From edb27d7e15161e3b12af0fa5b2c3bde8295bb5d7 Mon Sep 17 00:00:00 2001
From: Craig Jennings <c@cjennings.net>
Date: Tue, 30 Jun 2026 17:42:21 -0400
Subject: 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.
---
 tests/test-calendar-sync--syncing-p.el | 84 ++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 tests/test-calendar-sync--syncing-p.el

(limited to 'tests')

diff --git a/tests/test-calendar-sync--syncing-p.el b/tests/test-calendar-sync--syncing-p.el
new file mode 100644
index 000000000..b346bf776
--- /dev/null
+++ b/tests/test-calendar-sync--syncing-p.el
@@ -0,0 +1,84 @@
+;;; test-calendar-sync--syncing-p.el --- Tests for the in-flight sync guard  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `calendar-sync--syncing-p' (the per-calendar in-flight check
+;; that lets the dispatcher skip an overlapping timer tick) and for the
+;; load-state sanitize that clears a stale `syncing' status in a fresh process.
+
+;;; Code:
+
+(require 'ert)
+(require 'calendar-sync)
+
+(defun test-cs-syncing--reset ()
+  "Clear the module's per-calendar state hash."
+  (clrhash calendar-sync--calendar-states))
+
+;;; calendar-sync--syncing-p
+
+(ert-deftest test-calendar-sync--syncing-p-normal-true-when-syncing ()
+  "Normal: a calendar whose status is `syncing' reads as in-flight."
+  (test-cs-syncing--reset)
+  (calendar-sync--set-calendar-state "google" '(:status syncing))
+  (should (calendar-sync--syncing-p "google")))
+
+(ert-deftest test-calendar-sync--syncing-p-boundary-nil-when-no-state ()
+  "Boundary: a calendar with no recorded state is not in-flight."
+  (test-cs-syncing--reset)
+  (should-not (calendar-sync--syncing-p "never-seen")))
+
+(ert-deftest test-calendar-sync--syncing-p-error-nil-for-terminal-status ()
+  "Error: a terminal status (ok / error) is not in-flight."
+  (test-cs-syncing--reset)
+  (calendar-sync--set-calendar-state "google" '(:status ok))
+  (should-not (calendar-sync--syncing-p "google"))
+  (calendar-sync--set-calendar-state "proton" '(:status error))
+  (should-not (calendar-sync--syncing-p "proton")))
+
+;;; Dispatcher guard: an in-flight calendar skips both leaf syncers
+
+(ert-deftest test-calendar-sync--sync-calendar-skips-when-in-flight ()
+  "Normal: `calendar-sync--sync-calendar' does not launch a second sync for a
+calendar already marked syncing, so an overlapping timer tick is a no-op."
+  (test-cs-syncing--reset)
+  (let ((api-calls '()) (ics-calls '()))
+    (cl-letf (((symbol-function 'calendar-sync--sync-calendar-api)
+               (lambda (cal) (push cal api-calls)))
+              ((symbol-function 'calendar-sync--sync-calendar-ics)
+               (lambda (cal) (push cal ics-calls))))
+      (calendar-sync--set-calendar-state "proton" '(:status syncing))
+      (calendar-sync--sync-calendar '(:name "proton" :url "https://x/y.ics"
+                                            :file "/tmp/c.org"))
+      (should (null api-calls))
+      (should (null ics-calls)))))
+
+(ert-deftest test-calendar-sync--sync-calendar-dispatches-when-idle ()
+  "Boundary: an idle calendar (no in-flight status) still dispatches normally."
+  (test-cs-syncing--reset)
+  (let ((ics-calls '()))
+    (cl-letf (((symbol-function 'calendar-sync--sync-calendar-ics)
+               (lambda (cal) (push cal ics-calls))))
+      (calendar-sync--sync-calendar '(:name "proton" :url "https://x/y.ics"
+                                            :file "/tmp/c.org"))
+      (should (= 1 (length ics-calls))))))
+
+;;; load-state sanitize: a persisted `syncing' status is cleared on load
+
+(ert-deftest test-calendar-sync--load-state-clears-stale-syncing ()
+  "Error: a `syncing' status persisted before a crash is reset on load, so the
+in-flight guard cannot skip that calendar forever in the new session."
+  (test-cs-syncing--reset)
+  (let* ((dir (make-temp-file "cs-state-" t))
+         (calendar-sync--state-file (expand-file-name "state.el" dir)))
+    (unwind-protect
+        (progn
+          (with-temp-file calendar-sync--state-file
+            (prin1 '((timezone-offset . nil)
+                     (calendar-states . (("google" . (:status syncing)))))
+                   (current-buffer)))
+          (calendar-sync--load-state)
+          (should-not (calendar-sync--syncing-p "google")))
+      (delete-directory dir t))))
+
+(provide 'test-calendar-sync--syncing-p)
+;;; test-calendar-sync--syncing-p.el ends here
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