Add support for packing sparse files

This commit adds support for packing sparse files into squashfs images
as follows:

 - In the data writer: simply detect zero blocks and write a zero to the
   block size field and don't emit any data. Record the number of bytes
   saved this way. For fragments, set the fragment offset to invalid.
 - In the inode writer: write out the number of bytes saved for sparse
   files. If there should be a fragment but there is none, append a block
   count of 0.
 - In the data reader: if the block size is 0, read nothing from disk and
   emit an empty block. Do the same if the fragment is missing.
 - In the inode reader: restore the number of bytes saved for sparse files.

The sparse files can be packed and unpacked, but the unpacking will not
create sparse files for now.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/fstree.h b/include/fstree.h
index 040136e..6c2ec0b 100644
--- a/include/fstree.h
+++ b/include/fstree.h
@@ -58,6 +58,9 @@ struct file_info_t {
 
 	uint64_t size;
 
+	/* Number of bytes not written to disk because they are 0 */
+	uint64_t sparse;
+
 	/* Absolute position of the first data block. */
 	uint64_t startblock;