graph/db: add options infrastructure for iterator configuration

In this commit, we introduce a new options pattern for configuring
iterator behavior in the graph database. This includes configuration
for batch sizes when iterating over channel and node updates, as well
as an option to filter for public nodes only.

The new functional options pattern allows callers to customize iterator
behavior without breaking existing APIs. Default batch sizes are set to
1000 entries for both channel and node updates, which provides a good
balance between memory usage and performance.

graph/db/options.go

@@ -20,6 +20,60 @@ const (
 	DefaultPreAllocCacheNumNodes = 15000
 )
 
+// IteratorOption is a functional option used to change the per-call
+// configuration for iterators.
+type IteratorOption func(*iterConfig)
+
+// iterConfig holds the configuration for graph operations.
+type iterConfig struct {
+	// chanUpdateIterBatchSize is the batch size to use when reading out
+	// channel updates to send a peer a backlog.
+	chanUpdateIterBatchSize int
+
+	// nodeUpdateIterBatchSize is the batch size to use when reading out
+	// node updates to send to a peer backlog.
+	nodeUpdateIterBatchSize int
+
+	// iterPublicNodes is used to make an iterator that only iterates over
+	// public nodes.
+	iterPublicNodes bool
+}
+
+// defaultIteratorConfig returns the default configuration.
+func defaultIteratorConfig() *iterConfig {
+	return &iterConfig{
+		chanUpdateIterBatchSize: 1_000,
+		nodeUpdateIterBatchSize: 1_000,
+	}
+}
+
+// WithChanUpdateIterBatchSize sets the batch size for channel update
+// iterators.
+func WithChanUpdateIterBatchSize(size int) IteratorOption {
+	return func(cfg *iterConfig) {
+		if size > 0 {
+			cfg.chanUpdateIterBatchSize = size
+		}
+	}
+}
+
+// WithNodeUpdateIterBatchSize set the batch size for node ann iterators.
+func WithNodeUpdateIterBatchSize(size int) IteratorOption {
+	return func(cfg *iterConfig) {
+		if size > 0 {
+			cfg.nodeUpdateIterBatchSize = size
+		}
+	}
+}
+
+// WithIterPublicNodesOnly is used to create an iterator that only iterates over
+// public nodes.
+func WithIterPublicNodesOnly() IteratorOption {
+	return func(cfg *iterConfig) {
+		cfg.iterPublicNodes = true
+	}
+}
+
 // chanGraphOptions holds parameters for tuning and customizing the
 // ChannelGraph.
 type chanGraphOptions struct {