fix(langgraph): never store exit-mode delta writes under the null task id (#9229)

In exit mode, a run stores its DeltaChannel writes on the checkpoint it
started from, under synthetic task ids that put the superstep first. For
a `Command`'s writes in a run whose first superstep is 0, that id came
out as `NULL_TASK_ID` itself. Readers take writes under `NULL_TASK_ID`
for the checkpoint's own pending writes, so after
`invoke(Command(update=...), durability="exit")`:

- on a new thread, `get_state` on the empty first checkpoint showed the
update in the DeltaChannel but not in a plain channel;
- on a thread whose first checkpoint came from `update_state(...,
as_node="__input__")`, `get_state` on that checkpoint showed the update
the same way, and a replay or fork from it applied the update to the
DeltaChannel only.

`exit_delta_task_id` now never returns `NULL_TASK_ID`. Threads already
saved this way stay as they are.

## Tests

- `test_command_update_on_an_input_checkpoint_matches_a_plain_channel`
compares every checkpoint in the history, and a replay from the input
checkpoint, with a plain channel, on every checkpointer and durability.
The exit cases fail without the fix.
- `test_exit_command_update_on_a_new_thread_matches_a_plain_channel`
compares every checkpoint in a new thread's history with a plain
channel, on every checkpointer, and fails without the fix. It runs in
exit mode only: in sync and async the two channels already differ on
`main`, because of a separate bug that applies a new thread's `Command`
update twice.

`make format`, `make lint` and the langgraph suite pass.

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -26,6 +26,7 @@ from langgraph._internal._constants import (
     CONFIG_KEY_CHECKPOINT_ID,
     NS_END,
     NS_SEP,
+    NULL_TASK_ID,
     PUSH,
     SNAPSHOT_BUMPS,
 )
@@ -64,10 +65,14 @@ def exit_delta_task_id(step: int, task_id: str) -> str:
 
     Embeds the superstep in the first UUID group so `ORDER BY task_id, idx`
     preserves chronological order while remaining a valid RFC UUID (required by
-    Postgres `checkpoint_writes.task_id uuid` columns).
+    Postgres `checkpoint_writes.task_id uuid` columns). Never `NULL_TASK_ID`:
+    readers apply writes under it as the anchor checkpoint's own pending writes.
     """
     parts = str(uuid.UUID(task_id)).split("-")
-    return f"{step:08d}-{parts[1]}-{parts[2]}-{parts[3]}-{parts[4]}"
+    synthetic = f"{step:08d}-{parts[1]}-{parts[2]}-{parts[3]}-{parts[4]}"
+    if synthetic == NULL_TASK_ID:
+        return f"{step:08d}-0000-0000-0000-000000000001"
+    return synthetic
 
 
 def exit_delta_late_task_id(step: int, task_id: str) -> str:

libs/langgraph/tests/test_delta_channel_exit_mode.py

@@ -17,6 +17,7 @@ from langgraph.checkpoint.memory import InMemorySaver
 from langgraph.checkpoint.serde.types import _DeltaSnapshot
 from typing_extensions import TypedDict
 
+from langgraph._internal._constants import NULL_TASK_ID
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import START, StateGraph
 from langgraph.graph.message import _messages_delta_reducer
@@ -38,6 +39,7 @@ def test_exit_delta_task_id_is_valid_uuid_and_ordered() -> None:
     assert id1.split("-")[0] == "00000001"
     assert id7.split("-")[0] == "00000007"
     assert id1.endswith("-0270-bf16-1ef8-fb321bef9f3d")
+    assert exit_delta_task_id(0, NULL_TASK_ID) != NULL_TASK_ID
 
     with pytest.raises(ValueError):
         uuid.UUID(f"00000001-{tid}")
@@ -463,6 +465,43 @@ def test_resume_with_a_command_update_replays_its_write_once(
     assert state.values["log"] == state.values["plain"] == ["in", "cmd", "ask", "done"]
 
 
+def test_command_update_on_an_input_checkpoint_matches_a_plain_channel(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    builder = StateGraph(_ResumeState)
+    builder.add_node("node", lambda state: _both("node"))
+    builder.add_edge(START, "node")
+    graph = builder.compile(checkpointer=sync_checkpointer)
+    config = {"configurable": {"thread_id": "t"}}
+    graph.update_state(config, _both("in"), as_node="__input__")
+
+    graph.invoke(Command(update=_both("cmd")), config, durability=durability)
+
+    history = list(graph.get_state_history(config))
+    assert [s.values.get("log", []) for s in history] == [
+        s.values.get("plain", []) for s in history
+    ]
+    replayed = graph.invoke(None, history[-1].config, durability=durability)
+    assert replayed["log"] == replayed["plain"]
+
+
+def test_exit_command_update_on_a_new_thread_matches_a_plain_channel(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    builder = StateGraph(_ResumeState)
+    builder.add_node("node", lambda state: _both("node"))
+    builder.add_edge(START, "node")
+    graph = builder.compile(checkpointer=sync_checkpointer)
+    config = {"configurable": {"thread_id": "t"}}
+
+    graph.invoke(Command(update=_both("cmd")), config, durability="exit")
+
+    history = list(graph.get_state_history(config))
+    assert [s.values.get("log", []) for s in history] == [
+        s.values.get("plain", []) for s in history
+    ]
+
+
 class _FlagState(_ResumeState, total=False):
     extra: Annotated[list, DeltaChannel(_append)]
     flag: bool