fix: atomic claim prevents scheduled tasks from executing twice (#657)

* fix: atomic claim prevents scheduled tasks from executing twice (#138)

Replace the two-phase getDueTasks() + deferred updateTaskAfterRun() with
an atomic SQLite transaction (claimDueTasks) that advances next_run
BEFORE dispatching tasks to the queue. This eliminates the race window
where subsequent scheduler polls re-discover in-progress tasks.

Key changes:
- claimDueTasks(): SELECT + UPDATE in a single db.transaction(), so no
  poll can read stale next_run values. Once-tasks get next_run=NULL;
  recurring tasks get next_run advanced to the future.
- computeNextRun(): anchors interval tasks to the scheduled time (not
  Date.now()) to prevent cumulative drift. Includes a while-loop to
  skip missed intervals and a guard against invalid interval values.
- updateTaskAfterRun(): simplified to only record last_run/last_result
  since next_run is already handled by the claim.

Closes #138, #211, #300, #578

Co-authored-by: @taslim (PR #601)
Co-authored-by: @baijunjie (Issue #138)
Co-authored-by: @Michaelliv (Issue #300)
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply prettier formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: track running task ID in GroupQueue to prevent duplicate execution (#138)

Previous commits implemented an "atomic claim" approach (claimDueTasks)
that advanced next_run before execution. Per Gavriel's review, this
solved the symptom at the wrong layer and introduced crash-recovery
risks for once-tasks.

This commit reverts claimDueTasks and instead fixes the actual bug:
GroupQueue.enqueueTask() only checked pendingTasks for duplicates, but
running tasks had already been shifted out. Adding runningTaskId to
GroupState closes that gap with a 3-line fix at the correct layer.

The computeNextRun() drift fix is retained, applied post-execution
where it belongs.

Closes #138, #211, #300, #578

Co-authored-by: @taslim (PR #601)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add changelog entry for scheduler duplicate fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add contributors for scheduler race condition fix

Co-Authored-By: Taslim <9999802+taslim@users.noreply.github.com>
Co-Authored-By: BaiJunjie <7956480+baijunjie@users.noreply.github.com>
Co-Authored-By: Michael <13676242+Michaelliv@users.noreply.github.com>
Co-Authored-By: Kyle Zhike Chen <3477852+kk17@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: gavrielc <gabicohen22@yahoo.com>
Co-authored-by: Taslim <9999802+taslim@users.noreply.github.com>
Co-authored-by: BaiJunjie <7956480+baijunjie@users.noreply.github.com>
Co-authored-by: Michael <13676242+Michaelliv@users.noreply.github.com>
Co-authored-by: Kyle Zhike Chen <3477852+kk17@users.noreply.github.com>

src/task-scheduler.ts

@@ -21,6 +21,47 @@ import { resolveGroupFolderPath } from './group-folder.js';
 import { logger } from './logger.js';
 import { RegisteredGroup, ScheduledTask } from './types.js';
 
+/**
+ * Compute the next run time for a recurring task, anchored to the
+ * task's scheduled time rather than Date.now() to prevent cumulative
+ * drift on interval-based tasks.
+ *
+ * Co-authored-by: @community-pr-601
+ */
+export function computeNextRun(task: ScheduledTask): string | null {
+  if (task.schedule_type === 'once') return null;
+
+  const now = Date.now();
+
+  if (task.schedule_type === 'cron') {
+    const interval = CronExpressionParser.parse(task.schedule_value, {
+      tz: TIMEZONE,
+    });
+    return interval.next().toISOString();
+  }
+
+  if (task.schedule_type === 'interval') {
+    const ms = parseInt(task.schedule_value, 10);
+    if (!ms || ms <= 0) {
+      // Guard against malformed interval that would cause an infinite loop
+      logger.warn(
+        { taskId: task.id, value: task.schedule_value },
+        'Invalid interval value',
+      );
+      return new Date(now + 60_000).toISOString();
+    }
+    // Anchor to the scheduled time, not now, to prevent drift.
+    // Skip past any missed intervals so we always land in the future.
+    let next = new Date(task.next_run!).getTime() + ms;
+    while (next <= now) {
+      next += ms;
+    }
+    return new Date(next).toISOString();
+  }
+
+  return null;
+}
+
 export interface SchedulerDependencies {
   registeredGroups: () => Record<string, RegisteredGroup>;
   getSessions: () => Record<string, string>;
@@ -187,18 +228,7 @@ async function runTask(
     error,
   });
 
-  let nextRun: string | null = null;
-  if (task.schedule_type === 'cron') {
-    const interval = CronExpressionParser.parse(task.schedule_value, {
-      tz: TIMEZONE,
-    });
-    nextRun = interval.next().toISOString();
-  } else if (task.schedule_type === 'interval') {
-    const ms = parseInt(task.schedule_value, 10);
-    nextRun = new Date(Date.now() + ms).toISOString();
-  }
-  // 'once' tasks have no next run
-
+  const nextRun = computeNextRun(task);
   const resultSummary = error
     ? `Error: ${error}`
     : result