|
| 1 | +"""Deferred-construction wrapper for :class:`IdempotencyBackend`. |
| 2 | +
|
| 3 | +Mirrors the JS SDK ``createLazyBackend`` (adcp-client#2136). Use this when the |
| 4 | +real backend depends on application infrastructure that resolves asynchronously |
| 5 | +*after* the SDK server is constructed — for example a Postgres pool or Redis |
| 6 | +client produced by an async bootstrap:: |
| 7 | +
|
| 8 | + from adcp.server.idempotency import ( |
| 9 | + IdempotencyStore, |
| 10 | + LazyBackend, |
| 11 | + PgBackend, |
| 12 | + ) |
| 13 | +
|
| 14 | + async def _resolve() -> IdempotencyBackend: |
| 15 | + pool = await app.get_pg_pool() |
| 16 | + backend = PgBackend(pool=pool) |
| 17 | + await backend.create_schema() |
| 18 | + return backend |
| 19 | +
|
| 20 | + store = IdempotencyStore(backend=LazyBackend(_resolve), ttl_seconds=86400) |
| 21 | +
|
| 22 | +The underlying backend is resolved on first use and memoized (resolve-once). |
| 23 | +Concurrent first calls share a single factory invocation. If the factory |
| 24 | +raises, the wrapper forgets that failed attempt so a later call can retry. |
| 25 | +
|
| 26 | +``clear_all`` is **not** exposed by default: per the JS contract, the presence |
| 27 | +of a bulk-clear method is treated as the backend's explicit "safe to flush" |
| 28 | +signal, so it must be opted into via ``allow_clear_all=True``. Enable it only |
| 29 | +when every backend the factory can return safely permits bulk clearing (for |
| 30 | +example a dedicated test/dev :class:`MemoryBackend`, never a shared Redis). |
| 31 | +""" |
| 32 | + |
| 33 | +from __future__ import annotations |
| 34 | + |
| 35 | +import asyncio |
| 36 | +from collections.abc import Awaitable, Callable |
| 37 | + |
| 38 | +from adcp.server.idempotency.backends import CachedResponse, IdempotencyBackend |
| 39 | + |
| 40 | +# A factory may be sync (returns a backend) or async (returns an awaitable that |
| 41 | +# resolves to a backend) — both are normalized through ``_resolve`` below. |
| 42 | +LazyBackendFactory = Callable[[], "IdempotencyBackend | Awaitable[IdempotencyBackend]"] |
| 43 | + |
| 44 | + |
| 45 | +class LazyBackend(IdempotencyBackend): |
| 46 | + """Resolve an :class:`IdempotencyBackend` lazily on first use. |
| 47 | +
|
| 48 | + :param factory: A zero-arg callable returning an :class:`IdempotencyBackend` |
| 49 | + or an awaitable that resolves to one. Invoked at most once across the |
| 50 | + wrapper's lifetime once it succeeds; re-invoked only if a prior attempt |
| 51 | + raised. |
| 52 | + :param allow_clear_all: When ``True``, expose :meth:`clear_all`, delegating |
| 53 | + to the resolved backend's ``clear_all`` or ``clear`` method. Defaults to |
| 54 | + ``False`` because bulk clearing is dangerous on shared production stores |
| 55 | + and the SDK uses method presence as the reset-safety contract. |
| 56 | +
|
| 57 | + Concurrency: the first ``get``/``put``/``delete_expired`` triggers |
| 58 | + resolution. Multiple concurrent first operations share a single factory |
| 59 | + invocation via an :class:`asyncio.Lock`; later callers reuse the cached |
| 60 | + instance without locking on the hot path. |
| 61 | + """ |
| 62 | + |
| 63 | + def __init__( |
| 64 | + self, |
| 65 | + factory: LazyBackendFactory, |
| 66 | + *, |
| 67 | + allow_clear_all: bool = False, |
| 68 | + ) -> None: |
| 69 | + self._factory = factory |
| 70 | + self._allow_clear_all = allow_clear_all |
| 71 | + self._backend: IdempotencyBackend | None = None |
| 72 | + self._lock = asyncio.Lock() |
| 73 | + |
| 74 | + async def _resolve(self) -> IdempotencyBackend: |
| 75 | + """Return the resolved backend, invoking the factory once on first use. |
| 76 | +
|
| 77 | + Resolve-once + concurrency-safe: the fast path returns the memoized |
| 78 | + instance without locking. The slow path holds ``_lock`` so concurrent |
