incusd/storage/generic: Use proper custom volume size for backup if possible

Signed-off-by: Benjamin Somers <benjamin.somers@imt-atlantique.fr>

internal/instancewriter/instance_raw_writer.go

@@ -25,6 +25,6 @@ func (crw *InstanceRawWriter) WriteFile(name string, srcPath string, fi os.FileI
 
 // WriteFileFromReader streams a file into the target file.
 func (crw *InstanceRawWriter) WriteFileFromReader(src io.Reader, fi os.FileInfo) error {
-	_, err := io.Copy(crw.rawWriter, src)
+	_, err := io.CopyN(crw.rawWriter, src, fi.Size())
 	return err
 }

internal/server/storage/drivers/generic_vfs.go

@@ -8,6 +8,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -602,9 +603,15 @@ func genericVFSBackupVolume(d Driver, vol Volume, writer instancewriter.Instance
 
 				defer func() { _ = from.Close() }()
 
+				var fileSize int64
+				fileSize, err = strconv.ParseInt(vol.config["size"], 10, 64)
+				if err != nil {
+					fileSize = blockDiskSize
+				}
+
 				fi := instancewriter.FileInfo{
 					FileName:    name,
-					FileSize:    blockDiskSize,
+					FileSize:    fileSize,
 					FileMode:    0o600,
 					FileModTime: time.Now(),
 				}