fix: check blob exist before copying layers samller than chunk size (goharbor#21883)

`copyBlobByChunk()` should like `copyBlob()`, first try to mount an
exists layer, if not mounted or exist, then copy the layer monolithic
or by chunks.

Signed-off-by: Bin Liu <[email protected]>
Signed-off-by: Bin Liu <[email protected]>

Author: liubin

src/controller/replication/transfer/image/transfer.go

@@ -403,11 +403,6 @@ func (t *transfer) copyBlobByMonolithic(srcRepo, dstRepo, digest string, sizeFro
 // copyBlobByChunk copy blob by chunk with specified start and end range.
 // The <range> refers to the byte range of the chunk, and MUST be inclusive on both ends. The first chunk's range MUST begin with 0.
 func (t *transfer) copyBlobByChunk(srcRepo, dstRepo, digest string, sizeFromDescriptor int64, start, end *int64, location *string, speed int32) error {
-	// fallback to copy by monolithic if the blob size is equal or less than chunk size.
-	if sizeFromDescriptor <= replicationChunkSize {
-		return t.copyBlobByMonolithic(srcRepo, dstRepo, digest, sizeFromDescriptor, speed)
-	}
-
 	mounted, err := t.tryMountBlob(srcRepo, dstRepo, digest)
 	if err != nil {
 		return err
@@ -417,6 +412,11 @@ func (t *transfer) copyBlobByChunk(srcRepo, dstRepo, digest string, sizeFromDesc
 		return nil
 	}
 
+	// fallback to copy by monolithic if the blob size is equal or less than chunk size.
+	if sizeFromDescriptor <= replicationChunkSize {
+		return t.copyBlobByMonolithic(srcRepo, dstRepo, digest, sizeFromDescriptor, speed)
+	}
+
 	// end range should equal (blobSize - 1)
 	endRange := sizeFromDescriptor - 1
 	for {