cultcache-py: read the slot-nil vector; changelog for the store lock and msgpack

tests/vectors/slot-nil was read by TS only. The Python reader accepts the same 15 rows
and refuses the same 2 as the C# reference and rewrites the reference's bytes.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

packages/cultcache-py/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+### Changed
+
+- `msgpack` is a hard dependency, as `pyproject.toml` declared it. The optional-import fallbacks and the `cultcache-py[msgpack]` extra they named (which never existed) are gone: a missing msgpack is an `ImportError` at import.
+- The single-file store's `push`, `push_all` and `delete` take the cross-process lock on `<path>.lock` (flock; LockFileEx over the whole file on Windows) across the read and the replace, waiting up to `lock_timeout` seconds (30 by default, `TimeoutError` after), and exclude the C#, Rust and TypeScript runtimes' writers. The staged file is `<path>.tmp-<pid>-<uuid>`, fsynced before the replace, with a directory fsync after it off Windows. A fixed `<path>.tmp` is no longer used.
+
 ### Breaking
 
 - A single-file store written as `cultcache.store.v3` is read and written: a record that holds an element id (an owned list element with an `[CultElementId]` member, carried through every write) marks the store v3, and a v3 store is carried through a rewrite with its ids. A runtime from before this change refuses a v3 store by name.

packages/cultcache-py/tests/test_slot_nil.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+
+import msgpack
+
+from cultcache_py import database_entry_field, define_database_entry_type, define_record_codec, leaves as t
+
+# tests/vectors/slot-nil is written only by the C# reference (SlotNilVectorTests): what it reads and rewrites for a nil, a
+# str in a byte[] slot, and a record short of its last slots. This runtime accepts what the reference accepts and rewrites
+# the same bytes.
+VECTORS = Path(__file__).resolve().parents[3] / "tests" / "vectors" / "slot-nil"
+
+
+def _probe():
+    inner = define_record_codec("SlotNilInner", [database_entry_field("x", 0, default="", codec=t.string)])
+    return define_database_entry_type(
+        "tests.slot-nil",
+        [
+            database_entry_field("s", 0, default="", codec=t.nullable(t.string)),
+            database_entry_field("i", 1, default=0, codec=t.int32),
+            database_entry_field("b", 2, default=b"", codec=t.nullable(t.bytes)),
+            database_entry_field("r", 3, default=None, codec=t.nullable(inner)),
+            database_entry_field("l", 4, default=[], codec=t.nullable(t.list(t.string))),
+        ],
+    )
+
+
+class SlotNilVectorTests(unittest.TestCase):
+    def test_slots_read_and_rewrite_exactly_what_the_reference_does_for_nil_str_in_bytes_and_short_records(self) -> None:
+        rows = msgpack.unpackb((VECTORS / "reads.msgpack").read_bytes(), raw=False)
+        probe = _probe()
+        self.assertEqual(17, len(rows))
+        accepted = 0
+        for name, payload, was_accepted, rewritten in rows:
+            with self.subTest(name):
+                if was_accepted:
+                    self.assertEqual(rewritten.hex(), probe.encode_payload(probe.decode_payload(payload)).hex())
+                    accepted += 1
+                else:
+                    with self.assertRaises(Exception):
+                        probe.decode_payload(payload)
+        self.assertEqual(15, accepted)
+
+    def test_a_character_outside_the_bmp_is_four_utf8_bytes(self) -> None:
+        rows = {name: (payload, rewritten) for name, payload, _, rewritten in msgpack.unpackb((VECTORS / "reads.msgpack").read_bytes(), raw=False)}
+        probe = _probe()
+        self.assertEqual("\U0001F600", probe.decode_payload(rows["string-astral"][0])["s"])
+        self.assertEqual(b"\xf0\x9f\x98\x80", probe.decode_payload(rows["bytes-str-astral"][0])["b"])
+
+
+if __name__ == "__main__":
+    unittest.main()