From 0e2d33ad86c11a26abda216027e78831646d04c4 Mon Sep 17 00:00:00 2001
From: Tim Vaillancourt <timvaillancourt@github.com>
Date: Tue, 20 Oct 2020 16:08:49 +0200
Subject: [PATCH] Merge in https://github.com/github/gh-ost/pull/755

---
 go/logic/applier.go  | 14 +++++++++-----
 go/logic/migrator.go |  8 ++++++--
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/go/logic/applier.go b/go/logic/applier.go
index 089978c..8aa1c9c 100644
--- a/go/logic/applier.go
+++ b/go/logic/applier.go
@@ -17,6 +17,7 @@ import (
 	"github.com/github/gh-ost/go/sql"
 
 	"github.com/outbrain/golib/sqlutils"
+	"sync"
 )
 
 const (
@@ -787,7 +788,7 @@ func (this *Applier) CreateAtomicCutOverSentryTable() error {
 }
 
 // AtomicCutOverMagicLock
-func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocked chan<- error, okToUnlockTable <-chan bool, tableUnlocked chan<- error) error {
+func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocked chan<- error, okToUnlockTable <-chan bool, tableUnlocked chan<- error, dropCutOverSentryTableOnce *sync.Once) error {
 	tx, err := this.db.Begin()
 	if err != nil {
 		tableLocked <- err
@@ -865,10 +866,13 @@ func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocke
 		sql.EscapeName(this.migrationContext.DatabaseName),
 		sql.EscapeName(this.migrationContext.GetOldTableName()),
 	)
-	if _, err := tx.Exec(query); err != nil {
-		this.migrationContext.Log.Errore(err)
-		// We DO NOT return here because we must `UNLOCK TABLES`!
-	}
+
+	dropCutOverSentryTableOnce.Do(func() {
+		if _, err := tx.Exec(query); err != nil {
+			this.migrationContext.Log.Errore(err)
+			// We DO NOT return here because we must `UNLOCK TABLES`!
+		}
+	})
 
 	// Tables still locked
 	this.migrationContext.Log.Infof("Releasing lock from %s.%s, %s.%s",
diff --git a/go/logic/migrator.go b/go/logic/migrator.go
index 70af08d..85dae29 100644
--- a/go/logic/migrator.go
+++ b/go/logic/migrator.go
@@ -11,6 +11,7 @@ import (
 	"math"
 	"os"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -606,9 +607,12 @@ func (this *Migrator) atomicCutOver() (err error) {
 	defer atomic.StoreInt64(&this.migrationContext.InCutOverCriticalSectionFlag, 0)
 
 	okToUnlockTable := make(chan bool, 4)
+	var dropCutOverSentryTableOnce sync.Once
 	defer func() {
 		okToUnlockTable <- true
-		this.applier.DropAtomicCutOverSentryTableIfExists()
+		dropCutOverSentryTableOnce.Do(func() {
+			this.applier.DropAtomicCutOverSentryTableIfExists()
+		})
 	}()
 
 	atomic.StoreInt64(&this.migrationContext.AllEventsUpToLockProcessedInjectedFlag, 0)
@@ -617,7 +621,7 @@ func (this *Migrator) atomicCutOver() (err error) {
 	tableLocked := make(chan error, 2)
 	tableUnlocked := make(chan error, 2)
 	go func() {
-		if err := this.applier.AtomicCutOverMagicLock(lockOriginalSessionIdChan, tableLocked, okToUnlockTable, tableUnlocked); err != nil {
+		if err := this.applier.AtomicCutOverMagicLock(lockOriginalSessionIdChan, tableLocked, okToUnlockTable, tableUnlocked, &dropCutOverSentryTableOnce); err != nil {
 			this.migrationContext.Log.Errore(err)
 		}
 	}()