fix: use cut-over-lock-timeout for instant DDL (#1468)

* fix: use cut-over-lock-timeout for instant DDL

Addresses #1386

by reusing the cut-over-lock-timeout from the cutover code.
The lock wait timeout in the original code is actually set to double
the setting, so we keep that consistent.

* add new usage to the arg description

* Rename variable `query` to `lockTimeoutQuery`

---------

Co-authored-by: meiji163 <meiji163@github.com>

Author: artemvovk

Diff for: go/cmd/gh-ost/main.go

@@ -107,7 +107,7 @@ func main() {
 	chunkSize := flag.Int64("chunk-size", 1000, "amount of rows to handle in each iteration (allowed range: 10-100,000)")
 	dmlBatchSize := flag.Int64("dml-batch-size", 10, "batch size for DML events to apply in a single transaction (range 1-100)")
 	defaultRetries := flag.Int64("default-retries", 60, "Default number of retries for various operations before panicking")
-	cutOverLockTimeoutSeconds := flag.Int64("cut-over-lock-timeout-seconds", 3, "Max number of seconds to hold locks on tables while attempting to cut-over (retry attempted when lock exceeds timeout)")
+	cutOverLockTimeoutSeconds := flag.Int64("cut-over-lock-timeout-seconds", 3, "Max number of seconds to hold locks on tables while attempting to cut-over (retry attempted when lock exceeds timeout) or attempting instant DDL")
 	niceRatio := flag.Float64("nice-ratio", 0, "force being 'nice', imply sleep time per chunk time; range: [0.0..100.0]. Example values: 0 is aggressive. 1: for every 1ms spent copying rows, sleep additional 1ms (effectively doubling runtime); 0.7: for every 10ms spend in a rowcopy chunk, spend 7ms sleeping immediately after")
 
 	maxLagMillis := flag.Int64("max-lag-millis", 1500, "replication lag at which to throttle operation")

Diff for: go/logic/applier.go

@@ -255,6 +255,15 @@ func (this *Applier) ValidateOrDropExistingTables() error {
 func (this *Applier) AttemptInstantDDL() error {
 	query := this.generateInstantDDLQuery()
 	this.migrationContext.Log.Infof("INSTANT DDL query is: %s", query)
+
+	// Reuse cut-over-lock-timeout from regular migration process to reduce risk
+	// in situations where there may be long-running transactions.
+	tableLockTimeoutSeconds := this.migrationContext.CutOverLockTimeoutSeconds * 2
+	this.migrationContext.Log.Infof("Setting LOCK timeout as %d seconds", tableLockTimeoutSeconds)
+	lockTimeoutQuery := fmt.Sprintf(`set /* gh-ost */ session lock_wait_timeout:=%d`, tableLockTimeoutSeconds)
+	if _, err := this.db.Exec(lockTimeoutQuery); err != nil {
+		return err
+	}
 	// We don't need a trx, because for instant DDL the SQL mode doesn't matter.
 	_, err := this.db.Exec(query)
 	return err