Fork of a fork (nested fork) errors on read: LanceError(IO) Object not found

[milliondreams]

Summary

Building a fork from a fork session (a nested fork, fork.fork("y").build()) produces a fork
that cannot be read — querying it raises a LanceError(IO).

Environment

• uni-db 2.2.5 (PyPI), Python 3.12, Linux
• uni_db.Uni.in_memory()

Expected

A fork of a fork branches from the parent fork's visible state and can be read like any other fork —
the natural shape for tree-structured hypotheticals (MCTS rollouts, nested ABM ticks, nested
ASSUME).

Actual (2.2.5)

import uni_db

uni = uni_db.Uni.in_memory()
s = uni.session()
with s.tx() as tx:
    tx.execute("CREATE (:A {k: 1})-[:R {key: 11}]->(:B {k: 2})")
    tx.commit()

fork = s.fork("x").build()
print(fork.query("MATCH (a:A) RETURN count(a) AS c")[0]["c"])   # 1 — first-level fork is fine

nested = fork.fork("y").build()
print(nested.query("MATCH (a:A) RETURN count(a) AS c")[0]["c"])  # raises

Error:

uni_db.UniQueryError: Query error: Execution error: Failed to collect scan results:
Wrapped error: LanceError(IO): Generic N/A error: Cloned error: Cloned error:
LanceError(IO): Object [not found]

Standalone repro (only depends on uni-db)

"""Repro: a fork of a fork (nested fork) errors on read (uni-db 2.2.5). Depends only on uni-db.
    pytest test_fork_nested.py   # FAILS on 2.2.5
    python test_fork_nested.py   # PASS/FAIL summary; exit 1 if bug present
"""

import uni_db


def test_nested_fork_reads_inherited_data():
    uni = uni_db.Uni.in_memory()
    session = uni.session()
    with session.tx() as tx:
        tx.execute("CREATE (:A {k: 1})-[:R {key: 11}]->(:B {k: 2})")
        tx.commit()

    fork = session.fork("x").build()
    assert fork.query("MATCH (a:A) RETURN count(a) AS c")[0]["c"] == 1  # first-level fork is fine

    nested = fork.fork("y").build()
    try:
        count = nested.query("MATCH (a:A) RETURN count(a) AS c")[0]["c"]
    finally:
        del nested
        try:
            uni.drop_fork("y")
        except Exception:
            pass
        del fork
        try:
            uni.drop_fork("x")
        except Exception:
            pass

    assert count == 1, f"nested fork saw {count} nodes; expected 1"

Impact

Tree-structured fork usage is blocked — e.g. MCTS rollouts that fork per node down a path, or nested
hypothetical scenarios. Single-level forks work; only nesting fails.

[milliondreams]

Fixed. This was the same root cause as #106 (resolved in commit 9b787fcd8), even though that commit was framed as a vector/FTS fix.

Why one fix covered both: a fork is a Lance branch whose reads traverse a base_paths chain (child -> parent -> main). Lance resolves data fragments across that chain, but it does not resolve a scalar (BTree) index's _indices/<id>/page_lookup.lance metadata across more than one level. So any filtered branch scan that engages scalar-index pushdown breaks at the 2nd fork level — that includes vector/FTS queries (#106) and a plain MATCH (a:A) ..., where the :A label becomes a predicate. The fix disables scalar-index pushdown on every branch scan (execute_branch_scan); it is result-set neutral, since the _version <= HWM AND _deleted = false (+ user) predicate still matches the same rows via a sequential scan.

I verified the exact repro from this issue (schemaless (:A)-[:R {key: 11}]->(:B), fork("x") -> fork("y"), MATCH (a:A) RETURN count(a)) now returns 1 instead of raising LanceError(IO): Object not found, and added it as a regression test (issue_103_nested_fork_schemaless_edge_reads_inherited) that fails before 9b787fcd8 and passes after.

Closing as fixed.