| 79 | + first callers share a single factory invocation; the double-check inside |
| 80 | + the lock means a caller that waited on the lock sees the instance the |
| 81 | + winner resolved. A factory that raises is not memoized — the next call |
| 82 | + retries. |
| 83 | + """ |
| 84 | + cached = self._backend |
| 85 | + if cached is not None: |
| 86 | + return cached |
| 87 | + async with self._lock: |
| 88 | + # Re-read under the lock: a task that lost the race to acquire it |
| 89 | + # must observe the winner's resolved instance, not re-run the |
| 90 | + # factory. (Read into a local so the narrowing is on the local, |
| 91 | + # not the instance attribute another task may have mutated.) |
| 92 | + cached = self._backend |
| 93 | + if cached is not None: |
| 94 | + return cached |
| 95 | + result = self._factory() |
| 96 | + resolved = await result if isinstance(result, Awaitable) else result |
| 97 | + if not isinstance(resolved, IdempotencyBackend): |
| 98 | + raise TypeError( |
| 99 | + "LazyBackend factory must resolve to an IdempotencyBackend, " |
| 100 | + f"got {type(resolved).__name__}" |
| 101 | + ) |
| 102 | + self._backend = resolved |
| 103 | + return resolved |
| 104 | + |
| 105 | + async def get(self, scope_key: str, key: str) -> CachedResponse | None: |
| 106 | + return await (await self._resolve()).get(scope_key, key) |
| 107 | + |
| 108 | + async def put(self, scope_key: str, key: str, entry: CachedResponse) -> None: |
| 109 | + await (await self._resolve()).put(scope_key, key, entry) |
| 110 | + |
| 111 | + async def delete_expired(self, now_epoch: float | None = None) -> int: |
| 112 | + return await (await self._resolve()).delete_expired(now_epoch) |
| 113 | + |
| 114 | + async def _clear_all(self) -> None: |
| 115 | + """Delegate a bulk clear to the resolved backend. |
| 116 | +
|
| 117 | + Resolves the backend (so the factory runs if it hasn't yet) and |
| 118 | + delegates to its ``clear_all`` or ``clear`` method, raising if the |
| 119 | + resolved backend supports neither. Exposed as ``clear_all`` only when |
| 120 | + the wrapper is constructed with ``allow_clear_all=True`` (see |
| 121 | + :meth:`__getattr__`). |
| 122 | + """ |
| 123 | + backend = await self._resolve() |
| 124 | + clear = getattr(backend, "clear_all", None) or getattr(backend, "clear", None) |
| 125 | + if clear is None: |
| 126 | + raise NotImplementedError( |
| 127 | + f"Resolved backend {type(backend).__name__} does not support " |
| 128 | + "clear_all() or clear()." |
| 129 | + ) |
| 130 | + await clear() |
| 131 | + |
| 132 | + def __getattr__(self, name: str) -> object: |
| 133 | + """Expose ``clear_all`` only when opted in. |
| 134 | +
|
| 135 | + Mirrors the JS wrapper, which attaches ``clearAll`` to the returned |
| 136 | + object solely when ``{ clearAll: true }`` — reset-safety code uses |
| 137 | + ``hasattr``/method presence as the "safe to flush" contract, so the |
| 138 | + attribute must genuinely be absent otherwise. ``__getattr__`` is only |
| 139 | + consulted for names not found normally, so this never shadows the |
| 140 | + delegating methods above. |
| 141 | + """ |
| 142 | + if name == "clear_all" and self.__dict__.get("_allow_clear_all"): |
| 143 | + return self._clear_all |
| 144 | + raise AttributeError(f"{type(self).__name__!r} object has no attribute {name!r}") |
| 145 | + |
| 146 | + |
| 147 | +def create_lazy_backend( |
| 148 | + factory: LazyBackendFactory, |
| 149 | + *, |
| 150 | + allow_clear_all: bool = False, |
| 151 | +) -> LazyBackend: |
| 152 | + """Construct a :class:`LazyBackend` — functional alias mirroring the JS |
| 153 | + ``createLazyBackend`` factory shape. |
| 154 | +
|
| 155 | + See :class:`LazyBackend` for semantics and parameter documentation. |
| 156 | + """ |
| 157 | + return LazyBackend(factory, allow_clear_all=allow_clear_all) |
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