From 5a715cc60ad66ab0d9e7737631159ace79615228 Mon Sep 17 00:00:00 2001
From: Gani Georgiev <gani.georgiev@gmail.com>
Date: Wed, 21 Feb 2024 19:49:52 +0200
Subject: [PATCH] [#4394] reschedule the first cron tick to start at 00 second

---
 CHANGELOG.md       |  3 +++
 tools/cron/cron.go | 43 ++++++++++++++++++++++++++++++-------------
 2 files changed, 33 insertions(+), 13 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a67a3a16..776031b4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,9 @@
   oauth2
   ```
 
+- Updated the `cron.Start()` to start the ticker at the `00` second of the cron interval ([#4394](https://github.com/pocketbase/pocketbase/discussions/4394)).
+  _Note that the cron format has only minute granularity and there is still no guarantee that the sheduled job will be always executed at the `00` second._
+
 - Upgraded to `aws-sdk-go-v2` and added special handling for GCS to workaround the previous [GCS headers signature issue](https://github.com/pocketbase/pocketbase/issues/2231) that we had with v2.
   _This should also fix the SVG/JSON zero response when using Cloudflare R2 ([#4287](https://github.com/pocketbase/pocketbase/issues/4287#issuecomment-1925168142), [#2068](https://github.com/pocketbase/pocketbase/discussions/2068), [#2952](https://github.com/pocketbase/pocketbase/discussions/2952))._
   _If you are using S3 for uploaded files or backups, please verify that you have a green check in the Admin UI for your S3 configuration (I've tested the new version with GCS, MinIO, Cloudflare R2 and Wasabi)._
diff --git a/tools/cron/cron.go b/tools/cron/cron.go
index d3f1d1d0..c9e4f46c 100644
--- a/tools/cron/cron.go
+++ b/tools/cron/cron.go
@@ -22,12 +22,13 @@ type job struct {
 
 // Cron is a crontab-like struct for tasks/jobs scheduling.
 type Cron struct {
-	sync.RWMutex
+	timezone   *time.Location
+	ticker     *time.Ticker
+	startTimer *time.Timer
+	jobs       map[string]*job
+	interval   time.Duration
 
-	interval time.Duration
-	timezone *time.Location
-	ticker   *time.Ticker
-	jobs     map[string]*job
+	sync.RWMutex
 }
 
 // New create a new Cron struct with default tick interval of 1 minute
@@ -132,6 +133,11 @@ func (c *Cron) Stop() {
 	c.Lock()
 	defer c.Unlock()
 
+	if c.startTimer != nil {
+		c.startTimer.Stop()
+		c.startTimer = nil
+	}
+
 	if c.ticker == nil {
 		return // already stopped
 	}
@@ -146,15 +152,26 @@ func (c *Cron) Stop() {
 func (c *Cron) Start() {
 	c.Stop()
 
-	c.Lock()
-	c.ticker = time.NewTicker(c.interval)
-	c.Unlock()
+	// delay the ticker to start at 00 of 1 c.interval duration
+	now := time.Now()
+	next := now.Add(c.interval).Truncate(c.interval)
+	delay := next.Sub(now)
 
-	go func() {
-		for t := range c.ticker.C {
-			c.runDue(t)
-		}
-	}()
+	c.startTimer = time.AfterFunc(delay, func() {
+		c.Lock()
+		c.ticker = time.NewTicker(c.interval)
+		c.Unlock()
+
+		// run immediately at 00
+		c.runDue(time.Now())
+
+		// run after each tick
+		go func() {
+			for t := range c.ticker.C {
+				c.runDue(t)
+			}
+		}()
+	})
 }
 
 // HasStarted checks whether the current Cron ticker has been started.