test(core.utils): cover FileStreamProgramCache size cap

Exercises the opportunistic mtime-based eviction sweep that runs on each
write and confirms that omitting the cap keeps the cache unbounded.

Part of issue #178.

Author: cpcloud

cuda_core/tests/test_program_cache.py

@@ -559,3 +559,33 @@ def test_filestream_cache_accepts_str_keys(tmp_path):
         cache["my-key"] = _fake_object_code(b"v")
         assert "my-key" in cache
         assert b"my-key" in cache
+
+
+def test_filestream_cache_size_cap_evicts_oldest(tmp_path):
+    from cuda.core.utils import FileStreamProgramCache
+
+    # Big payloads, small cap; after the third entry, the cap is exceeded and
+    # the oldest file (a) must be evicted.
+    cap = 3000
+    root = tmp_path / "fc"
+    with FileStreamProgramCache(root, max_size_bytes=cap) as cache:
+        cache[b"a"] = _fake_object_code(b"A" * 2000, name="a")
+        time.sleep(0.02)
+        cache[b"b"] = _fake_object_code(b"B" * 2000, name="b")
+        time.sleep(0.02)
+        cache[b"c"] = _fake_object_code(b"C" * 2000, name="c")
+
+    with FileStreamProgramCache(root, max_size_bytes=cap) as cache:
+        # 'a' was oldest by mtime; the opportunistic sweep on the third write
+        # must have removed it. 'c' is the newest and must remain.
+        assert b"a" not in cache
+        assert b"c" in cache
+
+
+def test_filestream_cache_unbounded_by_default(tmp_path):
+    from cuda.core.utils import FileStreamProgramCache
+
+    with FileStreamProgramCache(tmp_path / "fc") as cache:
+        for i in range(20):
+            cache[f"k{i}".encode()] = _fake_object_code(b"X" * 1024, name=f"n{i}")
+        assert len(cache) == 20