Prevent exponential backoff from overflowing.

When using `-cut-over-exponential-backoff` with `-default-retries`
greater than `64`, the sleep interval will overflow and immediately
begin retrying in rapid succession. This commit corrects the backoff
algorithm and adds test for both "retry operation" functions.

go/logic/migrator.go

@@ -24,6 +24,7 @@ import (
 
 var (
 	ErrMigratorUnsupportedRenameAlter = errors.New("ALTER statement seems to RENAME the table. This is not supported, and you should run your RENAME outside gh-ost.")
+	RetrySleepFn                      = time.Sleep
 )
 
 type ChangelogState string
@@ -135,7 +136,7 @@ func (this *Migrator) retryOperation(operation func() error, notFatalHint ...boo
 	for i := 0; i < maxRetries; i++ {
 		if i != 0 {
 			// sleep after previous iteration
-			time.Sleep(1 * time.Second)
+			RetrySleepFn(1 * time.Second)
 		}
 		err = operation()
 		if err == nil {
@@ -155,16 +156,16 @@ func (this *Migrator) retryOperation(operation func() error, notFatalHint ...boo
 // attempts are reached. Wait intervals between attempts obey a maximum of
 // `ExponentialBackoffMaxInterval`.
 func (this *Migrator) retryOperationWithExponentialBackoff(operation func() error, notFatalHint ...bool) (err error) {
-	var interval int64
 	maxRetries := int(this.migrationContext.MaxRetries())
 	maxInterval := this.migrationContext.ExponentialBackoffMaxInterval
 	for i := 0; i < maxRetries; i++ {
-		newInterval := int64(math.Exp2(float64(i - 1)))
-		if newInterval <= maxInterval {
-			interval = newInterval
-		}
+		interval := math.Min(
+			float64(maxInterval),
+			math.Max(1, math.Exp2(float64(i-1))),
+		)
+
 		if i != 0 {
-			time.Sleep(time.Duration(interval) * time.Second)
+			RetrySleepFn(time.Duration(interval) * time.Second)
 		}
 		err = operation()
 		if err == nil {

go/logic/migrator_test.go

@@ -18,6 +18,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 	"github.com/testcontainers/testcontainers-go"
@@ -378,6 +379,72 @@ func (suite *MigratorTestSuite) TestFoo() {
 	suite.Require().Equal("_testing_del", tableName)
 }
 
+func TestMigratorRetry(t *testing.T) {
+	oldRetrySleepFn := RetrySleepFn
+	defer func() { RetrySleepFn = oldRetrySleepFn }()
+
+	migrationContext := base.NewMigrationContext()
+	migrationContext.SetDefaultNumRetries(100)
+	migrator := NewMigrator(migrationContext, "1.2.3")
+
+	var sleeps = 0
+	RetrySleepFn = func(duration time.Duration) {
+		assert.Equal(t, 1*time.Second, duration)
+		sleeps++
+	}
+
+	var tries = 0
+	retryable := func() error {
+		tries++
+		if tries < int(migrationContext.MaxRetries()) {
+			return errors.New("Backoff")
+		}
+		return nil
+	}
+
+	result := migrator.retryOperation(retryable, false)
+	assert.NoError(t, result)
+	assert.Equal(t, sleeps, 99)
+	assert.Equal(t, tries, 100)
+}
+
+func TestMigratorRetryWithExponentialBackoff(t *testing.T) {
+	oldRetrySleepFn := RetrySleepFn
+	defer func() { RetrySleepFn = oldRetrySleepFn }()
+
+	migrationContext := base.NewMigrationContext()
+	migrationContext.SetDefaultNumRetries(100)
+	migrationContext.SetExponentialBackoffMaxInterval(42)
+	migrator := NewMigrator(migrationContext, "1.2.3")
+
+	var sleeps = 0
+	expected := []int{
+		1, 2, 4, 8, 16, 32, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
+		42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
+		42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
+		42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
+		42, 42, 42, 42, 42, 42,
+	}
+	RetrySleepFn = func(duration time.Duration) {
+		assert.Equal(t, time.Duration(expected[sleeps])*time.Second, duration)
+		sleeps++
+	}
+
+	var tries = 0
+	retryable := func() error {
+		tries++
+		if tries < int(migrationContext.MaxRetries()) {
+			return errors.New("Backoff")
+		}
+		return nil
+	}
+
+	result := migrator.retryOperationWithExponentialBackoff(retryable, false)
+	assert.NoError(t, result)
+	assert.Equal(t, sleeps, 99)
+	assert.Equal(t, tries, 100)
+}
+
 func TestMigrator(t *testing.T) {
 	suite.Run(t, new(MigratorTestSuite))
 }