discovery: flatten future message cache

This commit removes the slice used when saving future messages into the
cache. Instead, each message is now saved independently into the cache
with a monotonically increasing integer as its ID.

discovery/gossiper.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -428,8 +429,9 @@ type AuthenticatedGossiper struct {
 	// height specified in the future. We will save them and resend it to
 	// the chan networkMsgs once the block height has reached. The cached
 	// map format is,
-	//   {blockHeight: [msg1, msg2, ...], ...}
-	futureMsgs *lru.Cache[uint32, *cachedNetworkMsg]
+	//   {msgID1: msg1, msgID2: msg2, ...}
+	futureMsgs  *lru.Cache[uint64, *cachedFutureMsg]
+	futureMsgID atomic.Uint64
 
 	// chanPolicyUpdates is a channel that requests to update the
 	// forwarding policy of a set of channels is sent over.
@@ -486,7 +488,7 @@ func New(cfg Config, selfKeyDesc *keychain.KeyDescriptor) *AuthenticatedGossiper
 		selfKeyLoc:  selfKeyDesc.KeyLocator,
 		cfg:         &cfg,
 		networkMsgs: make(chan *networkMsg),
-		futureMsgs: lru.NewCache[uint32, *cachedNetworkMsg](
+		futureMsgs: lru.NewCache[uint64, *cachedFutureMsg](
 			maxFutureMessages,
 		),
 		quit:              make(chan struct{}),
@@ -636,33 +638,64 @@ func (d *AuthenticatedGossiper) syncBlockHeight() {
 	}
 }
 
+// cachedFutureMsg is a future message that's saved to the `futureMsgs` cache.
+type cachedFutureMsg struct {
+	// msg is the network message.
+	msg *networkMsg
+
+	// height is the block height.
+	height uint32
+}
+
+// Size returns the size of the message.
+func (c *cachedFutureMsg) Size() (uint64, error) {
+	// Return a constant 1.
+	return 1, nil
+}
+
 // resendFutureMessages takes a block height, resends all the future messages
-// found at that height and deletes those messages found in the gossiper's
-// futureMsgs.
+// found below and equal to that height and deletes those messages found in the
+// gossiper's futureMsgs.
 func (d *AuthenticatedGossiper) resendFutureMessages(height uint32) {
-	result, err := d.futureMsgs.Get(height)
+	var (
+		// msgs are the target messages.
+		msgs []*networkMsg
+
+		// keys are the target messages' caching keys.
+		keys []uint64
+	)
+
+	// filterMsgs is the visitor used when iterating the future cache.
+	filterMsgs := func(k uint64, cmsg *cachedFutureMsg) bool {
+		if cmsg.height <= height {
+			msgs = append(msgs, cmsg.msg)
+			keys = append(keys, k)
+		}
+
+		return true
+	}
+
+	// Filter out the target messages.
+	d.futureMsgs.Range(filterMsgs)
 
 	// Return early if no messages found.
-	if err == cache.ErrElementNotFound {
+	if len(msgs) == 0 {
 		return
 	}
 
-	// The error must nil, we will log an error and exit.
-	if err != nil {
-		log.Errorf("Reading future messages got error: %v", err)
-		return
+	// Remove the filtered messages.
+	for _, key := range keys {
+		d.futureMsgs.Delete(key)
 	}
 
-	msgs := result.msgs
-
 	log.Debugf("Resending %d network messages at height %d",
 		len(msgs), height)
 
-	for _, pMsg := range msgs {
+	for _, msg := range msgs {
 		select {
-		case d.networkMsgs <- pMsg.msg:
+		case d.networkMsgs <- msg:
 		case <-d.quit:
-			pMsg.msg.err <- ErrGossiperShuttingDown
+			msg.err <- ErrGossiperShuttingDown
 		}
 	}
 }
@@ -1879,23 +1912,11 @@ func (d *AuthenticatedGossiper) isPremature(chanID lnwire.ShortChannelID,
 		return false
 	}
 
-	// Add the premature message to our future messages which will
-	// be resent once the block height has reached.
+	// Add the premature message to our future messages which will be
+	// resent once the block height has reached.
 	//
-	// Init an empty cached message and overwrite it if there are cached
-	// messages found.
-	cachedMsgs := &cachedNetworkMsg{
-		msgs: make([]*processedNetworkMsg, 0),
-	}
-
-	result, err := d.futureMsgs.Get(msgHeight)
-	// No error returned means we have old messages cached.
-	if err == nil {
-		cachedMsgs = result
-	}
-
-	// Copy the networkMsgs since the old message's err chan will
-	// be consumed.
+	// Copy the networkMsgs since the old message's err chan will be
+	// consumed.
 	copied := &networkMsg{
 		peer:              msg.peer,
 		source:            msg.source,
@@ -1905,15 +1926,15 @@ func (d *AuthenticatedGossiper) isPremature(chanID lnwire.ShortChannelID,
 		err:               make(chan error, 1),
 	}
 
-	// The processed boolean is unused in the futureMsgs case.
-	pMsg := &processedNetworkMsg{msg: copied}
+	// Create the cached message.
+	cachedMsg := &cachedFutureMsg{
+		msg:    copied,
+		height: msgHeight,
+	}
 
-	// Add the network message.
-	msgs := cachedMsgs.msgs
-	msgs = append(msgs, pMsg)
-	_, err = d.futureMsgs.Put(msgHeight, &cachedNetworkMsg{
-		msgs: msgs,
-	})
+	// Increment the msg ID and add it to the cache.
+	nextMsgID := d.futureMsgID.Add(1)
+	_, err := d.futureMsgs.Put(nextMsgID, cachedMsg)
 	if err != nil {
 		log.Errorf("Adding future message got error: %v", err)
 	}