fix(calendar-sync): skip overlapping syncs for the same calendar

A timer tick that fired while a calendar's previous fetch was still running launched a second concurrent sync for that calendar, wasting work and racing to write the same org file. The dispatcher now skips a calendar whose status is already syncing and logs the skipped tick.

The sentinel resets the status on process exit, so the skip clears on its own. load-state also clears a stale syncing status left by a crash, so a calendar can't be skipped forever.

2 files changed, 28 insertions, 5 deletions

diff --git a/modules/calendar-sync-source.el b/modules/calendar-sync-source.el
index d9efc885b..15c91c594 100644
--- a/modules/calendar-sync-source.el
+++ b/modules/calendar-sync-source.el
@@ -90,7 +90,13 @@ Hash table mapping calendar name (string) to state plist with:
             (let ((cal-states (alist-get 'calendar-states state)))
               (clrhash calendar-sync--calendar-states)
               (dolist (entry cal-states)
-                (puthash (car entry) (cdr entry) calendar-sync--calendar-states)))))
+                (let ((st (cdr entry)))
+                  ;; A persisted `syncing' status is stale in a fresh process
+                  ;; (no sync is actually running), so reset it; otherwise the
+                  ;; in-flight guard would skip that calendar forever.
+                  (when (eq (plist-get st :status) 'syncing)
+                    (setq st (plist-put (copy-sequence st) :status 'never)))
+                  (puthash (car entry) st calendar-sync--calendar-states))))))
       (error
        (calendar-sync--log-silently "calendar-sync: Error loading state: %s" (error-message-string err))))))
 
@@ -98,6 +104,13 @@ Hash table mapping calendar name (string) to state plist with:
   "Get state plist for CALENDAR-NAME, or nil if not found."
   (gethash calendar-name calendar-sync--calendar-states))
 
+(defun calendar-sync--syncing-p (calendar-name)
+  "Return non-nil when CALENDAR-NAME has an in-flight sync.
+Used to skip an overlapping sync when a timer tick fires while the previous
+sync for the same calendar is still running."
+  (eq (plist-get (calendar-sync--get-calendar-state calendar-name) :status)
+      'syncing))
+
 (defun calendar-sync--set-calendar-state (calendar-name state)
   "Set STATE plist for CALENDAR-NAME."
   (puthash calendar-name state calendar-sync--calendar-states))
diff --git a/modules/calendar-sync.el b/modules/calendar-sync.el
index 297d1fe61..804d71faf 100644
--- a/modules/calendar-sync.el
+++ b/modules/calendar-sync.el
@@ -211,10 +211,20 @@ fetcher) or :account + :calendar-id (the \\='api fetcher).  Dispatches on the
 :fetcher key, defaulting to the .ics path.
 Updates calendar state and saves to disk on completion.
 The fetch and conversion run in external processes so parsing and writing large
-calendar files do not block the interactive Emacs thread."
-  (if (eq (plist-get calendar :fetcher) 'api)
-      (calendar-sync--sync-calendar-api calendar)
-    (calendar-sync--sync-calendar-ics calendar)))
+calendar files do not block the interactive Emacs thread.
+
+Skips a calendar whose previous sync is still in flight, so a timer tick that
+fires before a slow fetch finishes does not launch a second overlapping sync for
+the same calendar."
+  (let ((name (plist-get calendar :name)))
+    (cond
+     ((calendar-sync--syncing-p name)
+      (calendar-sync--log-silently
+       "calendar-sync: [%s] sync already in flight; skipping overlapping tick" name))
+     ((eq (plist-get calendar :fetcher) 'api)
+      (calendar-sync--sync-calendar-api calendar))
+     (t
+      (calendar-sync--sync-calendar-ics calendar)))))
 
 (defun calendar-sync--require-calendars ()
   "Return non-nil if calendars are configured, else warn and return nil."