mater: CARv2 archive not padded to the power of 2?

[th7nder]

➜  polka-storage git:(feat/410/generate-porep) ✗ cat test-data.txt
12345678
➜  polka-storage git:(feat/410/generate-porep) ✗ ./target/debug/mater-cli convert test-data.txt
Converted test-data.txt and saved the CARv2 file at test-data.car with a CID of bafkreibggtbqs74y4nugl4gfoiajyt75omywxsfyrth6rumwv427i3rdsq
➜  polka-storage git:(feat/410/generate-porep) ✗ ./target/debug/polka-storage-provider utils commp test-data.car
Piece commitment CID: baga6ea4seaqbnqij5xlom6q37ebwxnc3com4dr3e36wc6ch2ld7pbhfog3lvwky
➜  polka-storage git:(feat/410/generate-porep) ✗ ./target/debug/polka-storage-provider utils po-rep test-data.car baga6ea4seaqbnqij5xlom6q37ebwxnc3com4dr3e36wc6ch2ld7pbhfog3lvwky
thread 'main' panicked at /home/th7nder/workspace/eiger/polka-storage/lib/polka-storage-proofs/src/porep/mod.rs:49:81:
called `Result::unwrap()` on an `Err` value: Bit-padded piece size must be a power of 2 (PaddedBytesAmount(228))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

➜  polka-storage git:(feat/410/generate-porep) ✗ xxd test-data.txt
00000000: 3132 3334 3536 3738 0a                   12345678.
➜  polka-storage git:(feat/410/generate-porep) ✗ ls -lh test-data.car 
-rw-r--r-- 1 th7nder th7nder 226 Sep 30 19:21 test-data.car
➜  polka-storage git:(feat/410/generate-porep) ✗ ls -lh test-data.txt
-rw-r--r-- 1 th7nder th7nder 9 Sep 30 19:14 test-data.txt

Mater creates a 226 bytes long CARv2 archive, out of 9 bytes file.
Moreover, rust reads it as 228 bytes.

It should be a power of two, because: https://github.com/filecoin-project/rust-fil-proofs/blob/5a0523ae1ddb73b415ce2fa819367c7989aaf73f/filecoin-proofs/src/api/mod.rs#L471

Reproducible on feat/410/generate-porep branch.

[jmg-duarte]

We figured out the padding isn't mater's issue and it's added outside of the file by commp and friends