From 37d639064206cb89b50c0f3387179926f9c83a68 Mon Sep 17 00:00:00 2001
From: Elle Mouton <elle.mouton@gmail.com>
Date: Sun, 29 Jun 2025 16:33:44 +0200
Subject: [PATCH] discovery: use a no-op hash accumluator for local networks

If LND is running on a local network, then use deterministic sampling so
that we can have deterministic peer bootstrapping.
---
 discovery/bootstrapper.go                  | 51 +++++++++++++++++++---
 docs/release-notes/release-notes-0.20.0.md |  4 +-
 server.go                                  |  4 +-
 3 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/discovery/bootstrapper.go b/discovery/bootstrapper.go
index 81576df3d..dbfdbc7d2 100644
--- a/discovery/bootstrapper.go
+++ b/discovery/bootstrapper.go
@@ -138,13 +138,26 @@ var _ NetworkPeerBootstrapper = (*ChannelGraphBootstrapper)(nil)
 // backed by an active autopilot.ChannelGraph instance. This type of network
 // peer bootstrapper will use the authenticated nodes within the known channel
 // graph to bootstrap connections.
-func NewGraphBootstrapper(cg autopilot.ChannelGraph) (NetworkPeerBootstrapper,
-	error) {
+func NewGraphBootstrapper(cg autopilot.ChannelGraph,
+	deterministicSampling bool) (NetworkPeerBootstrapper, error) {
 
-	hashAccumulator, err := newRandomHashAccumulator()
-	if err != nil {
-		return nil, fmt.Errorf("unable to create hash accumulator: %w",
-			err)
+	var (
+		hashAccumulator hashAccumulator
+		err             error
+	)
+	if deterministicSampling {
+		// If we're using deterministic sampling, then we'll use a
+		// no-op hash accumulator that will always return false for
+		// skipNode.
+		hashAccumulator = newNoOpHashAccumulator()
+	} else {
+		// Otherwise, we'll use a random hash accumulator to sample
+		// nodes from the channel graph.
+		hashAccumulator, err = newRandomHashAccumulator()
+		if err != nil {
+			return nil, fmt.Errorf("unable to create hash "+
+				"accumulator: %w", err)
+		}
 	}
 
 	return &ChannelGraphBootstrapper{
@@ -602,3 +615,29 @@ func (r *randomHashAccumulator) rotate() {
 func (r *randomHashAccumulator) skipNode(pub route.Vertex) bool {
 	return bytes.Compare(r.hash[:], pub[1:]) > 0
 }
+
+// noOpHashAccumulator is a no-op implementation of the hashAccumulator
+// interface. This is used when we want deterministic behavior and don't
+// want to sample nodes randomly from the channel graph.
+type noOpHashAccumulator struct{}
+
+// newNoOpHashAccumulator returns a new instance of a noOpHashAccumulator.
+func newNoOpHashAccumulator() *noOpHashAccumulator {
+	return &noOpHashAccumulator{}
+}
+
+// rotate is a no-op for the noOpHashAccumulator.
+//
+// NOTE: this is part of the hashAccumulator interface.
+func (*noOpHashAccumulator) rotate() {}
+
+// skipNode always returns false, meaning that no nodes will be skipped.
+//
+// NOTE: this is part of the hashAccumulator interface.
+func (*noOpHashAccumulator) skipNode(route.Vertex) bool {
+	return false
+}
+
+// A compile-time assertion to ensure that noOpHashAccumulator meets the
+// hashAccumulator interface.
+var _ hashAccumulator = (*noOpHashAccumulator)(nil)
diff --git a/docs/release-notes/release-notes-0.20.0.md b/docs/release-notes/release-notes-0.20.0.md
index 29c8ba532..d0e163c45 100644
--- a/docs/release-notes/release-notes-0.20.0.md
+++ b/docs/release-notes/release-notes-0.20.0.md
@@ -144,7 +144,9 @@ reader of a payment request.
   disabling has now been 
   [removed](https://github.com/lightningnetwork/lnd/pull/9967) meaning that any 
   test network scripts that rely on bootstrapping being disabled will need to 
-  explicitly define the `--nobootstrap` flag.
+  explicitly define the `--nobootstrap` flag. Bootstrapping will now also be
+  [deterministic](https://github.com/lightningnetwork/lnd/pull/10003) on local 
+  test networks so that bootstrapping behaviour can be tested for.
 
 ## Database
 
diff --git a/server.go b/server.go
index f6884a1fe..a0089ea38 100644
--- a/server.go
+++ b/server.go
@@ -3064,7 +3064,9 @@ func initNetworkBootstrappers(s *server) ([]discovery.NetworkPeerBootstrapper, e
 	// this can be used by default if we've already partially seeded the
 	// network.
 	chanGraph := autopilot.ChannelGraphFromDatabase(s.graphDB)
-	graphBootstrapper, err := discovery.NewGraphBootstrapper(chanGraph)
+	graphBootstrapper, err := discovery.NewGraphBootstrapper(
+		chanGraph, s.cfg.Bitcoin.IsLocalNetwork(),
+	)
 	if err != nil {
 		return nil, err
 	}