routing/chainview/neutrino: cache filter entries

This commit alters the neutrino chainview such that it
caches the filter entries corresponding to watched
outpoints at the moment they are added to the filter.
Previously, we would rederive each filter entry when
reconstructing the relevant filter entries, which
would lead to unnecessary work on the gc. Now, each is
created at most once, and reused across subsequent
reconstructions.

routing/chainview/neutrino.go

@@ -42,7 +42,7 @@ type CfFilteredChainView struct {
 
 	// chainFilter is the
 	filterMtx   sync.RWMutex
-	chainFilter map[wire.OutPoint]struct{}
+	chainFilter map[wire.OutPoint][]byte
 
 	quit chan struct{}
 	wg   sync.WaitGroup
@@ -62,7 +62,7 @@ func NewCfFilteredChainView(node *neutrino.ChainService) (*CfFilteredChainView,
 		blockQueue:    newBlockEventQueue(),
 		quit:          make(chan struct{}),
 		rescanErrChan: make(chan error),
-		chainFilter:   make(map[wire.OutPoint]struct{}),
+		chainFilter:   make(map[wire.OutPoint][]byte),
 		p2pNode:       node,
 	}, nil
 }
@@ -250,9 +250,8 @@ func (c *CfFilteredChainView) FilterBlock(blockHash *chainhash.Hash) (*FilteredB
 	// filters.
 	c.filterMtx.RLock()
 	relevantPoints := make([][]byte, 0, len(c.chainFilter))
-	for op := range c.chainFilter {
-		opBytes := builder.OutPointToFilterEntry(op)
-		relevantPoints = append(relevantPoints, opBytes)
+	for _, filterEntry := range c.chainFilter {
+		relevantPoints = append(relevantPoints, filterEntry)
 	}
 	c.filterMtx.RUnlock()
 
@@ -324,7 +323,7 @@ func (c *CfFilteredChainView) UpdateFilter(ops []wire.OutPoint,
 	// UTXO's, ignoring duplicates in the process.
 	c.filterMtx.Lock()
 	for _, op := range ops {
-		c.chainFilter[op] = struct{}{}
+		c.chainFilter[op] = builder.OutPointToFilterEntry(op)
 	}
 	c.filterMtx.Unlock()