fix: merge fix that removed deadlock issue when waiting for event up …

…to lock

This PR contains a fix for the deadlock issue described in github#992 (which is also discussed here github#1171). The fix is introduced in the original repo here https://github.com/github/gh-ost/pull/1180/files. This PR contains a cherry-pick of that fix

go/logic/applier.go

@@ -8,7 +8,6 @@ package logic
 import (
 	gosql "database/sql"
 	"fmt"
-	"sync"
 	"strings"
 	"sync/atomic"
 	"time"
@@ -885,7 +884,7 @@ func (this *Applier) CreateAtomicCutOverSentryTable() error {
 }
 
 // AtomicCutOverMagicLock
-func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocked chan<- error, okToUnlockTable <-chan bool, tableUnlocked chan<- error, dropCutOverSentryTableOnce *sync.Once) error {
+func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocked chan<- error, okToUnlockTable <-chan bool, tableUnlocked chan<- error) error {
 	tx, err := this.db.Begin()
 	if err != nil {
 		tableLocked <- err
@@ -896,6 +895,7 @@ func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocke
 		tableLocked <- fmt.Errorf("Unexpected error in AtomicCutOverMagicLock(), injected to release blocking channel reads")
 		tableUnlocked <- fmt.Errorf("Unexpected error in AtomicCutOverMagicLock(), injected to release blocking channel reads")
 		tx.Rollback()
+		this.DropAtomicCutOverSentryTableIfExists()
 	}()
 
 	var sessionId int64
@@ -964,12 +964,10 @@ func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocke
 		sql.EscapeName(this.migrationContext.GetOldTableName()),
 	)
 
-	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`!
-		}
-	})
+	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",

go/logic/migrator.go

@@ -12,7 +12,6 @@ import (
 	"math"
 	"os"
 	"strings"
-	"sync"
 	"sync/atomic"
 	"time"
 
@@ -650,12 +649,8 @@ 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
-		dropCutOverSentryTableOnce.Do(func() {
-			this.applier.DropAtomicCutOverSentryTableIfExists()
-		})
 	}()
 
 	atomic.StoreInt64(&this.migrationContext.AllEventsUpToLockProcessedInjectedFlag, 0)
@@ -664,7 +659,7 @@ 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, &dropCutOverSentryTableOnce); err != nil {
+		if err := this.applier.AtomicCutOverMagicLock(lockOriginalSessionIdChan, tableLocked, okToUnlockTable, tableUnlocked); err != nil {
 			this.migrationContext.Log.Errore(err)
 		}
 	}()