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, 18 insertions, 0 deletions

diff --git a/lib/sqfs/data_writer.c b/lib/sqfs/data_writer.c
index 753e5d4..70fe884 100644
--- a/lib/sqfs/data_writer.c
+++ b/lib/sqfs/data_writer.c
@@ -78,6 +78,11 @@ static int grow_fragment_table(data_writer_t *data)
 	return 0;
 }
 
+static bool is_zero_block(unsigned char *ptr, size_t size)
+{
+	return ptr[0] == 0 && memcmp(ptr, ptr + 1, size - 1) == 0;
+}
+
 int data_writer_flush_fragments(data_writer_t *data)
 {
 	uint64_t offset;
@@ -115,6 +120,7 @@ int write_data_from_fd(data_writer_t *data, file_info_t *fi, int infd)
 	size_t diff;
 
 	fi->startblock = data->super->bytes_used;
+	fi->sparse = 0;
 
 	while (count != 0) {
 		diff = count > (uint64_t)data->super->block_size ?
@@ -126,6 +132,18 @@ int write_data_from_fd(data_writer_t *data, file_info_t *fi, int infd)
 		if ((size_t)ret < diff)
 			goto fail_trunc;
 
+		if (is_zero_block(data->block, diff)) {
+			if (diff < data->super->block_size) {
+				fi->fragment_offset = 0xFFFFFFFF;
+				fi->fragment = 0xFFFFFFFF;
+			} else {
+				fi->blocksizes[blk_idx++] = 0;
+			}
+			fi->sparse += diff;
+			count -= diff;
+			continue;
+		}
+
 		if (diff < data->super->block_size) {
 			if (data->frag_offset + diff > data->super->block_size) {
 				if (data_writer_flush_fragments(data))