Avoid incrementing cache hits for expired entries (#2159)

.changeset/forty-coins-hug.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Avoid incrementing cache hits for expired entries

packages/effect/src/internal/cache.ts

@@ -551,11 +551,11 @@ class CacheImpl<in out Key, in out Error, in out Value> implements Cache.Cache<K
       switch (value._tag) {
         case "Complete": {
           this.trackAccess(value.key)
-          this.trackHit()
           if (this.hasExpired(clock, value.timeToLiveMillis)) {
             MutableHashMap.remove(this.cacheState.map, value.key.current)
             return core.succeed(Option.none<Value>())
           }
+          this.trackHit()
           return core.map(value.exit, Option.some)
         }
         case "Pending": {

packages/effect/test/Cache.test.ts

@@ -0,0 +1,22 @@
+import * as it from "effect-test/utils/extend"
+import * as Cache from "effect/Cache"
+import * as Effect from "effect/Effect"
+import * as TestClock from "effect/TestClock"
+import { describe, expect } from "vitest"
+
+describe("Cache", () => {
+  it.effect("should not increment cache hits on expired entries", () =>
+    Effect.gen(function*(_) {
+      const cache = yield* _(Cache.make({
+        capacity: 100,
+        timeToLive: "1 seconds",
+        lookup: (n: number) => Effect.succeed(n)
+      }))
+      yield* _(cache.get(42))
+      yield* _(TestClock.adjust("2 seconds"))
+      yield* _(cache.get(42))
+      const { hits, misses } = yield* _(cache.cacheStats)
+      expect(hits).toBe(0)
+      expect(misses).toBe(2)
+    }))
+})