From e4f9fe9b8e75e3b723a5d1074aa4b9a460ddde85 Mon Sep 17 00:00:00 2001
From: Shaunak Kashyap <ycombinator@gmail.com>
Date: Mon, 9 Dec 2019 12:49:26 -0800
Subject: [PATCH] Compute last timestamp while accounting for system time going
 backwards

---
 .../add_id/generator/es_generator.go          | 47 ++++++++++---------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/libbeat/processors/add_id/generator/es_generator.go b/libbeat/processors/add_id/generator/es_generator.go
index a626b13dc589..53f1a7d4314b 100644
--- a/libbeat/processors/add_id/generator/es_generator.go
+++ b/libbeat/processors/add_id/generator/es_generator.go
@@ -36,7 +36,8 @@ func ESTimeBasedUUIDGenerator() IDGenerator {
 
 var (
 	sequenceNumber uint32
-	lastTimestamp  time.Time
+	lastTimestamp  uint64
+	delta          uint64
 	once           sync.Once
 	mac            []byte
 	mu             sync.Mutex
@@ -68,7 +69,7 @@ func initOnce() {
 	})
 }
 
-func nextIDData() (int64, uint32) {
+func nextIDData() (uint64, uint32) {
 	mu.Lock()
 	defer mu.Unlock()
 
@@ -77,35 +78,39 @@ func nextIDData() (int64, uint32) {
 	// We only use bottom 3 bytes for the sequence number.
 	s := sequenceNumber & 0xffffff
 
-	timestamp := getTimestamp()
-	if s == 0 {
-		// Always force the clock to increment whenever sequence number is 0, in case we have a long time-slip backwards.
-		timestamp.Add(1 * time.Millisecond)
-	}
-	lastTimestamp = timestamp
-
-	t := timestamp.UnixNano() / 1000 // timestamp in ms-since-epoch
+	t := timestamp()
 	return t, s
 }
 
-func getTimestamp() time.Time {
-	// Don't let timestamp go backwards, at least "on our watch" (while this process is running).  We are still vulnerable if we are
-	// shut down, clock goes backwards, and we restart... for this we randomize the sequenceNumber on init to decrease chance of
-	// collision.
-	now := time.Now()
+// timestamp returns the next timestamp to use, while accounting for system time going
+// backwards (e.g. due to a DST change).
+func timestamp() uint64 {
+	now := uint64(time.Now().UnixNano() / 1000)
+	if lastTimestamp == 0 {
+		// Last timestamp has not been previously initialized
+		lastTimestamp = now
+		return lastTimestamp
+	}
 
-	if lastTimestamp.IsZero() {
-		return now
+	// Normally now should be later than lastTimestamp, but if the system time went backwards
+	// (e.g. due to DST change), we should compute a delta to account for this change, so we always return
+	// a value that's greater than the last call to this function.
+	if now < lastTimestamp {
+		delta = lastTimestamp - now + 1
+		lastTimestamp = now
+		return now + delta
 	}
 
-	if lastTimestamp.After(now) {
-		return lastTimestamp
+	// If the system time was reset, reset delta as well
+	if now-lastTimestamp >= delta {
+		delta = 0
 	}
 
-	return now
+	lastTimestamp = now
+	return lastTimestamp
 }
 
-func packID(buf []byte, ts int64, seq uint32) {
+func packID(buf []byte, ts uint64, seq uint32) {
 	//// We have auto-generated ids, which are usually used for append-only workloads.
 	//// So we try to optimize the order of bytes for indexing speed (by having quite
 	//// unique bytes close to the beginning of the ids so that sorting is fast) and