[PATCH 2/3] inject: Support skipping messages

[Kyle Meyer <kyle@kyleam.com>]

piem-inject-thread-into-maildir allows pulling a thread from a
public-inbox URL, but repeating the process to get new messages in the
thread leads to duplicates in the local store.  Add a predicate that
the caller can configure to avoid this.

Note that the predicate has the entire message available to make its
decision, but it's called with the message ID for convenience because
that's likely the only information needed to determine if the message
is already in the local store.
---
 piem.el | 63 ++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 45 insertions(+), 18 deletions(-)

diff --git a/piem.el b/piem.el
index 5fc7f04..aa0ec0c 100644
--- a/piem.el
+++ b/piem.el
@@ -145,6 +145,16 @@ (defcustom piem-maildir-directory nil
 If non-nil, this must be an existing Maildir directory."
   :type 'string)
 
+(defcustom piem-mail-injection-skipif-predicate nil
+  "Predicate to decide whether to skip injecting a message.
+
+The function is called with the message ID (no surrounding
+brackets) within a buffer that is narrowed to the message.  The
+function does not need to worry about saving point.  A non-nil
+return value signals that `piem-inject-thread-into-maildir'
+should skip the message."
+  :type 'function)
+
 (defcustom piem-after-mail-injection-functions nil
   "Functions called after writing messages to `piem-maildir-directory'.
 Functions should accept one argument, the message ID given to
@@ -406,7 +416,8 @@ (defun piem-download-and-decompress (url)
     (url-retrieve url #'piem--decompress-callback)))
 
 (defun piem--write-mbox-to-maildir ()
-  (let ((n-messages 0))
+  (let ((n-added 0)
+        (n-skipped 0))
     (while (and (not (eobp))
                 (search-forward "From mboxrd@z" nil t))
       (let* ((beg (line-beginning-position 2))
@@ -417,21 +428,31 @@ (defun piem--write-mbox-to-maildir ()
              (basename (piem-maildir-make-uniq-maildir-id))
              (tmpfile (concat piem-maildir-directory "/tmp/" basename)))
         (goto-char beg)
-        ;; TODO: Consider supporting a caller-specified predicate that
-        ;; could be used, for example, to skip messages already in
-        ;; their local mail.
-        (let ((case-fold-search nil))
-          (while (re-search-forward
-                  (rx line-start ">" (group (zero-or-more ">") "From "))
-                  end t)
-            (replace-match "\\1" t)))
-        (write-region beg end tmpfile nil nil nil 'excl)
-        (piem-maildir-move-tmp-to-new piem-maildir-directory
-                                      basename)
-        (delete-file tmpfile)
-        (cl-incf n-messages)
+        (if (and (functionp piem-mail-injection-skipif-predicate)
+                 (save-excursion
+                   (save-restriction
+                     (narrow-to-region beg end)
+                     (funcall piem-mail-injection-skipif-predicate
+                              (replace-regexp-in-string
+                               "\\`<\\(.*\\)>\\'" "\\1"
+                               (or (save-restriction
+                                     (save-excursion
+                                       (message-narrow-to-head-1)
+                                       (message-fetch-field "message-id" t)))
+                                   (error "Message lacks message-ID")))))))
+            (cl-incf n-skipped)
+          (let ((case-fold-search nil))
+            (while (re-search-forward
+                    (rx line-start ">" (group (zero-or-more ">") "From "))
+                    end t)
+              (replace-match "\\1" t)))
+          (write-region beg end tmpfile nil nil nil 'excl)
+          (piem-maildir-move-tmp-to-new piem-maildir-directory
+                                        basename)
+          (delete-file tmpfile)
+          (cl-incf n-added))
         (goto-char end)))
-    n-messages))
+    (cons n-added n-skipped)))
 
 (defun piem--inject-thread-callback (status mid message-only)
   (let ((buffer (current-buffer)))
@@ -442,9 +463,15 @@ (defun piem--inject-thread-callback (status mid message-only)
             (piem--url-remove-header)
             (unless message-only
               (piem--url-decompress))
-            (let ((message-count (piem--write-mbox-to-maildir)))
-              (message "%d message(s) for %s moved to %s"
-                       message-count mid
+            (pcase-let ((`(,added-count . ,skipped-count)
+                         (piem--write-mbox-to-maildir)))
+              (message "Added %d message%s%s for %s to %s"
+                       added-count
+                       (if (= added-count 1) "" "s")
+                       (if (> skipped-count 0)
+                           (format " (skipping %d)" skipped-count)
+                         "")
+                       mid
                        (abbreviate-file-name piem-maildir-directory)))
             (run-hook-with-args 'piem-after-mail-injection-functions mid)))
       (and (buffer-live-p buffer)
-- 
2.28.0