[Rust] 'Unaligned' error when reading a newly created buffer

[sergiimk]

After upgrading to flatbuffers = "0.8.0" and flatc 1.12.0 (master) I started getting occasional "Unaligned" errors when trying to read back a flatbuffer that was just written.

Example schema:

table MetadataBlock {
  block_hash: [ubyte];
}

Example code:

mod fb_generated;
use fb_generated::*;

fn main() {
    let mut fb = flatbuffers::FlatBufferBuilder::new_with_capacity(33);  // Happens without odd capacity on some of my objects too
    let block_hash: Vec<u8> = vec![0, 10];
    let block_hash_offset = fb.create_vector(&block_hash);
    let mut builder = MetadataBlockBuilder::new(&mut fb);
    builder.add_block_hash(block_hash_offset);
    let offset = builder.finish();
    fb.finish(offset, None);
    let (buf, head) = fb.collapse();
    println!("{} {:?}", head, buf);

    flatbuffers::root::<MetadataBlock>(&buf[head..]).unwrap();
}

Produces:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Unaligned { position: 0, unaligned_type: "u32", error_trace: ErrorTrace([]) }', src/main.rs:15:54

The previous version flatbuffers = "0.7.0" had no such issue. Albeit flatbuffers::get_root was not returning a Result before all data was read correctly.

Is it reasonable to expect fb itself to produce a properly aligned data?

[CasperN]

Yes its reasonable to expect that. Its happening because the flatbufferbuilder builds buffers in a Vec<u8> which, unlike C++'s std::vector<uint_8>, is only 1byte aligned. We've historically been producing unaligned data (see issues related to undefined behavior and MIRI), its just now explicit. Getting the builder to be based on a Vec<u128> (which will be 16byte aligned) is next up on my list