add cache utility (#1790)

* add simple caching utility

* test cache

* remove prefix, default on get

* check expiry in pop aswell

* remove unnecessary type

* improve invalidation task

increase default interval to 10 seconds - doesnt have to check that often.
instead of recreating the dict everytime mutate the existing one

tests/core/test_cache.py

@@ -0,0 +1,56 @@
+import asyncio
+
+import pytest
+
+from lnbits.cache import Cache
+from tests.conftest import pytest_asyncio
+
+
+@pytest_asyncio.fixture(scope="session")
+async def cache():
+    cache = Cache()
+
+    task = asyncio.create_task(cache.invalidate_forever(interval=0.1))
+    yield cache
+    task.cancel()
+
+
+key = "foo"
+value = "bar"
+
+
+@pytest.mark.asyncio
+async def test_cache_get_set(cache):
+    cache.set(key, value)
+    assert cache.get(key) == value
+    assert cache.get(key, default="default") == value
+    assert cache.get("i-dont-exist", default="default") == "default"
+
+
+@pytest.mark.asyncio
+async def test_cache_expiry(cache):
+    cache.set(key, value, expiry=0.1)
+    await asyncio.sleep(0.2)
+    assert not cache.get(key)
+
+
+@pytest.mark.asyncio
+async def test_cache_pop(cache):
+    cache.set(key, value)
+    assert cache.pop(key) == value
+    assert not cache.get(key)
+    assert cache.pop(key, default="a") == "a"
+
+
+@pytest.mark.asyncio
+async def test_cache_coro(cache):
+    called = 0
+
+    async def test():
+        nonlocal called
+        called += 1
+        return called
+
+    await cache.save_result(test, key="test")
+    result = await cache.save_result(test, key="test")
+    assert result == called == 1