incusd/storage/drivers/common: Truncate/Discard ahead of sparse write

As we use a sparse writer which will skip any zero blocks, we need to
discard (block) or truncate (file) prior to processing the data or we
risk having leftover data from a previous snapshot interfere with the
new state.

Closes #1264

Signed-off-by: Stéphane Graber <[email protected]>

internal/server/storage/drivers/generic_vfs.go

@@ -332,6 +332,12 @@ func genericVFSCreateVolumeFromMigration(d Driver, initVolume func(vol Volume) (
 
 		defer func() { _ = to.Close() }()
 
+		// Reset the disk.
+		err = clearDiskData(path, to)
+		if err != nil {
+			return err
+		}
+
 		// Setup progress tracker.
 		fromPipe := io.ReadCloser(conn)
 		if wrapper != nil {
@@ -787,6 +793,13 @@ func genericVFSBackupUnpack(d Driver, sysOS *sys.OS, vol Volume, snapshots []str
 						logMsg = "Unpacking custom block volume"
 					}
 
+					// Reset the disk.
+					err = clearDiskData(targetPath, to)
+					if err != nil {
+						return err
+					}
+
+					// Copy the data.
 					d.Logger().Debug(logMsg, logger.Ctx{"source": srcFile, "target": targetPath})
 					_, err = io.Copy(NewSparseFileWrapper(to), tr)
 					if err != nil {

internal/server/storage/drivers/utils.go

@@ -957,3 +957,38 @@ func roundAbove(above, val int64) int64 {
 
 	return rounded
 }
+
+// clearDiskData resets a disk file or device to a zero value.
+func clearDiskData(diskPath string, disk *os.File) error {
+	st, err := disk.Stat()
+	if err != nil {
+		return err
+	}
+
+	if linux.IsBlockdev(st.Mode()) {
+		// If dealing with a block device, discard its current content.
+		// This saves space and avoids issues with leaving zero blocks to their original value.
+		_, err = subprocess.RunCommand("blkdiscard", diskPath)
+		if err != nil {
+			return err
+		}
+	} else {
+		// Otherwise truncate the file.
+		err = disk.Truncate(0)
+		if err != nil {
+			return err
+		}
+
+		err = disk.Truncate(st.Size())
+		if err != nil {
+			return err
+		}
+
+		_, err = disk.Seek(0, 0)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}