Quoth Dmitry Kurochkin on Jan 26 at  9:06 am:
> The patch adds <tab> completion to "tag all" operation bound to "*"
> (`notmuch-search-operate-all' function).
> ---
> 
> On a second thought, `notmuch-select-tag-with-completion' should never
> need `prefixes' argument at all.  So I reverted the API and related
> changes.
> 
> Changes:
> 
> v4:
> 
> * do not change `notmuch-select-tag-with-completion' API, revert
>   related changes
> 
> v3:
> 
> * fixed comments from Austin's review [1]
> 
> v2:
> 
> * s/thistag/this_tag/ for consistency with "that_tag", since we touch
>   the line anyway
> 
> Regards,
>   Dmitry
> 
> [1] id:"20120126013727.GB1176@mit.edu"
> 
>  emacs/notmuch.el |   53 ++++++++++++++++++++++++++++++++++++++++-------------
>  1 files changed, 40 insertions(+), 13 deletions(-)
> 
> diff --git a/emacs/notmuch.el b/emacs/notmuch.el
> index e02966f..d2af630 100644
> --- a/emacs/notmuch.el
> +++ b/emacs/notmuch.el
> @@ -48,6 +48,7 @@
>  ;; required, but is available from http://notmuchmail.org).
>  
>  (eval-when-compile (require 'cl))
> +(require 'crm)
>  (require 'mm-view)
>  (require 'message)
>  
> @@ -75,12 +76,36 @@ For example:
>  (defvar notmuch-query-history nil
>    "Variable to store minibuffer history for notmuch queries")
>  
> -(defun notmuch-select-tag-with-completion (prompt &rest search-terms)
> +(defun notmuch-tag-completions (&optional prefixes search-terms)
>    (let ((tag-list
>  	 (with-output-to-string
>  	   (with-current-buffer standard-output
>  	     (apply 'call-process notmuch-command nil t nil "search-tags" search-terms)))))
> -    (completing-read prompt (split-string tag-list "\n+" t) nil nil nil)))
> +    (setq tag-list (split-string tag-list "\n+" t))

Since this setq is unconditional, you can do the split-string right in
the let binding, around the with-output-to-string.

> +    (if (null prefixes)
> +	tag-list
> +      (apply #'append
> +	     (mapcar (lambda (tag)
> +		       (mapcar (lambda (prefix)
> +				 (concat prefix tag)) prefixes))
> +		     tag-list)))))
> +
> +(defun notmuch-select-tag-with-completion (prompt &rest search-terms)
> +  (let ((tag-list (notmuch-tag-completions nil search-terms)))
> +    (completing-read prompt tag-list)))
> +
> +(defun notmuch-select-tags-with-completion (prompt &optional prefixes &rest search-terms)
> +  (let ((tag-list (notmuch-tag-completions prefixes search-terms))
> +	(crm-separator " ")
> +	(crm-local-completion-map
> +	 (let ((map (make-sparse-keymap)))
> +	   (set-keymap-parent map crm-local-completion-map)
> +	   map)))
> +    ;; By default, space is bound to "complete word" function.
> +    ;; Re-bind it to insert a space instead.  Note that <tab> still
> +    ;; does the completion.
> +    (define-key crm-local-completion-map " " 'self-insert-command)

You could do the define-key inside the (let ((map ..)) ..) so you get
back the fully formed keymap.  Your call.

> +    (completing-read-multiple prompt tag-list)))
>  
>  (defun notmuch-foreach-mime-part (function mm-handle)
>    (cond ((stringp (car mm-handle))
> @@ -849,7 +874,7 @@ non-authors is found, assume that all of the authors match."
>  	      (goto-char found-target)))
>        (delete-process proc))))
>  
> -(defun notmuch-search-operate-all (action)
> +(defun notmuch-search-operate-all (&rest actions)
>    "Add/remove tags from all matching messages.
>  
>  This command adds or removes tags from all messages matching the
> @@ -860,16 +885,18 @@ will prompt for tags to be added or removed. Tags prefixed with
>  Each character of the tag name may consist of alphanumeric
>  characters as well as `_.+-'.
>  "
> -  (interactive "sOperation (+add -drop): notmuch tag ")
> -  (let ((action-split (split-string action " +")))
> -    ;; Perform some validation
> -    (let ((words action-split))
> -      (when (null words) (error "No operation given"))
> -      (while words
> -	(unless (string-match-p "^[-+][-+_.[:word:]]+$" (car words))
> -	  (error "Action must be of the form `+thistag -that_tag'"))
> -	(setq words (cdr words))))
> -    (apply 'notmuch-tag notmuch-search-query-string action-split)))
> +  (interactive (notmuch-select-tags-with-completion
> +		"Operations (+add -drop): notmuch tag "
> +		'("+" "-")))
> +  (setq actions (delete "" actions))

Either this line isn't necessary or
notmuch-select-tags-with-completion can do something funny that it
should take care of internally.

> +  ;; Perform some validation
> +  (let ((words actions))
> +    (when (null words) (error "No operations given"))
> +    (while words
> +      (unless (string-match-p "^[-+][-+_.[:word:]]+$" (car words))
> +	(error "Action must be of the form `+this_tag' or `-that_tag'"))
> +      (setq words (cdr words))))
> +  (apply 'notmuch-tag notmuch-search-query-string actions))
>  
>  (defun notmuch-search-buffer-title (query)
>    "Returns the title for a buffer with notmuch search results."