OFS_DELTA base distance

Error message

OFS_DELTA base distance {base_distance} is invalid for pack offset {pack_offset}

What it means

When resolving pack entries by offset, gix-pack validates each OFS_DELTA entry: the base distance must resolve to a valid base pack offset relative to the delta's own offset (`Header::verified_base_pack_offset`). If verification fails, the data is structurally inconsistent and iteration aborts with an `InvalidData` io error wrapped as `Error::Io` with the message 'invalid OFS_DELTA base distance'.

Solutions

Verify the pack's integrity (trailer SHA/checksum) and re-fetch or re-clone the repository if it fails.
Run `gix pack` verify/index operations on the pack to locate the corrupt entry.
If handling untrusted packs, catch the error and reject the pack instead of panicking or proceeding.

Defensive patterns

Strategy: try-catch

Try / catch

match pack_file::iter::FromOffsets::new(...) {
    Err(e) => /* reject the pack: check e for 'invalid OFS_DELTA base distance' and verify the pack checksum */,
    ok => /* proceed */,
}

Prevention

Verify pack checksums before delta iteration.
Re-fetch or regenerate packs that fail structural validation.
Never iterate raw pack bytes at hand-computed offsets; use the crate's parsed headers.

When it happens

Trigger: Iterating entries via `gix_pack::data::File::stream_from`/offset iteration (from_offsets) over a pack containing an OFS_DELTA whose encoded distance points before the start of the pack or is otherwise impossible.

Common situations: Corrupted or truncated packfiles on disk; manually edited or fuzzed pack data; reading bytes at a wrong offset so a delta header is misparsed.

Related errors

AI-assisted analysis of GitoxideLabs/gitoxide@e73179060b (2026-09-08). Data as JSON: /api/errors/bd4c1a76746aff8d. Report an issue: GitHub.

Appendix: source

Thrown at gix-pack/src/cache/delta/from_offsets.rs:110

            use crate::data::entry::Header::*;
            match entry.header {
                Tree | Blob | Commit | Tag => {
                    tree.add_root(pack_offset, data)?;
                }
                RefDelta { base_id } => {
                    resolve_in_pack_id(base_id.as_ref())
                        .ok_or(Error::UnresolvedRefDelta { id: base_id })
                        .and_then(|base_pack_offset| {
                            tree.add_child(base_pack_offset, pack_offset, data).map_err(Into::into)
                        })?;
                }
                OfsDelta { base_distance } => {
                    let Some(base_pack_offset) =
                        crate::data::entry::Header::verified_base_pack_offset(pack_offset, base_distance)
                    else {
                        return Err(Error::Io {
                            source: io::Error::new(
                                io::ErrorKind::InvalidData,
                                format!(
                                    "OFS_DELTA base distance {base_distance} is invalid for pack offset {pack_offset}"
                                ),
                            ),
                            message: "invalid OFS_DELTA base distance",
                        });
                    };
                    tree.add_child(base_pack_offset, pack_offset, data)?;
                }
            }
            progress.inc();
            if idx % 10_000 == 0 && should_interrupt.load(Ordering::SeqCst) {
                return Err(Error::Interrupted);
            }
        }

        progress.show_throughput(then);

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