From 42f9f7fb5dec74f8e5a95b19c1efd0c8b846e486 Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Fri, 1 May 2020 12:19:19 +0100
Subject: [PATCH] lib/cache: add Rename, Pin and Unpin

---
 lib/cache/cache.go      | 45 ++++++++++++++++++++-
 lib/cache/cache_test.go | 89 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 132 insertions(+), 2 deletions(-)

diff --git a/lib/cache/cache.go b/lib/cache/cache.go
index 59d853af4..7fa66392f 100644
--- a/lib/cache/cache.go
+++ b/lib/cache/cache.go
@@ -33,6 +33,7 @@ type cacheEntry struct {
 	err      error       // creation error
 	key      string      // key
 	lastUsed time.Time   // time used for expiry
+	pinCount int         // non zero if the entry should not be removed
 }
 
 // CreateFunc is called to create new values.  If the create function
@@ -74,6 +75,26 @@ func (c *Cache) Get(key string, create CreateFunc) (value interface{}, err error
 	return entry.value, entry.err
 }
 
+func (c *Cache) addPin(key string, count int) {
+	c.mu.Lock()
+	entry, ok := c.cache[key]
+	if ok {
+		entry.pinCount += count
+		c.used(entry)
+	}
+	c.mu.Unlock()
+}
+
+// Pin a value in the cache if it exists
+func (c *Cache) Pin(key string) {
+	c.addPin(key, 1)
+}
+
+// Unpin a value in the cache if it exists
+func (c *Cache) Unpin(key string) {
+	c.addPin(key, -1)
+}
+
 // Put puts an value named key into the cache
 func (c *Cache) Put(key string, value interface{}) {
 	c.mu.Lock()
@@ -98,13 +119,35 @@ func (c *Cache) GetMaybe(key string) (value interface{}, found bool) {
 	return entry.value, found
 }
 
+// Rename renames the item at oldKey to newKey.
+//
+// If there was an existing item at newKey then it takes precedence
+// and is returned otherwise the item (if any) at oldKey is returned.
+func (c *Cache) Rename(oldKey, newKey string) (value interface{}, found bool) {
+	c.mu.Lock()
+	if newEntry, newFound := c.cache[newKey]; newFound {
+		// If new entry is found use that
+		delete(c.cache, oldKey)
+		value, found = newEntry.value, newFound
+		c.used(newEntry)
+	} else if oldEntry, oldFound := c.cache[oldKey]; oldFound {
+		// If old entry is found rename it to new and use that
+		c.cache[newKey] = oldEntry
+		delete(c.cache, oldKey)
+		c.used(oldEntry)
+		value, found = oldEntry.value, oldFound
+	}
+	c.mu.Unlock()
+	return value, found
+}
+
 // cacheExpire expires any entries that haven't been used recently
 func (c *Cache) cacheExpire() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	now := time.Now()
 	for key, entry := range c.cache {
-		if now.Sub(entry.lastUsed) > c.expireDuration {
+		if entry.pinCount <= 0 && now.Sub(entry.lastUsed) > c.expireDuration {
 			delete(c.cache, key)
 		}
 	}
diff --git a/lib/cache/cache_test.go b/lib/cache/cache_test.go
index cfdc28023..d2811e66d 100644
--- a/lib/cache/cache_test.go
+++ b/lib/cache/cache_test.go
@@ -106,22 +106,74 @@ func TestCacheExpire(t *testing.T) {
 
 	c.mu.Lock()
 	entry := c.cache["/"]
-
 	assert.Equal(t, 1, len(c.cache))
 	c.mu.Unlock()
+
 	c.cacheExpire()
+
 	c.mu.Lock()
 	assert.Equal(t, 1, len(c.cache))
 	entry.lastUsed = time.Now().Add(-c.expireDuration - 60*time.Second)
 	assert.Equal(t, true, c.expireRunning)
 	c.mu.Unlock()
+
 	time.Sleep(250 * time.Millisecond)
+
 	c.mu.Lock()
 	assert.Equal(t, false, c.expireRunning)
 	assert.Equal(t, 0, len(c.cache))
 	c.mu.Unlock()
 }
 
+func TestCachePin(t *testing.T) {
+	c, create := setup(t)
+
+	_, err := c.Get("/", create)
+	require.NoError(t, err)
+
+	// Pin a non existent item to show nothing happens
+	c.Pin("notfound")
+
+	c.mu.Lock()
+	entry := c.cache["/"]
+	assert.Equal(t, 1, len(c.cache))
+	c.mu.Unlock()
+
+	c.cacheExpire()
+
+	c.mu.Lock()
+	assert.Equal(t, 1, len(c.cache))
+	c.mu.Unlock()
+
+	// Pin the entry and check it does not get expired
+	c.Pin("/")
+
+	// Reset last used to make the item expirable
+	c.mu.Lock()
+	entry.lastUsed = time.Now().Add(-c.expireDuration - 60*time.Second)
+	c.mu.Unlock()
+
+	c.cacheExpire()
+
+	c.mu.Lock()
+	assert.Equal(t, 1, len(c.cache))
+	c.mu.Unlock()
+
+	// Unpin the entry and check it does get expired now
+	c.Unpin("/")
+
+	// Reset last used
+	c.mu.Lock()
+	entry.lastUsed = time.Now().Add(-c.expireDuration - 60*time.Second)
+	c.mu.Unlock()
+
+	c.cacheExpire()
+
+	c.mu.Lock()
+	assert.Equal(t, 0, len(c.cache))
+	c.mu.Unlock()
+}
+
 func TestClear(t *testing.T) {
 	c, create := setup(t)
 
@@ -172,3 +224,38 @@ func TestGetMaybe(t *testing.T) {
 	assert.Equal(t, false, found)
 	assert.Nil(t, value)
 }
+
+func TestCacheRename(t *testing.T) {
+	c := New()
+	create := func(path string) (interface{}, bool, error) {
+		return path, true, nil
+	}
+
+	existing1, err := c.Get("existing1", create)
+	require.NoError(t, err)
+	_, err = c.Get("existing2", create)
+	require.NoError(t, err)
+
+	assert.Equal(t, 2, c.Entries())
+
+	// rename to non existent
+	value, found := c.Rename("existing1", "EXISTING1")
+	assert.Equal(t, true, found)
+	assert.Equal(t, existing1, value)
+
+	assert.Equal(t, 2, c.Entries())
+
+	// rename to existent and check existing value is returned
+	value, found = c.Rename("existing2", "EXISTING1")
+	assert.Equal(t, true, found)
+	assert.Equal(t, existing1, value)
+
+	assert.Equal(t, 1, c.Entries())
+
+	// rename non existent
+	value, found = c.Rename("notfound", "NOTFOUND")
+	assert.Equal(t, false, found)
+	assert.Nil(t, value)
+
+	assert.Equal(t, 1, c.Entries())
+}