+;; Part content ID handling
+
+(defvar notmuch-show--cids nil
+ "Alist from raw content ID to (MSG PART).")
+(make-variable-buffer-local 'notmuch-show--cids)
+
+(defun notmuch-show--register-cids (msg part)
+ "Register content-IDs in PART and all of PART's sub-parts."
+ (let ((content-id (plist-get part :content-id)))
+ (when content-id
+ ;; Note that content-IDs are globally unique, except when they
+ ;; aren't: RFC 2046 section 5.1.4 permits children of a
+ ;; multipart/alternative to have the same content-ID, in which
+ ;; case the MUA is supposed to pick the best one it can render.
+ ;; We simply add the content-ID to the beginning of our alist;
+ ;; so if this happens, we'll take the last (and "best")
+ ;; alternative (even if we can't render it).
+ (push (list content-id msg part) notmuch-show--cids)))
+ ;; Recurse on sub-parts
+ (let ((ctype (notmuch-split-content-type
+ (downcase (plist-get part :content-type)))))
+ (cond ((equal (first ctype) "multipart")
+ (mapc (apply-partially #'notmuch-show--register-cids msg)
+ (plist-get part :content)))
+ ((equal ctype '("message" "rfc822"))
+ (notmuch-show--register-cids
+ msg
+ (first (plist-get (first (plist-get part :content)) :body)))))))
+
+(defun notmuch-show--get-cid-content (cid)
+ "Return a list (CID-content content-type) or nil.
+
+This will only find parts from messages that have been inserted
+into the current buffer. CID must be a raw content ID, without
+enclosing angle brackets, a cid: prefix, or URL encoding. This
+will return nil if the CID is unknown or cannot be retrieved."
+ (let ((descriptor (cdr (assoc cid notmuch-show--cids))))
+ (when descriptor
+ (let* ((msg (first descriptor))
+ (part (second descriptor))
+ ;; Request caching for this content, as some messages
+ ;; reference the same cid: part many times (hundreds!).
+ (content (notmuch-get-bodypart-binary
+ msg part notmuch-show-process-crypto 'cache))
+ (content-type (plist-get part :content-type)))
+ (list content content-type)))))
+
+(defun notmuch-show-setup-w3m ()
+ "Instruct w3m how to retrieve content from a \"related\" part of a message."
+ (interactive)
+ (if (boundp 'w3m-cid-retrieve-function-alist)
+ (unless (assq 'notmuch-show-mode w3m-cid-retrieve-function-alist)
+ (push (cons 'notmuch-show-mode #'notmuch-show--cid-w3m-retrieve)
+ w3m-cid-retrieve-function-alist)))
+ (setq mm-inline-text-html-with-images t))
+
+(defvar w3m-current-buffer) ;; From `w3m.el'.
+(defun notmuch-show--cid-w3m-retrieve (url &rest args)
+ ;; url includes the cid: prefix and is URL encoded (see RFC 2392).
+ (let* ((cid (url-unhex-string (substring url 4)))
+ (content-and-type
+ (with-current-buffer w3m-current-buffer
+ (notmuch-show--get-cid-content cid))))
+ (when content-and-type
+ (insert (first content-and-type))
+ (second content-and-type))))
+