Remove Checkpoint.pending_sends

- Instead store sends in a Topic channel, removing the need to fetch sends as writes against the parent checkpoint
- Remove deprecated/unused functions in langgraph-checkpoint (will require bumping min range for langgraph-checkpoint in langgraph lib)
- Implement migration of old pending sends in langgraph-checkpoint-postgres
- Ensure parent config of `checkpoint_during=False` checkpoints always points to checkpoints that were also saved

libs/checkpoint-postgres/langgraph/checkpoint/postgres/__init__.py

@@ -1,4 +1,5 @@
 import threading
+from collections import defaultdict
 from collections.abc import Iterator, Sequence
 from contextlib import contextmanager
 from typing import Any, Optional
@@ -142,8 +143,36 @@ class PostgresSaver(BasePostgresSaver):
             query += f" LIMIT {limit}"
         # if we change this to use .stream() we need to make sure to close the cursor
         with self._cursor() as cur:
-            cur.execute(query, args, binary=True)
-            for value in cur:
+            cur.execute(query, args)
+            values = cur.fetchall()
+            if not values:
+                return
+            # migrate pending sends if necessary
+            if to_migrate := [
+                v
+                for v in values
+                if v["checkpoint"]["v"] < 4 and v["parent_checkpoint_id"]
+            ]:
+                cur.execute(
+                    self.SELECT_PENDING_SENDS_SQL,
+                    (
+                        values[0]["thread_id"],
+                        [v["parent_checkpoint_id"] for v in to_migrate],
+                    ),
+                )
+                grouped_by_parent = defaultdict(list)
+                for value in to_migrate:
+                    grouped_by_parent[value["parent_checkpoint_id"]].append(value)
+                for sends in cur:
+                    for value in grouped_by_parent[sends["checkpoint_id"]]:
+                        if value["channel_values"] is None:
+                            value["channel_values"] = []
+                        self._migrate_pending_sends(
+                            sends["sends"],
+                            value["checkpoint"],
+                            value["channel_values"],
+                        )
+            for value in values:
                 yield CheckpointTuple(
                     {
                         "configurable": {
@@ -155,7 +184,6 @@ class PostgresSaver(BasePostgresSaver):
                     self._load_checkpoint(
                         value["checkpoint"],
                         value["channel_values"],
-                        value["pending_sends"],
                     ),
                     self._load_metadata(value["metadata"]),
                     (
@@ -221,37 +249,52 @@ class PostgresSaver(BasePostgresSaver):
             cur.execute(
                 self.SELECT_SQL + where,
                 args,
-                binary=True,
             )
+            value = cur.fetchone()
+            if value is None:
+                return None
 
-            for value in cur:
-                return CheckpointTuple(
+            # migrate pending sends if necessary
+            if value["checkpoint"]["v"] < 4 and value["parent_checkpoint_id"]:
+                cur.execute(
+                    self.SELECT_PENDING_SENDS_SQL,
+                    (thread_id, [value["parent_checkpoint_id"]]),
+                )
+                if sends := cur.fetchone():
+                    if value["channel_values"] is None:
+                        value["channel_values"] = []
+                    self._migrate_pending_sends(
+                        sends["sends"],
+                        value["checkpoint"],
+                        value["channel_values"],
+                    )
+
+            return CheckpointTuple(
+                {
+                    "configurable": {
+                        "thread_id": thread_id,
+                        "checkpoint_ns": checkpoint_ns,
+                        "checkpoint_id": value["checkpoint_id"],
+                    }
+                },
+                self._load_checkpoint(
+                    value["checkpoint"],
+                    value["channel_values"],
+                ),
+                self._load_metadata(value["metadata"]),
+                (
                     {
                         "configurable": {
                             "thread_id": thread_id,
                             "checkpoint_ns": checkpoint_ns,
-                            "checkpoint_id": value["checkpoint_id"],
+                            "checkpoint_id": value["parent_checkpoint_id"],
                         }
-                    },
-                    self._load_checkpoint(
-                        value["checkpoint"],
-                        value["channel_values"],
-                        value["pending_sends"],
-                    ),
-                    self._load_metadata(value["metadata"]),
-                    (
-                        {
-                            "configurable": {
-                                "thread_id": thread_id,
-                                "checkpoint_ns": checkpoint_ns,
-                                "checkpoint_id": value["parent_checkpoint_id"],
-                            }
-                        }
-                        if value["parent_checkpoint_id"]
-                        else None
-                    ),
-                    self._load_writes(value["pending_writes"]),
-                )
+                    }
+                    if value["parent_checkpoint_id"]
+                    else None
+                ),
+                self._load_writes(value["pending_writes"]),
+            )
 
     def put(
         self,