Remove TTL from cacheEntryExpiry

This should very slightly reduce the amount of memory required to store each item in
the cache.

It will also enable setting different TTLs based on the type of result. For example
we may want to use a shorter TTL when the result indicates the resource does not exist,
as storing these types of records could easily lead to a DOS caused by
OOM.

agent/cache/cache.go

@@ -317,7 +317,7 @@ RETRY_GET:
 
 		// Touch the expiration and fix the heap.
 		c.entriesLock.Lock()
-		entry.Expiry.Reset()
+		entry.Expiry.Update(tEntry.Opts.LastGetTTL)
 		c.entriesExpiryHeap.Fix(entry.Expiry)
 		c.entriesLock.Unlock()
 
@@ -582,11 +582,8 @@ func (c *Cache) fetch(tEntry typeEntry, key string, r Request, allowNew bool, at
 		// initial expiry information and insert. If we're already in
 		// the heap we do nothing since we're reusing the same entry.
 		if newEntry.Expiry == nil || newEntry.Expiry.HeapIndex == -1 {
-			newEntry.Expiry = &cacheEntryExpiry{
-				Key: key,
-				TTL: tEntry.Opts.LastGetTTL,
-			}
-			newEntry.Expiry.Reset()
+			newEntry.Expiry = &cacheEntryExpiry{Key: key}
+			newEntry.Expiry.Update(tEntry.Opts.LastGetTTL)
 			heap.Push(c.entriesExpiryHeap, newEntry.Expiry)
 		}
 
@@ -717,13 +714,6 @@ func (c *Cache) Close() error {
 // AutoEncrypt.TLS is turned on. The cache itself cannot fetch that the first
 // time because it requires a special RPCType. Subsequent runs are fine though.
 func (c *Cache) Prepopulate(t string, res FetchResult, dc, token, k string) error {
-	// Check the type that we're prepolulating
-	c.typesLock.RLock()
-	tEntry, ok := c.types[t]
-	c.typesLock.RUnlock()
-	if !ok {
-		return fmt.Errorf("unknown type in cache: %s", t)
-	}
 	key := makeEntryKey(t, dc, token, k)
 	newEntry := cacheEntry{
 		Valid:     true,
@@ -732,7 +722,7 @@ func (c *Cache) Prepopulate(t string, res FetchResult, dc, token, k string) erro
 		Index:     res.Index,
 		FetchedAt: time.Now(),
 		Waiter:    make(chan struct{}),
-		Expiry:    &cacheEntryExpiry{Key: key, TTL: tEntry.Opts.LastGetTTL},
+		Expiry:    &cacheEntryExpiry{Key: key},
 	}
 	c.entriesLock.Lock()
 	c.entries[key] = newEntry

agent/cache/entry.go

@@ -49,13 +49,12 @@ type cacheEntry struct {
 type cacheEntryExpiry struct {
 	Key       string    // Key in the cache map
 	Expires   time.Time // Time when entry expires (monotonic clock)
-	TTL       time.Duration // TTL for this entry to extend when resetting
 	HeapIndex int       // Index in the heap
 }
 
-// Reset resets the expiration to be the ttl duration from now.
-func (e *cacheEntryExpiry) Reset() {
-	e.Expires = time.Now().Add(e.TTL)
+// Update the expiry to d time from now.
+func (e *cacheEntryExpiry) Update(d time.Duration) {
+	e.Expires = time.Now().Add(d)
 }
 
 // expiryHeap is a heap implementation that stores information about