Add test to ensure backfilling results does not lead to evictions

Partial results are back-filled (new domains added) by re-setting them
in the cache. With a sufficiently incorrect implementation, the cache
can evict entries on that occasion even though it does not need
to (because we're replacing an existing entry).

Exactly that should have been fixed by #204, but was not tested at the
time.

Fixes #199

Author: masklinn

tests/test_caches.py

@@ -1,25 +1,24 @@
 from collections import OrderedDict
 
+import pytest  # type: ignore
+
 from ua_parser import (
-    BasicResolver,
     CachingResolver,
-    Device,
     Domain,
-    OS,
     Parser,
     PartialResult,
-    UserAgent,
 )
-from ua_parser.caching import Lru
-from ua_parser.matchers import DeviceMatcher, OSMatcher, UserAgentMatcher
+from ua_parser.caching import Lru, S3Fifo, Sieve
 
 
 def test_lru():
     """Tests that the cache entries do get moved when accessed, and are
     popped LRU-first.
     """
     cache = Lru(2)
-    p = Parser(CachingResolver(BasicResolver(([], [], [])), cache))
+    p = Parser(
+        CachingResolver(lambda s, d: PartialResult(d, None, None, None, s), cache)
+    )
 
     p.parse("a")
     p.parse("b")
@@ -41,37 +40,36 @@ def test_lru():
     )
 
 
-def test_backfill():
-    """Tests that caches handle partial parsing correctly, by updating the
-    existing entry when new parts get parsed.
+@pytest.mark.parametrize("cache", [Lru, S3Fifo, Sieve])
+def test_backfill(cache):
+    """Tests that caches handle partial parsing correctly, by updating
+    the existing entry when new parts get parsed, without evicting
+    still-fitting entries.
     """
-    cache = Lru(2)
-    p = Parser(
-        CachingResolver(
-            BasicResolver(
-                (
-                    [UserAgentMatcher("(a)")],
-                    [OSMatcher("(a)")],
-                    [DeviceMatcher("(a)")],
-                )
-            ),
-            cache,
-        )
-    )
+    misses = 0
+
+    def resolver(ua: str, domains: Domain, /) -> PartialResult:
+        nonlocal misses
+        misses += 1
+        return PartialResult(domains, None, None, None, ua)
+
+    p = Parser(CachingResolver(resolver, cache(10)))
 
+    # fill the cache first, no need to hit the entries twice because
+    # S3 waits until it needs space in the main cache before demotes
+    # (or promotes) from the probationary cache.
+    for s in map(str, range(9)):
+        p.parse(s)
+    assert misses == 9
+    # add a partial entry
     p.parse_user_agent("a")
-    assert cache.cache == {
-        "a": PartialResult(Domain.USER_AGENT, UserAgent("a"), None, None, "a"),
-    }
-    p("a", Domain.OS)
-    assert cache.cache == {
-        "a": PartialResult(
-            Domain.USER_AGENT | Domain.OS, UserAgent("a"), OS("a"), None, "a"
-        ),
-    }
-    p.parse("a")
-    assert cache.cache == {
-        "a": PartialResult(
-            Domain.ALL, UserAgent("a"), OS("a"), Device("a", None, "a"), "a"
-        ),
-    }
+    # fill the partial entry, counts as a miss since it needs to
+    # resolve the new bit
+    p.parse_os("a")
+    assert misses == 11
+
+    misses = 0
+    # check that the original entries are all hits
+    for s in map(str, range(9)):
+        p.parse(s)
+    assert misses == 0