mempool: Keep cache hashmap and linked list in sync (#2188)

* mempool: Keep cache hashmap and linked list in sync

This removes bugs with the linked list being full, but hashmap empty

* address PR comments

* switch clist back to list

mempool/mempool.go

@@ -488,7 +488,7 @@ type txCache interface {
 type mapTxCache struct {
 	mtx  sync.Mutex
 	size int
-	map_ map[string]struct{}
+	map_ map[string]*list.Element
 	list *list.List // to remove oldest tx when cache gets too big
 }
 
@@ -498,7 +498,7 @@ var _ txCache = (*mapTxCache)(nil)
 func newMapTxCache(cacheSize int) *mapTxCache {
 	return &mapTxCache{
 		size: cacheSize,
-		map_: make(map[string]struct{}, cacheSize),
+		map_: make(map[string]*list.Element, cacheSize),
 		list: list.New(),
 	}
 }
@@ -506,7 +506,7 @@ func newMapTxCache(cacheSize int) *mapTxCache {
 // Reset resets the cache to an empty state.
 func (cache *mapTxCache) Reset() {
 	cache.mtx.Lock()
-	cache.map_ = make(map[string]struct{}, cache.size)
+	cache.map_ = make(map[string]*list.Element, cache.size)
 	cache.list.Init()
 	cache.mtx.Unlock()
 }
@@ -524,20 +524,26 @@ func (cache *mapTxCache) Push(tx types.Tx) bool {
 	if cache.list.Len() >= cache.size {
 		popped := cache.list.Front()
 		poppedTx := popped.Value.(types.Tx)
-		// NOTE: the tx may have already been removed from the map
-		// but deleting a non-existent element is fine
 		delete(cache.map_, string(poppedTx))
-		cache.list.Remove(popped)
+		if popped != nil {
+			cache.list.Remove(popped)
+		}
 	}
-	cache.map_[string(tx)] = struct{}{}
 	cache.list.PushBack(tx)
+	cache.map_[string(tx)] = cache.list.Back()
 	return true
 }
 
 // Remove removes the given tx from the cache.
 func (cache *mapTxCache) Remove(tx types.Tx) {
 	cache.mtx.Lock()
-	delete(cache.map_, string(tx))
+	stx := string(tx)
+	popped := cache.map_[stx]
+	delete(cache.map_, stx)
+	if popped != nil {
+		cache.list.Remove(popped)
+	}
+
 	cache.mtx.Unlock()
 }