introduce RWLock for caching obj.Stat()

src/restic/backend/s3/s3.go

@@ -7,6 +7,7 @@ import (
 	"path"
 	"restic"
 	"strings"
+	"sync"
 
 	"restic/backend"
 	"restic/errors"
@@ -24,8 +25,8 @@ type s3 struct {
 	connChan     chan struct{}
 	bucketname   string
 	prefix       string
-	cacheObjName string
-	cacheSize    int64
+	cacheMutex   sync.RWMutex
+	cacheObjSize map[string]int64
 }
 
 // Open opens the S3 backend at bucket and region. The bucket is created if it
@@ -38,7 +39,12 @@ func Open(cfg Config) (restic.Backend, error) {
 		return nil, errors.Wrap(err, "minio.New")
 	}
 
-	be := &s3{client: client, bucketname: cfg.Bucket, prefix: cfg.Prefix}
+	be := &s3{
+		client:       client,
+		bucketname:   cfg.Bucket,
+		prefix:       cfg.Prefix,
+		cacheObjSize: make(map[string]int64),
+	}
 
 	tr := &http.Transport{MaxIdleConnsPerHost: connLimit}
 	client.SetCustomTransport(tr)
@@ -189,17 +195,20 @@ func (be *s3) Load(h restic.Handle, length int, offset int64) (io.ReadCloser, er
 	}()
 
 	// otherwise use a buffer with ReadAt
-	if be.cacheObjName == objName {
-		size = be.cacheSize
-	} else {
+	be.cacheMutex.RLock()
+	size, cacheHit := be.cacheObjSize[objName]
+	be.cacheMutex.RUnlock()
+
+	if !cacheHit {
 		info, err := obj.Stat()
 		if err != nil {
 			_ = obj.Close()
 			return nil, errors.Wrap(err, "obj.Stat")
 		}
 		size = info.Size
-		be.cacheObjName = objName
-		be.cacheSize = size
+		be.cacheMutex.Lock()
+		be.cacheObjSize[objName] = size
+		be.cacheMutex.Unlock()
 	}
 
 	if offset > size {