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2b93e8b79f
Fixes langchain-ai/langgraph#8382 `DeltaChannel` rebuilds its value by replaying ancestor writes through the reducer. Every saver ordered a checkpoint's writes by `(task_id, idx)`, but live execution applies a super-step's writes in task-path order (`apply_writes` sorts tasks by `task_path_str(task.path[:3])`). `task_id` is a hash of the path, so when two or more tasks write one `DeltaChannel` in the same super-step, replay applies them in an arbitrary permutation. Reducers must be batching-invariant, not order-invariant, so the permutation changes the value: `get_state` disagrees with what `invoke` returned, and continuing the thread saves the reordered value as the base for every later write. ## Fix Replay orders by `(task_path, task_id, idx)`, the order `SELECT_PENDING_SENDS_SQL` already uses for sends. `apply_writes` and every saver now sort with one `writes_sort_key` from `langgraph.checkpoint.base`, in Python, so live and replay can't drift apart and don't depend on a database collation. `apply_writes` sorts by the full task path, the string `put_writes` stores, instead of `path[:3]`. The paths that reach `apply_writes` no longer carry task ids, so the live order doesn't change. `BaseCheckpointSaver`'s default `get_delta_channel_history` can't sort by path, since `PendingWrite` has no task path, so it replays each checkpoint's writes in `get_tuple` order. `get_tuple` now has to return `pending_writes` in `writes_sort_key` order, and memory, SQLite and Postgres do, so the default walk is right for any saver that follows that. `AsyncPostgresSaver` also gets the sync `get_delta_channel_history` that `AsyncSqliteSaver` already had; sync reads used to fall back to the default walk. Memory and Postgres already stored `task_path`. SQLite accepted it on `put_writes` and dropped it, so `writes` gains the column and `setup()` adds it to existing databases. Writes without a task path (graph input, `update_state` updates, rows written before the column existed) sort first by `task_id`, which is also where live execution applies input. ## Compatibility - Memory and Postgres already had `task_path` on disk, so existing threads replay in the corrected order after upgrade. - On SQLite, rows from before the upgrade read back as `''` and keep their old order. Only new writes get the corrected order, since there is no stored path to recover. - SQLite has no `ADD COLUMN IF NOT EXISTS`, so `setup()` runs the `ALTER` and treats `duplicate column name` as success, which also covers two processes migrating one file. That error comes while SQLite prepares the statement, before it asks for the write lock, so `setup()` on an up-to-date database doesn't wait on other writers. A read-only database from before the column is read with empty paths instead of failing setup. - Downgrade is safe: older code's `INSERT` omits the column and the column has a default. - `get_tuple` and `list` return `pending_writes` in `writes_sort_key` order instead of task id order (SQLite, Postgres) or put order (memory). Nothing in langgraph depends on the old order. - Postgres reads `task_path` and `idx` with each pending write, so a subclass that pairs `SELECT_SQL` with its own `_load_writes`, or the other way round, has to match the new row shape. - A third-party saver has to replay in `writes_sort_key` order: in its own `get_delta_channel_history` if it has one, otherwise by returning `pending_writes` from `get_tuple` in that order. Otherwise it fails the new conformance tests; the `get_tuple` one is a base test, so it runs for every saver. - langgraph, checkpoint-sqlite and checkpoint-postgres import `writes_sort_key` from langgraph-checkpoint, so this bumps langgraph-checkpoint to 4.3.0 and requires it there. 4.3.0 has to be released first. ## Limits - Several updates in one `bulk_update_state` super-step share a task id, so the saver keeps only the first one's writes and the `DeltaChannel` loses the rest (plain channels are fine). That's on main too; #9128 fixes it and stores a task path per update, so with this PR those updates also replay in the order given. - Exit durability stores its writes differently; resume ordering there is fixed in #9114, stacked on this PR. - langgraphjs applies `DeltaChannel` writes in task id order, live and on replay (langchain-ai/langgraphjs#2544), so its reads match too. Moving JS to task path order is a follow-up. ## Tests Each fails on main and passes here: - `test_delta_channel_parallel_order.py`, across memory, SQLite and Postgres: parallel writers and a `Send` fan-out, comparing `get_state` with the live result, including sync `get_state` on the async savers. - Three conformance tests whose task ids sort opposite to their task paths, so they cannot pass by chance: two on `get_delta_channel_history`, one on `get_tuple`'s `pending_writes` order. - SQLite migration tests from a pre-column database, sync and async, including a read-only file. One more that passes on main too: SQLite `setup()` while another connection holds the write lock, with no busy timeout, sync and async. It keeps the `ALTER` from waiting on that lock. Thanks to @ErenAta16 for the report, the reproduction, and for tracing `task_path` through all three backends, and to @ragnarok268 for the conformance-test design with task ids ordered opposite to their paths. --------- Co-authored-by: ErenAta16 <149434812+ErenAta16@users.noreply.github.com> Co-authored-by: ragnarok268 <58264829+ragnarok268@users.noreply.github.com>
91 lines
2.0 KiB
TOML
91 lines
2.0 KiB
TOML
[build-system]
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requires = ["hatchling"]
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build-backend = "hatchling.build"
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[project]
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name = "langgraph-checkpoint-postgres"
