fix: gc works unexpeted in high concurrent

When acceld converts multiple images with large size differences, for example : alpine and WordPress.
Obviously alpine will finish the task first, and WordPress is still in the pulling phase.
If alpine triggers gc(decide by configuration), GC will clear the blobs of WordPress,
because the lease of WordPress blob had cached in lease and only used once(in commit).
To solve this problem, now gc will wait all converting tasks completed.

fix #174

Signed-off-by: Yadong Ding <ding_yadong@foxmail.com>

pkg/adapter/adapter.go

@@ -113,9 +113,12 @@ func (adp *LocalAdapter) Convert(ctx context.Context, source string) error {
 	if err = adp.content.NewRemoteCache(cacheRef); err != nil {
 		return err
 	}
+	adp.content.GcMutex.RLock()
 	if _, err = adp.cvt.Convert(ctx, source, target, cacheRef); err != nil {
+		adp.content.GcMutex.RUnlock()
 		return err
 	}
+	adp.content.GcMutex.RUnlock()
 	if err := adp.content.GC(ctx); err != nil {
 		return err
 	}

pkg/content/content.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"strconv"
+	"sync"
 	"time"
 
 	ctrcontent "github.com/containerd/containerd/content"
@@ -47,6 +48,8 @@ type Content struct {
 	lm leases.Manager
 	// gcSingleflight help to resolve concurrent gc
 	gcSingleflight *singleflight.Group
+	// GcMutex works between gc and convert
+	GcMutex *sync.RWMutex
 	// lc cache the used count and reference order of lease
 	lc *leaseCache
 	// store is the local content store wrapped inner db
@@ -95,6 +98,7 @@ func NewContent(contentDir string, databaseDir string, threshold string, useRemo
 		db:             db,
 		lm:             metadata.NewLeaseManager(db),
 		gcSingleflight: &singleflight.Group{},
+		GcMutex:        &sync.RWMutex{},
 		lc:             lc,
 		store:          db.ContentStore(),
 		threshold:      int64(t),
@@ -137,6 +141,13 @@ func (content *Content) GC(ctx context.Context) error {
 	// if the local content size over eighty percent of threshold, gc start
 	if size > (content.threshold*int64(80))/100 {
 		if _, err, _ := content.gcSingleflight.Do(accelerationServiceNamespace, func() (interface{}, error) {
+			content.GcMutex.Lock()
+			defer content.GcMutex.Unlock()
+			// recalculate the local cache size
+			size, err := content.Size()
+			if err != nil {
+				return nil, err
+			}
 			return nil, content.garbageCollect(ctx, size-(content.threshold*int64(80))/100)
 		}); err != nil {
 			return err