fix: synchronize in-memory store access

tests/test_store_memory.py

@@ -1,4 +1,7 @@
+import threading
+import time
 import uuid
+from concurrent.futures import ThreadPoolExecutor
 
 from agentmsgs.stores.memory import InMemoryStore
 
@@ -86,3 +89,57 @@ def test_memory_thread_info_for_empty_thread():
     assert state.unread_from_others_count == 0
     assert s.get_thread_info(uuid.uuid4()) is None
     assert s.get_thread_read_state(uuid.uuid4(), a) is None
+
+
+def test_memory_concurrent_get_or_create_agent_is_idempotent():
+    store = InMemoryStore()
+    start = threading.Barrier(2)
+
+    class SlowNameIndex(dict):
+        def __contains__(self, key):
+            present = super().__contains__(key)
+            if not present:
+                time.sleep(0.01)
+            return present
+
+    store._name_idx = SlowNameIndex()
+
+    def create_agent():
+        start.wait()
+        return store.get_or_create_agent("Alice")
+
+    with ThreadPoolExecutor(max_workers=2) as executor:
+        agents = list(executor.map(lambda _: create_agent(), range(2)))
+
+    assert len({agent.id for agent in agents}) == 1
+    assert len(store.list_agents()) == 1
+
+
+def test_memory_concurrent_append_assigns_unique_sequences():
+    store = InMemoryStore()
+    alice = store.get_or_create_agent("Alice")
+    bob = store.get_or_create_agent("Bob")
+    thread = store.create_thread({alice, bob})
+    gate = threading.Barrier(2, timeout=0.2)
+    local = threading.local()
+
+    class SlowMessageList(list):
+        def __len__(self):
+            length = super().__len__()
+            if not hasattr(local, "waited"):
+                try:
+                    gate.wait()
+                except threading.BrokenBarrierError:
+                    pass
+                local.waited = True
+            return length
+
+    store._msgs[thread.id] = SlowMessageList()
+
+    def append_message(index):
+        return store.append_message(thread.id, alice, str(index))
+
+    with ThreadPoolExecutor(max_workers=2) as executor:
+        messages = list(executor.map(append_message, range(2)))
+
+    assert sorted(message.seq for message in messages) == [1, 2]