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, 10 insertions, 2 deletions

diff --git a/lib/sqfs/write_inode.c b/lib/sqfs/write_inode.c
index fd14913..82663e7 100644
--- a/lib/sqfs/write_inode.c
+++ b/lib/sqfs/write_inode.c
@@ -86,7 +86,7 @@ int meta_writer_write_inode(fstree_t *fs, id_table_t *idtbl, meta_writer_t *im,
 		fi = node->data.file;
 
 		if (fi->startblock > 0xFFFFFFFFUL || fi->size > 0xFFFFFFFFUL ||
-		    hard_link_count(node) > 1) {
+		    hard_link_count(node) > 1 || fi->sparse > 0) {
 			type = SQFS_INODE_EXT_FILE;
 		}
 	}
@@ -185,7 +185,7 @@ int meta_writer_write_inode(fstree_t *fs, id_table_t *idtbl, meta_writer_t *im,
 		sqfs_inode_file_ext_t ext = {
 			.blocks_start = htole64(fi->startblock),
 			.file_size = htole64(fi->size),
-			.sparse = htole64(0),
+			.sparse = htole64(fi->sparse),
 			.nlink = htole32(hard_link_count(node)),
 			.fragment_idx = htole32(0xFFFFFFFF),
 			.fragment_offset = htole32(0xFFFFFFFF),
@@ -298,5 +298,13 @@ out_file_blocks:
 		if (meta_writer_append(im, &bs, sizeof(bs)))
 			return -1;
 	}
+
+	if ((fi->size % fs->block_size) != 0 &&
+	    (fi->fragment == 0xFFFFFFFF || fi->fragment_offset == 0xFFFFFFFF)) {
+		bs = htole32(0);
+
+		if (meta_writer_append(im, &bs, sizeof(bs)))
+			return -1;
+	}
 	return 0;
 }