From 391cae49f4992c963e25a238168de29bddb89f2f Mon Sep 17 00:00:00 2001
From: motoki317 <motoki317@gmail.com>
Date: Mon, 5 Jun 2023 22:33:46 +0900
Subject: [PATCH] Add GetIfExists method to cache

---
 cache.go      | 25 +++++++++++++++
 cache_test.go | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+)

diff --git a/cache.go b/cache.go
index 551dfbe..e096185 100644
--- a/cache.go
+++ b/cache.go
@@ -156,6 +156,31 @@ retry:
 	return cl.val.v, cl.err
 }
 
+// GetIfExists retrieves an item without triggering value replacements.
+//
+// This method doesn't wait for value replacement to finish, even if there is an ongoing one.
+func (c *cache[K, V]) GetIfExists(key K) (v V, ok bool) {
+	// Record time as soon as Get is called *before acquiring the lock* - this maximizes the reuse of values
+	calledAt := monoTimeNow()
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	val, ok := c.values.Get(key)
+
+	// value exists (includes stale values)
+	if ok && !val.isExpired(calledAt, c.ttl) {
+		if val.isFresh(calledAt, c.freshFor) {
+			c.stats.Hits++
+		} else {
+			c.stats.GraceHits++
+		}
+		return val.v, true
+	}
+
+	// value doesn't exist
+	c.stats.Misses++
+	return val.v, false
+}
+
 // Forget instructs the cache to forget about the key.
 // Corresponding item will be deleted, ongoing cache replacement results (if any) will not be added to the cache,
 // and any future Get calls will immediately retrieve a new item.
diff --git a/cache_test.go b/cache_test.go
index b84d502..ef0744d 100644
--- a/cache_test.go
+++ b/cache_test.go
@@ -349,6 +349,90 @@ func TestCache_Get_Error(t *testing.T) {
 	}
 }
 
+func TestCache_GetIfExists(t *testing.T) {
+	t.Parallel()
+
+	for _, c := range allCaches(10) {
+		c := c
+		t.Run(c.name, func(t *testing.T) {
+			t.Parallel()
+
+			var cnt int64
+			replaceFn := func(ctx context.Context, key string) (string, error) {
+				t.Log("replaceFn triggered")
+				atomic.AddInt64(&cnt, 1)
+				return "result-" + key, nil
+			}
+			cache, err := New[string, string](replaceFn, 500*time.Millisecond, 1*time.Second, c.cacheOpts...)
+			assert.NoError(t, err)
+
+			// Check empty
+			_, ok := cache.GetIfExists("k1")
+			assert.False(t, ok)
+			_, ok = cache.GetIfExists("k2")
+			assert.False(t, ok)
+			_, ok = cache.GetIfExists("k3")
+			assert.False(t, ok)
+			assert.EqualValues(t, 0, cnt)
+
+			// trigger value replacement
+			val, err := cache.Get(context.Background(), "k1")
+			assert.NoError(t, err)
+			assert.Equal(t, "result-k1", val)
+			assert.EqualValues(t, 1, cnt)
+			val, err = cache.Get(context.Background(), "k2")
+			assert.NoError(t, err)
+			assert.Equal(t, "result-k2", val)
+			assert.EqualValues(t, 2, cnt)
+
+			// Check k1 and k2 are present
+			val, ok = cache.GetIfExists("k1")
+			assert.True(t, ok)
+			assert.Equal(t, "result-k1", val)
+			val, ok = cache.GetIfExists("k2")
+			assert.True(t, ok)
+			assert.Equal(t, "result-k2", val)
+			_, ok = cache.GetIfExists("k3")
+			assert.False(t, ok)
+			assert.EqualValues(t, 2, cnt)
+
+			// test graceful hit
+			time.Sleep(750 * time.Millisecond)
+			val, ok = cache.GetIfExists("k1")
+			assert.True(t, ok)
+			assert.Equal(t, "result-k1", val)
+			val, ok = cache.GetIfExists("k2")
+			assert.True(t, ok)
+			assert.Equal(t, "result-k2", val)
+			_, ok = cache.GetIfExists("k3")
+			assert.False(t, ok)
+			assert.EqualValues(t, 2, cnt)
+
+			// test forget
+			cache.Forget("k2")
+
+			val, ok = cache.GetIfExists("k1")
+			assert.True(t, ok)
+			assert.Equal(t, "result-k1", val)
+			_, ok = cache.GetIfExists("k2")
+			assert.False(t, ok)
+			_, ok = cache.GetIfExists("k3")
+			assert.False(t, ok)
+			assert.EqualValues(t, 2, cnt)
+
+			// test expiration
+			time.Sleep(500 * time.Millisecond)
+			_, ok = cache.GetIfExists("k1")
+			assert.False(t, ok)
+			_, ok = cache.GetIfExists("k2")
+			assert.False(t, ok)
+			_, ok = cache.GetIfExists("k3")
+			assert.False(t, ok)
+			assert.EqualValues(t, 2, cnt)
+		})
+	}
+}
+
 // TestCache_Forget_Interrupt ensures that calling Cache.Forget will make later Get calls trigger replaceFn.
 func TestCache_Forget_Interrupt(t *testing.T) {
 	t.Parallel()