channeldb: implement a multi-source AddrSource

In this commit, we implement a version of the AddrSource interface which
merges the results of a set of AddrSource implementations. This will
later be used to merge the results of the channel db and graph db.

channeldb/addr_source.go

@@ -13,3 +13,56 @@ type AddrSource interface {
 	// key.
 	AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr, error)
 }
+
+// multiAddrSource is an implementation of AddrSource which gathers all the
+// known addresses for a given node from multiple backends and de-duplicates the
+// results.
+type multiAddrSource struct {
+	sources []AddrSource
+}
+
+// NewMultiAddrSource constructs a new AddrSource which will query all the
+// provided sources for a node's addresses and will then de-duplicate the
+// results.
+func NewMultiAddrSource(sources ...AddrSource) AddrSource {
+	return &multiAddrSource{
+		sources: sources,
+	}
+}
+
+// AddrsForNode returns all known addresses for the target node public key. It
+// queries all the address sources provided and de-duplicates the results.
+//
+// NOTE: this implements the AddrSource interface.
+func (c *multiAddrSource) AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr,
+	error) {
+
+	// The multiple address sources will likely contain duplicate addresses,
+	// so we use a map here to de-dup them.
+	dedupedAddrs := make(map[string]net.Addr)
+
+	// Iterate over all the address sources and query each one for the
+	// addresses it has for the node in question.
+	for _, src := range c.sources {
+		addrs, err := src.AddrsForNode(nodePub)
+		if err != nil {
+			return nil, err
+		}
+
+		for _, addr := range addrs {
+			dedupedAddrs[addr.String()] = addr
+		}
+	}
+
+	// Convert the map into a list we can return.
+	addrs := make([]net.Addr, 0, len(dedupedAddrs))
+	for _, addr := range dedupedAddrs {
+		addrs = append(addrs, addr)
+	}
+
+	return addrs, nil
+}
+
+// A compile-time check to ensure that multiAddrSource implements the AddrSource
+// interface.
+var _ AddrSource = (*multiAddrSource)(nil)