fix(gateway/api_server): deduplicate concurrent idempotent requests

tests/gateway/test_api_server.py

@@ -12,6 +12,7 @@ Tests cover:
 - Error handling (invalid JSON, missing fields)
 """
 
+import asyncio
 import json
 import time
 import uuid
@@ -25,6 +26,7 @@ from gateway.config import GatewayConfig, Platform, PlatformConfig
 from gateway.platforms.api_server import (
     APIServerAdapter,
     ResponseStore,
+    _IdempotencyCache,
     _CORS_HEADERS,
     _derive_chat_session_id,
     check_api_server_requirements,
@@ -104,6 +106,95 @@ class TestResponseStore:
         assert store.delete("resp_missing") is False
 
 
+# ---------------------------------------------------------------------------
+# _IdempotencyCache
+# ---------------------------------------------------------------------------
+
+
+class TestIdempotencyCache:
+    @pytest.mark.asyncio
+    async def test_concurrent_same_key_and_fingerprint_runs_once(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            started.set()
+            await gate.wait()
+            return ("response", {"total_tokens": 1})
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+
+        await started.wait()
+        assert calls == 1
+
+        gate.set()
+        first_result, second_result = await asyncio.gather(first, second)
+
+        assert first_result == second_result == ("response", {"total_tokens": 1})
+
+    @pytest.mark.asyncio
+    async def test_different_fingerprint_does_not_reuse_inflight_task(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            result = calls
+            if calls == 2:
+                started.set()
+            await gate.wait()
+            return result
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-2", compute))
+
+        await started.wait()
+        assert calls == 2
+
+        gate.set()
+        results = await asyncio.gather(first, second)
+
+        assert sorted(results) == [1, 2]
+
+    @pytest.mark.asyncio
+    async def test_cancelled_waiter_does_not_drop_shared_inflight_task(self):
+        cache = _IdempotencyCache()
+        gate = asyncio.Event()
+        started = asyncio.Event()
+        calls = 0
+
+        async def compute():
+            nonlocal calls
+            calls += 1
+            started.set()
+            await gate.wait()
+            return "response"
+
+        first = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+
+        await started.wait()
+        assert calls == 1
+
+        first.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await first
+
+        second = asyncio.create_task(cache.get_or_set("idem-key", "fp-1", compute))
+        await asyncio.sleep(0)
+        assert calls == 1
+
+        gate.set()
+        assert await second == "response"
+
+
 # ---------------------------------------------------------------------------
 # Adapter initialization
 # ---------------------------------------------------------------------------