Corrupting flac files

[phayes]

Reproducer

Summary

I've been running into issues with lofty corrupting flac files in the following situations:

1. It's a "bare" flac file (eg STREAMINFO is the final block, no tag metadata, no SEEKTABLE),

2. There's no padding or metadata, just a STREAMINFO and SEEKTABLE.

I'm not entirely sure of my understanding of the bug. I've laid out below what I think is the problem, but I dont have enough confidence in my understanding to offer a PR:

1. Right here (

   lofty-rs/lofty/src/flac/write.rs

   Line 83 in 2727c5c

   last_block_info = (block.byte, (end - start) as usize, end as usize)

   ). last_block_info is later used as (header_first_byte, header_offset, payload_end).
   You’re overwriting the header offset with (end - start) (the length), and the “header first byte” with block.byte (which is just the value of the first header byte, not an offset). This causes file_bytes[last_block_info.1] = first_byte; (

   lofty-rs/lofty/src/flac/write.rs

   Line 127 in 2727c5c

   file_bytes[last_block_info.1] = first_byte;

   ) to write the “is_last” flag into a length-sized index. The causes file corruption when we need to adding a padding block.

2. If there are no blocks to remove, we corrupt the file right here (

   lofty-rs/lofty/src/flac/write.rs

   Line 152 in 2727c5c

   file_bytes.splice(0..0, comment_blocks);

   ) file_bytes.splice(0..0, comment_blocks); overwrites the first 4 fLaC bytes in the file.

Expected behavior

No response

Assets

No response

[phayes]

I've opened a PR for this here: #550 - I would appreciate a careful review since I'm not 100% confident I'm following all the various accouting being done to keep track of things.