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version = "3.1.2"
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description = "Library with a Postgres implementation of LangGraph checkpoint saver."
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authors = []
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requires-python = ">=3.10"
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readme = "README.md"
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license = "MIT"
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license-files = ['LICENSE']
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dependencies = [
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"langgraph-checkpoint>=4.3.0,<5.0.0",
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"orjson>=3.11.5",
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"psycopg>=3.2.0",
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"psycopg-pool>=3.2.0",
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]
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[project.urls]
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Source = "https://github.com/langchain-ai/langgraph/tree/main/libs/checkpoint-postgres"
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Twitter = "https://x.com/langchain_oss"
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Slack = "https://www.langchain.com/join-community"
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Reddit = "https://www.reddit.com/r/LangChain/"
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[dependency-groups]
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test = [
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"pytest",
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"anyio",
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"pytest-asyncio",
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"pytest-mock",
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"psycopg[binary]",
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"langgraph-checkpoint",
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"langgraph-checkpoint-conformance",
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"pytest-watcher",
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]
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lint = [
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"ruff",
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"codespell",
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"ty",
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]
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dev = [
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{include-group = "test"},
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{include-group = "lint"},
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]
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[tool.uv]
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default-groups = ['dev']
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[tool.uv.sources]
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langgraph-checkpoint = { path = "../checkpoint", editable = true }
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langgraph-checkpoint-conformance = { path = "../checkpoint-conformance", editable = true }
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[tool.hatch.build.targets.wheel]
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include = ["langgraph"]
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[tool.pytest.ini_options]
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addopts = "--strict-markers --strict-config --durations=5 -vv"
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asyncio_mode = "auto"
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[tool.ruff]
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lint.select = [
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"E", # pycodestyle
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"F", # Pyflakes
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"UP", # pyupgrade
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"B", # flake8-bugbear
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"I", # isort
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"PLC0415", # import-outside-top-level
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"RUF100", # unused noqa directive
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"UP", # pyupgrade
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]
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lint.ignore = ["E501", "B008"]
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target-version = "py310"
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[tool.ty.rules]
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invalid-argument-type = "ignore"
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invalid-assignment = "ignore"
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invalid-key = "ignore"
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invalid-return-type = "ignore"
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invalid-typed-dict-field = "ignore"
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invalid-yield = "ignore"
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no-matching-overload = "ignore"
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unused-type-ignore-comment = "ignore"
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[tool.pytest-watcher]
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now = true
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delay = 0.1
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runner_args = ["--ff", "-x", "-v", "--tb", "short"]
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patterns = ["*.py"]
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