fix(langgraph): fix PostgresSaver crashing when loading older checkpoints (#6162)

### Description

https://github.com/langchain-ai/langgraph/issues/6137 and
https://github.com/langchain-ai/langgraph/issues/5677 reported issues
where older checkpoints read by AsyncPostgresSaver/PostgresSaver from
`langgraph-checkpoint-postgres==2.0.19` fail to read channel values,
throwing `NoneType object is not a mapping`. This was due to a bug in
how `channel_values` is assembled:
```python
"channel_values": {
    **value["checkpoint"].get("channel_values"),  # <--- if channel_values doesn't exist (old checkpoint), **None errors
    **self._load_blobs(value["channel_values"]),
},
```
This bug was observed for checkpoints generated by
`langgraph-checkpoint-postgres<=2.0.19`.

Fixed by providing a fallback to
`value["checkpoint"].get("channel_values")`:
```python
**value["checkpoint"],
"channel_values": {
    **(
        value["checkpoint"].get("channel_values") or {}
    ),  # 'or {}' needed for backwards compat with v3 checkpoints and below, as v4 introduced channel_values key
    **self._load_blobs(value["channel_values"]),
},
```

### Tests
Added test for AsyncPostgresSaver and test for PostgresSaver, using
monkeypatch to remove `channel_values` before CheckpointTuple is
assembled in `_load_checkpoint_tuple`.

### Solves
https://github.com/langchain-ai/langgraph/issues/6137 and
https://github.com/langchain-ai/langgraph/issues/5677

---------

Co-authored-by: Shahrukh Shaik <144558473+shahrukh-shaik@users.noreply.github.com>

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

@@ -450,7 +450,7 @@ class PostgresSaver(BasePostgresSaver):
             {
                 **value["checkpoint"],
                 "channel_values": {
-                    **value["checkpoint"].get("channel_values"),
+                    **(value["checkpoint"].get("channel_values") or {}),
                     **self._load_blobs(value["channel_values"]),
                 },
             },

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

@@ -409,7 +409,7 @@ class AsyncPostgresSaver(BasePostgresSaver):
             {
                 **value["checkpoint"],
                 "channel_values": {
-                    **value["checkpoint"].get("channel_values"),
+                    **(value["checkpoint"].get("channel_values") or {}),
                     **self._load_blobs(value["channel_values"]),
                 },
             },

libs/checkpoint-postgres/tests/test_async.py

@@ -344,3 +344,34 @@ async def test_pending_sends_migration(saver_name: str) -> None:
             TASKS: ["send-1", "send-2", "send-3"]
         }
         assert TASKS in search_results[0].checkpoint["channel_versions"]
+
+
+@pytest.mark.parametrize("saver_name", ["base", "pool", "pipe"])
+async def test_get_checkpoint_no_channel_values(
+    monkeypatch, saver_name: str, test_data
+) -> None:
+    """Backwards compatibility test that verifies a checkpoint with no channel_values key can be retrieved without throwing an error."""
+    async with _saver(saver_name) as saver:
+        config = {
+            "configurable": {
+                "thread_id": "thread-2",
+                "checkpoint_ns": "",
+                "__super_private_key": "super_private_value",
+            },
+            "metadata": {"run_id": "my_run_id"},
+        }
+        chkpnt: Checkpoint = create_checkpoint(empty_checkpoint(), {}, 1)
+        await saver.aput(config, chkpnt, {}, {})
+
+        load_checkpoint_tuple = saver._load_checkpoint_tuple
+
+        def patched_load_checkpoint_tuple(value):
+            value["checkpoint"].pop("channel_values", None)
+            return load_checkpoint_tuple(value)
+
+        monkeypatch.setattr(
+            saver, "_load_checkpoint_tuple", patched_load_checkpoint_tuple
+        )
+
+        checkpoint = await saver.aget_tuple(config)
+        assert checkpoint.checkpoint["channel_values"] == {}

libs/checkpoint-postgres/tests/test_sync.py

@@ -332,3 +332,33 @@ def test_pending_sends_migration(saver_name: str) -> None:
             TASKS: ["send-1", "send-2", "send-3"]
         }
         assert TASKS in search_results[0].checkpoint["channel_versions"]
+
+
+@pytest.mark.parametrize("saver_name", ["base", "pool", "pipe"])
+def test_get_checkpoint_no_channel_values(
+    monkeypatch, saver_name: str, test_data
+) -> None:
+    """Backwards compatibility test that verifies a checkpoint with no channel_values key can be retrieved without throwing an error."""
+    with _saver(saver_name) as saver:
+        config = {
+            "configurable": {
+                "thread_id": "thread-2",
+                "checkpoint_ns": "",
+                "__super_private_key": "super_private_value",
+            },
+        }
+        chkpnt: Checkpoint = create_checkpoint(empty_checkpoint(), {}, 1)
+        saver.put(config, chkpnt, {}, {})
+
+        load_checkpoint_tuple = saver._load_checkpoint_tuple
+
+        def patched_load_checkpoint_tuple(value):
+            value["checkpoint"].pop("channel_values", None)
+            return load_checkpoint_tuple(value)
+
+        monkeypatch.setattr(
+            saver, "_load_checkpoint_tuple", patched_load_checkpoint_tuple
+        )
+
+        checkpoint = saver.get_tuple(config)
+        assert checkpoint.checkpoint["channel_values"] == {}