store/tikv/gcworker: add manual gc back for some kv api users (#6123)

* use fake gc_worker instead of bringing funcs out of gc_worker
* fix comments

store/tikv/gcworker/gc_worker.go

@@ -666,6 +666,16 @@ func (w *GCWorker) genNextGCTask(bo *tikv.Backoffer, safePoint uint64, key kv.Ke
 }
 
 func (w *GCWorker) doGC(ctx context.Context, safePoint uint64) error {
+	concurrency, err := w.loadGCConcurrencyWithDefault()
+	if err != nil {
+		log.Errorf("[gc worker] %s failed to load gcConcurrency, err %s", w.uuid, err)
+		concurrency = gcDefaultConcurrency
+	}
+
+	return w.doGCInternal(ctx, safePoint, concurrency)
+}
+
+func (w *GCWorker) doGCInternal(ctx context.Context, safePoint uint64, concurrency int) error {
 	gcWorkerCounter.WithLabelValues("do_gc").Inc()
 
 	err := w.saveSafePoint(w.store.GetSafePointKV(), tikv.GcSavedSafePoint, safePoint)
@@ -676,12 +686,6 @@ func (w *GCWorker) doGC(ctx context.Context, safePoint uint64) error {
 	// Sleep to wait for all other tidb instances update their safepoint cache.
 	time.Sleep(gcSafePointCacheInterval)
 
-	concurrency, err := w.loadGCConcurrencyWithDefault()
-	if err != nil {
-		log.Errorf("[gc worker] %s failed to load gcConcurrency, err %s", w.uuid, err)
-		concurrency = gcDefaultConcurrency
-	}
-
 	log.Infof("[gc worker] %s start gc, concurrency %v, safePoint: %v.", w.uuid, concurrency, safePoint)
 	startTime := time.Now()
 	var successRegions int32
@@ -911,6 +915,24 @@ func (w *GCWorker) saveValueToSysTable(key, value string, s session.Session) err
 	return errors.Trace(err)
 }
 
+// RunGCJob sends GC command to KV. it is exported for kv api, do not use it with GCWorker at the same time.
+func RunGCJob(ctx context.Context, s tikv.Storage, safePoint uint64, identifier string) error {
+	gcWorker := &GCWorker{
+		store: s,
+		uuid:  identifier,
+	}
+
+	err := gcWorker.resolveLocks(ctx, safePoint)
+	if err != nil {
+		return errors.Trace(err)
+	}
+	err = gcWorker.doGCInternal(ctx, safePoint, gcDefaultConcurrency)
+	if err != nil {
+		return errors.Trace(err)
+	}
+	return nil
+}
+
 // MockGCWorker is for test.
 type MockGCWorker struct {
 	worker *GCWorker