GitoxideLabs/gitoxide · warning

at least one delta chain item

Error message

at least one delta chain item

What it means

A `expect()` panic asserting `last_result_size` is set after the delta application loop in `resolve_deltas`. The loop runs once per chain item, and the empty-chain case already returned earlier, so at least one delta must have been applied. A `None` here means the chain was mutated mid-flight — a decoder invariant violation.

Solutions

  1. Update gix-pack to a current release
  2. Ensure packs are not truncated/corrupted before decoding (run pack verification)
  3. File a bug with a reproducing pack if ever observed
Defensive patterns

Strategy: try-catch

Validate before calling

// validate the pack before decoding: open via gix_pack::data::File::at(path)? and run verify()

Try / catch

match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| pack.decode_entry(id, &mut out, ...))) { Ok(v) => v, Err(_) => /* fallback: read object via git or another pack */ }

Prevention

When it happens

Trigger: Only via a code defect: `decode_entry` -> `resolve_deltas` reaching the post-loop code with an empty delta chain, which the earlier `if chain.is_empty()` early-return should prevent.

Common situations: Essentially unreachable for library users; appears when modifying `resolve_deltas` or fuzzing deeply corrupted packs that defeat the early return.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of GitoxideLabs/gitoxide@e73179060b (2026-09-08). Data as JSON: /api/errors/cd50128c74f7522f. Report an issue: GitHub.

Appendix: source

Thrown at gix-pack/src/data/file/decode/entry.rs:436

            delta_idx,
            Delta {
                data,
                base_size,
                result_size,
                ..
            },
        ) in chain.into_iter().rev().enumerate()
        {
            let data = &mut instructions[data];
            if delta_idx + 1 == chain_len {
                last_result_size = Some(result_size);
            }
            delta::apply(&source_buf[..base_size], &mut target_buf[..result_size], data)?;
            // use the target as source for the next delta
            std::mem::swap(&mut source_buf, &mut target_buf);
        }

        let last_result_size = last_result_size.expect("at least one delta chain item");
        // uneven chains leave the target buffer after the source buffer
        // FIXME(Performance) If delta-chains are uneven, we know we will have to copy bytes over here
        //      Instead we could use a different start buffer, to naturally end up with the result in the
        //      right one.
        //      However, this is a bit more complicated than just that - you have to deal with the base
        //      object, which should also be placed in the second buffer right away. You don't have that
        //      control/knowledge for out-of-pack bases, so this is a special case to deal with, too.
        //      Maybe these invariants can be represented in the type system though.
        if chain_len % 2 == 1 {
            // this seems inverted, but remember: we swapped the buffers on the last iteration
            target_buf[..last_result_size].copy_from_slice(&source_buf[..last_result_size]);
        }
        debug_assert!(out.len() >= last_result_size);
        out.truncate(last_result_size);

        let object_kind = object_kind.expect("a base object as root of any delta chain that we are here to resolve");
        let consumed_input = consumed_input.expect("at least one decompressed delta object");
        cache.put(

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