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/base.py

@@ -14,7 +14,7 @@ from langgraph.checkpoint.base import (
     get_checkpoint_id,
 )
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
-from langgraph.checkpoint.serde.types import TASKS, ChannelProtocol
+from langgraph.checkpoint.serde.types import TASKS
 
 MetadataInput = Optional[dict[str, Any]]
 
@@ -72,7 +72,7 @@ MIGRATIONS = [
     """ALTER TABLE checkpoint_writes ADD COLUMN task_path TEXT NOT NULL DEFAULT '';""",
 ]
 
-SELECT_SQL = f"""
+SELECT_SQL = """
 select
     thread_id,
     checkpoint,
@@ -96,17 +96,20 @@ select
         where cw.thread_id = checkpoints.thread_id
             and cw.checkpoint_ns = checkpoints.checkpoint_ns
             and cw.checkpoint_id = checkpoints.checkpoint_id
-    ) as pending_writes,
-    (
-        select array_agg(array[cw.type::bytea, cw.blob] order by cw.task_path, cw.task_id, cw.idx)
-        from checkpoint_writes cw
-        where cw.thread_id = checkpoints.thread_id
-            and cw.checkpoint_ns = checkpoints.checkpoint_ns
-            and cw.checkpoint_id = checkpoints.parent_checkpoint_id
-            and cw.channel = '{TASKS}'
-    ) as pending_sends
+    ) as pending_writes
 from checkpoints """
 
+SELECT_PENDING_SENDS_SQL = f"""
+select
+    checkpoint_id,
+    array_agg(array[type::bytea, blob] order by task_path, task_id, idx) as sends
+from checkpoint_writes
+where thread_id = %s
+    and checkpoint_id = any(%s)
+    and channel = '{TASKS}'
+group by checkpoint_id
+"""
+
 UPSERT_CHECKPOINT_BLOBS_SQL = """
     INSERT INTO checkpoint_blobs (thread_id, checkpoint_ns, channel, version, type, blob)
     VALUES (%s, %s, %s, %s, %s, %s)
@@ -140,6 +143,7 @@ INSERT_CHECKPOINT_WRITES_SQL = """
 
 class BasePostgresSaver(BaseCheckpointSaver[str]):
     SELECT_SQL = SELECT_SQL
+    SELECT_PENDING_SENDS_SQL = SELECT_PENDING_SENDS_SQL
     MIGRATIONS = MIGRATIONS
     UPSERT_CHECKPOINT_BLOBS_SQL = UPSERT_CHECKPOINT_BLOBS_SQL
     UPSERT_CHECKPOINTS_SQL = UPSERT_CHECKPOINTS_SQL
@@ -149,22 +153,38 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
     jsonplus_serde = JsonPlusSerializer()
     supports_pipeline: bool
 
+    def _migrate_pending_sends(
+        self,
+        pending_sends: list[tuple[bytes, bytes]],
+        checkpoint: dict[str, Any],
+        channel_values: list[tuple[bytes, bytes, bytes]],
+    ) -> None:
+        if not pending_sends:
+            return
+        # add to values
+        enc, blob = self.serde.dumps_typed(
+            [self.serde.loads_typed((c.decode(), b)) for c, b in pending_sends],
+        )
+        channel_values.append((TASKS.encode(), enc.encode(), blob))
+        # add to versions
+        checkpoint["channel_versions"][TASKS] = (
+            max(checkpoint["channel_versions"].values())
+            if checkpoint["channel_versions"]
+            else self.get_next_version(None)
+        )
+
     def _load_checkpoint(
         self,
         checkpoint: dict[str, Any],
         channel_values: list[tuple[bytes, bytes, bytes]],
-        pending_sends: list[tuple[bytes, bytes]],
     ) -> Checkpoint:
         return {
             **checkpoint,
-            "pending_sends": [
-                self.serde.loads_typed((c.decode(), b)) for c, b in pending_sends or []
-            ],
             "channel_values": self._load_blobs(channel_values),
         }
 
     def _dump_checkpoint(self, checkpoint: Checkpoint) -> dict[str, Any]:
-        return {**checkpoint, "pending_sends": []}
+        return checkpoint
 
     def _load_blobs(
         self, blob_values: list[tuple[bytes, bytes, bytes]]
@@ -249,7 +269,7 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
         # NOTE: we're using JSON serializer (not msgpack), so we need to remove null characters before writing
         return serialized_metadata.decode().replace("\\u0000", "")
 
-    def get_next_version(self, current: Optional[str], channel: ChannelProtocol) -> str:
+    def get_next_version(self, current: Optional[str]) -> str:
         if current is None:
             current_v = 0
         elif isinstance(current, int):