chore: import upstream snapshot with attribution

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/_delta.py

@@ -0,0 +1,172 @@
+"""Shared helpers for `get_delta_channel_history` on sqlite savers.
+
+Mirrors the two-stage shape of `BasePostgresSaver` (ancestor walk +
+per-channel UNION ALL writes fetch), but adapted for sqlite's
+constraints. The structural differences:
+
+* No JSONB — to inspect `channel_values` for a checkpoint we must
+  deserialize the full blob. Stage 1 streams the cursor row-by-row and
+  deserializes only the rows the merged walk visits, freeing each blob
+  before advancing.
+* No separate blob table — `channel_values` lives inline in the
+  checkpoint, so seeds come back from stage 1 with no second fetch.
+* Single merged walk (not K independent walks): each visited cid is
+  deserialized exactly once, regardless of how many channels are still
+  seeking their seed.
+
+The streaming design keeps peak in-flight memory at roughly one
+deserialized checkpoint at a time, instead of holding the entire
+ancestor chain's worth of raw blobs as a `fetchall()`-materialized list.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Mapping, Sequence
+from typing import Any
+
+from langgraph.checkpoint.base import DeltaChannelHistory, PendingWrite
+
+# Stage 1 streams ancestors of `target_cid` newest-first. The `<=`
+# predicate keeps target itself in the stream so we can read its
+# `parent_checkpoint_id` from the first row without a separate lookup;
+# the caller skips target's own writes/seed (matches the
+# `BaseCheckpointSaver` contract).
+DELTA_STAGE1_SQL = (
+    "SELECT checkpoint_id, parent_checkpoint_id, type, checkpoint "
+    "FROM checkpoints "
+    "WHERE thread_id = ? AND checkpoint_ns = ? AND checkpoint_id <= ? "
+    "ORDER BY checkpoint_id DESC"
+)
+
+
+def build_delta_stage2_sql(*, chain_lens: Sequence[int]) -> str:
+    """Stage-2 per-channel UNION ALL fetching writes from `writes`.
+
+    One branch per channel with a non-empty chain. Each branch inlines its
+    own `IN (?, ?, ...)` placeholder list because sqlite has no array-bind
+    equivalent of postgres's `= ANY(%s)`. Caller passes parameters in
+    matching order: `[thread_id, checkpoint_ns, channel, *chain_cids]` per
+    branch.
+
+    Returns an empty string when no channel has a chain (caller skips
+    executing in that case). Per-channel UNION ALL avoids the over-fetch
+    of a single `channel = ANY(channels)` filter when channels have
+    different chain depths — same rationale as postgres.
+    """
+    branches: list[str] = []
+    for n in chain_lens:
+        cid_placeholders = ",".join("?" * n)
+        branches.append(
+            "SELECT checkpoint_id, channel, task_id, idx, type, value "
+            "FROM writes "
+            "WHERE thread_id = ? AND checkpoint_ns = ? AND channel = ? "
+            f"AND checkpoint_id IN ({cid_placeholders})"
+        )
+    return " UNION ALL ".join(branches)
+
+
+def step_walk_with_row(
+    *,
+    cid: str,
+    parent_cid: str | None,
+    type_tag: str,
+    blob: bytes,
+    target_id: str,
+    serde: Any,
+    chain_by_ch: dict[str, list[str]],
+    seed_val_by_ch: dict[str, Any],
+    walk_state: dict[str, Any],
+    seeded: set[str],
+    channels: Sequence[str],
+) -> bool:
+    """Process one streamed stage-1 row in the merged ancestor walk.
+
+    The cursor returns (cid, parent_cid, type, blob) rows in
+    `checkpoint_id` DESC order starting at target. The first row is
+    target itself; we read its parent_cid to seed the walk and otherwise
+    skip it (target's own writes/seed are not part of the contract).
+
+    For each subsequent row, if `cid` matches the walk's current
+    position, we deserialize the blob, append the cid to every
+    not-yet-seeded channel's chain, and check `channel_values` for
+    seeds. The deserialized checkpoint is dropped before advancing — no
+    cross-row cache, so peak in-flight is one deserialized checkpoint.
+
+    Off-path rows (different branch on the same thread) advance the
+    cursor without doing any work.
+
+    Returns True when every requested channel is seeded — the caller
+    can stop iterating and close the cursor.
+    """
+    if "started" not in walk_state:
+        if cid == target_id:
+            walk_state["started"] = True
+            walk_state["cur_cid"] = parent_cid
+            walk_state["active"] = {ch for ch in channels if ch not in seeded}
+        # Not target yet (or target not present): keep streaming.
+        return False
+    active: set[str] = walk_state["active"]
+    if not active:
+        return True
+    if cid != walk_state["cur_cid"]:
+        # Off-path row from a sibling branch — skip without deserializing.
+        return False
+    for ch in active:
+        chain_by_ch[ch].append(cid)
+    ckpt = serde.loads_typed((type_tag, blob))
+    channel_values: Mapping[str, Any] = ckpt.get("channel_values") or {}
+    for ch in [ch for ch in active if ch in channel_values]:
+        seed_val_by_ch[ch] = channel_values[ch]
+        seeded.add(ch)
+        active.discard(ch)
+    del ckpt, channel_values
+    walk_state["cur_cid"] = parent_cid
+    return not active
+
+
+def build_delta_channels_writes_history(
+    *,
+    channels: Sequence[str],
+    chain_by_ch: Mapping[str, list[str]],
+    seed_val_by_ch: Mapping[str, Any],
+    seeded: set[str],
+    stage2_rows: Sequence[tuple[str, str, str, int, str, bytes]],
+    serde: Any,
+) -> dict[str, DeltaChannelHistory]:
+    """Demux stage-2 rows per channel; produce per-channel histories.
+
+    Stage-2 rows are `(checkpoint_id, channel, task_id, idx, type, value)`.
+    Final write order is oldest→newest globally and `(task_id, idx)` within
+    a checkpoint, matching the contract on `DeltaChannelHistory.writes`.
+
+    `seed` is omitted when the walk reached a true root with no snapshot
+    found (channel never entered `seeded`); consumers treat absence as
+    "start empty".
+    """
+    writes_by_ch_by_cid: dict[str, dict[str, list[tuple[str, bytes, str, int]]]] = {
+        ch: {} for ch in channels
+    }
+    for cid, ch, task_id, idx, type_tag, value_blob in stage2_rows:
+        writes_by_ch_by_cid.setdefault(ch, {}).setdefault(cid, []).append(
+            (type_tag, value_blob, task_id, idx)
+        )
+    for cid_map in writes_by_ch_by_cid.values():
+        for ws in cid_map.values():
+            ws.sort(key=lambda w: (w[2], w[3]))
+
+    result: dict[str, DeltaChannelHistory] = {}
+    for ch in channels:
+        chain_cids = chain_by_ch.get(ch, [])
+        cid_writes = writes_by_ch_by_cid.get(ch, {})
+        collected: list[PendingWrite] = []
+        # Chain is newest-first; iterate oldest-first for the public order.
+        for cid in reversed(chain_cids):
+            for type_tag, value_blob, task_id, _idx in cid_writes.get(cid, []):
+                collected.append(
+                    (task_id, ch, serde.loads_typed((type_tag, value_blob)))
+                )
+        entry: DeltaChannelHistory = {"writes": collected}
+        if ch in seeded:
+            entry["seed"] = seed_val_by_ch[ch]
+        result[ch] = entry
+    return result