping for batch tagging.
+test_emacs "(notmuch-tag (concat \"$os_x_darwin_thread\" \" or \" (make-string notmuch-tag-argument-limit ?x)) (list \"+tag-from-%-large-query\"))"
+output=$(notmuch search $os_x_darwin_thread | notmuch_search_sanitize)
+test_expect_equal "$output" "thread:XXX   2009-11-18 [4/4] Jjgod Jiang, Alexander Botero-Lowry; [notmuch] Mac OS X/Darwin compatibility issues (inbox tag-from-%-large-query unread)"
+notmuch tag -tag-from-%-large-query $os_x_darwin_thread
+
 test_begin_subtest "notmuch-show: add single tag to single message"
 test_emacs "(notmuch-show \"$os_x_darwin_thread\")
 	    (execute-kbd-macro \"+tag-from-show-view\")"