Fails with sqlalchemy `KeyError: "Deferred loader for attribute 'id' failed to populate correctly"`

[ddorian]

Hi,

Here is a simple script to reproduce the bug:

# /// script
# requires-python = "==3.14.6"
# dependencies = [
#     "cinderx",
#     "sqlalchemy>=2.0",
# ]
# ///
"""Minimal reproducer for a CinderX (Meta's CPython JIT, alpha) miscompilation
that *silently* breaks SQLAlchemy 2.0.

    uv run cinderx_sqlalchemy_repro.py

What it does: against an in-memory SQLite database, in a loop, INSERT a row,
commit (which expires the ORM object's attributes), then read back the primary
key `.id`. That read triggers a "deferred load" SELECT to repopulate the
expired attribute. With `cinderx.jit.auto()` enabled, that reload path is
miscompiled and the value fails to populate, raising:

    KeyError: "Deferred loader for attribute 'id' failed to populate correctly"

Run the exact same loop with the JIT off (`NO_JIT=1`) and it completes cleanly,
proving the fault is the JIT, not the SQLAlchemy usage. Needs nothing external.

Env knobs:
    NO_JIT=1        run the control: do NOT enable the CinderX JIT
    ITERATIONS=N    loop count (default 3000)
"""

import os
import sys

# The JIT must be enabled BEFORE SQLAlchemy is imported so its bytecode becomes
# eligible for compilation. This is the *only* difference between the failing
# and passing runs.
ENABLE_JIT = os.environ.get("NO_JIT") != "1"
if ENABLE_JIT:
    import cinderx.jit

    cinderx.jit.auto()

import sqlalchemy as sa
from sqlalchemy.orm import DeclarativeBase, Mapped, Session, mapped_column
from sqlalchemy.pool import StaticPool


class Base(DeclarativeBase):
    pass


class Widget(Base):
    __tablename__ = "cinderx_repro_widget"

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(sa.String(50))


def main() -> int:
    iterations = int(os.environ.get("ITERATIONS", "3000"))
    jit_on: bool = ENABLE_JIT and cinderx.jit.is_enabled()

    # In-memory SQLite: keep ONE shared connection (StaticPool) so the table
    # persists across the per-iteration sessions.
    engine = sa.create_engine(
        "sqlite://", poolclass=StaticPool, connect_args={"check_same_thread": False}
    )
    # drop_all is a no-op on the fresh in-memory DB, but running it first warms
    # the code path that makes the JIT miscompile surface as the documented
    # "Deferred loader ... failed to populate correctly" rather than a FlushError.
    Base.metadata.drop_all(engine)
    Base.metadata.create_all(engine)

    failures = 0
    first_failure_at: int | None = None

    print(f"JIT enabled: {jit_on}; iterations: {iterations}")
    for i in range(iterations):
        try:
            with Session(engine) as session:
                w = Widget(name="x")
                session.add(w)
                session.commit()  # expire_on_commit=True -> all attrs expired
                _ = w.id  # deferred-reload SELECT -> the miscompiled path
        except Exception as exc:  # noqa: BLE001 - we are counting any failure
            failures += 1
            if first_failure_at is None:
                first_failure_at = i
                print(f"  first failure at iter {i}: {type(exc).__name__}: {exc}")
        if i % 1000 == 0:
            print(f"  iter {i}: failures so far = {failures}")

    engine.dispose()

    print()
    print(f"JIT enabled : {jit_on}")
    print(f"iterations  : {iterations}")
    print(f"failures    : {failures}")
    if failures:
        print(
            f"first failed: iter {first_failure_at}\n"
            "=> BUG REPRODUCED: deferred primary-key reload corrupted under the CinderX JIT."
        )
        return 1
    print("=> clean: no failures (expected when NO_JIT=1).")
    return 0


if __name__ == "__main__":
    sys.exit(main())

[ddorian]

A smaller script:

# /// script
# requires-python = "==3.14.6"
# dependencies = [
#     "cinderx",
#     "sqlalchemy>=2.0",
# ]
# ///
"""Minimal reproducer for a CinderX (Meta's CPython JIT, alpha) miscompilation
that silently breaks SQLAlchemy 2.0.

    uv run cinderx_sqlalchemy_repro.py

INSERT a row, commit (expiring the ORM attributes), then read back the primary
key `.id`. That read triggers a deferred-load SELECT to repopulate it; with
CinderX's auto-mode on, the value fails to populate and reading `.id` raises:

    KeyError: "Deferred loader for attribute 'id' failed to populate correctly"

A single operation is enough -- the fault is CinderX's interpreter tier, not
JIT codegen (it reproduces with zero functions JIT-compiled).
"""

import cinderx.jit

cinderx.jit.auto()

import sqlalchemy as sa
from sqlalchemy.orm import DeclarativeBase, Mapped, Session, mapped_column
from sqlalchemy.pool import StaticPool


class Base(DeclarativeBase):
    pass


class Widget(Base):
    __tablename__ = "cinderx_repro_widget"

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(sa.String(50))


engine = sa.create_engine(
    "sqlite://", poolclass=StaticPool, connect_args={"check_same_thread": False}
)
# drop_all is a no-op on the fresh in-memory DB, but running it first warms the
# code path so the miscompile surfaces as the documented "Deferred loader ..."
# KeyError rather than a FlushError (same root cause: the PK is silently dropped).
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)

with Session(engine) as session:
    w = Widget(name="x")
    session.add(w)
    session.commit()  # expire_on_commit=True -> all attrs expired
    print(f"w.id = {w.id}")  # deferred-reload SELECT -> raises under CinderX

❯ uv run cinderx_sqlalchemy_repro.py
Traceback (most recent call last):
  File "/home/guru/.cache/uv/environments-v2/cinderx-sqlalchemy-repro-c0ce3bae3f4a9518/lib/python3.14/site-packages/sqlalchemy/orm/attributes.py", line 1102, in get
    return dict_[key]
           ~~~~~^^^^^
KeyError: 'id'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/guru/Desktop/the_stream/stream-2/cinderx_sqlalchemy_repro.py", line 56, in <module>
    print(f"w.id = {w.id}")  # deferred-reload SELECT -> raises under CinderX
                    ^^^^
  File "/home/guru/.cache/uv/environments-v2/cinderx-sqlalchemy-repro-c0ce3bae3f4a9518/lib/python3.14/site-packages/sqlalchemy/orm/attributes.py", line 569, in __get__
    return self.impl.get(state, dict_)  # type: ignore[no-any-return]
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/guru/.cache/uv/environments-v2/cinderx-sqlalchemy-repro-c0ce3bae3f4a9518/lib/python3.14/site-packages/sqlalchemy/orm/attributes.py", line 1105, in get
    raise KeyError(
    ...<3 lines>...
    ) from err
KeyError: "Deferred loader for attribute 'id' failed to populate correctly"