feat(handler) : add format handler for BTRFS stream

[Corv0O]

#1372

A btrfs stream file is a binary format produced by btrfs send to transfer filesystem snapshots. It encodes filesystem operations as a sequence of TLV-based commands that btrfs receive replays to reconstruct the original snapshot.

Format

Stream header :

Magic : 62 74 72 66 73 2d 73 74 72 65 61 6d 00 (btrfs-stream\0)
Version : 1 or 2

After the stream header, the format consists of a sequence of commands replayed by btrfs receive. Each command has a fixed 10-byte header:

Command header :

Length of data : 4 bytes
Type of the command : 2 bytes
Crc32c : 4 bytes (use Castagnoli reflected polynomial, seed = 0x0, no final XOR)

Command payload (TLV attributes) :

Type : 2 bytes (PATH, UUID, ...)
Length : 2 bytes
Value : [length] bytes

The format has two modes:

• Full sends: complete snapshot serialization with all the commands to recreate the file systems
• Incremental sends: diff between two snapshots

This handler only handle Full send file

Sample :

setup

truncate -s 2G /tmp/btrfs.img && mkfs.btrfs /tmp/btrfs.img
sudo mount -o loop /tmp/btrfs.img /mnt/source
sudo btrfs subvolume create /mnt/source/vol
echo "hello" > /mnt/source/vol/hello.txt

full send

sudo btrfs subvolume snapshot -r /mnt/source/vol /mnt/source/snap
sudo btrfs send /mnt/source/snap -f sample.bin

you can verify the content of the btfrs stream with this command
btrfs receive --dump -f sample.bin

[References]

https://btrfs.readthedocs.io/en/latest/dev/dev-send-stream.html#btrfs-send-c-encoded-write-25
https://formats.kaitai.io/btrfs_stream/
https://archive.kernel.org/oldwiki/btrfs.wiki.kernel.org/index.php/Design_notes_on_Send/Receive.html

[qkaiser]

Don't forget unit tests. Add integration samples that exercise:

• different types of compression
• with prefix and suffix data

[qkaiser]

@Corv0O don't forget to rebase on upstream main for CI checks to pass

[qkaiser]

A few changes and we're good to go.