Cache size of last ReadAt on S3 for performance

Each obj.Stat() call adds another request to the S3 endpoint
for some commands a lot of ReadAt calls are made for the same
object in S3. This patch essentially cuts the number of calls
to S3 in this case in half. Speeding up the progress and lowering
costs to S3.

src/restic/backend/s3/s3.go

@@ -24,6 +24,8 @@ type s3 struct {
 	connChan     chan struct{}
 	bucketname   string
 	prefix       string
+	cacheObjName string
+	cacheSize    int64
 }
 
 // Open opens the S3 backend at bucket and region. The bucket is created if it
@@ -139,6 +141,7 @@ func (be *s3) Load(h restic.Handle, length int, offset int64) (io.ReadCloser, er
 	}
 
 	var obj *minio.Object
+	var size int64
 
 	objName := be.s3path(h)
 
@@ -186,20 +189,27 @@ 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 {
 		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
+	}
 
-	if offset > info.Size {
+	if offset > size {
 		_ = obj.Close()
 		return nil, errors.New("offset larger than file size")
 	}
 
 	l := int64(length)
-	if offset+l > info.Size {
-		l = info.Size - offset
+	if offset+l > size {
+		l = size - offset
 	}
 
 	buf := make([]byte, l)