add basic explorer API endpoint

api/client.go

@@ -58,3 +58,9 @@ func (c *Client) SyncerBroadcastBlock(b types.Block) (err error) {
 	err = c.c.POST("/syncer/broadcast/block", b, nil)
 	return
 }
+
+// Tip returns the current tip of the explorer.
+func (c *Client) Tip() (resp types.ChainIndex, err error) {
+	err = c.c.GET("/explorer/tip", &resp)
+	return
+}

api/server.go

@@ -36,10 +36,16 @@ type (
 		BroadcastV2TransactionSet(txns []types.V2Transaction)
 		BroadcastV2BlockOutline(bo gateway.V2BlockOutline)
 	}
+
+	// Explorer implements a Sia explorer.
+	Explorer interface {
+		Tip() (types.ChainIndex, error)
+	}
 )
 
 type server struct {
 	cm ChainManager
+	e  Explorer
 	s  Syncer
 
 	mu sync.Mutex
@@ -124,10 +130,17 @@ func (s *server) txpoolBroadcastHandler(jc jape.Context) {
 	}
 }
 
+func (s *server) explorerTipHandler(jc jape.Context) {
+	tip, err := s.e.Tip()
+	jc.Check("failed to get tip", err)
+	jc.Encode(tip)
+}
+
 // NewServer returns an HTTP handler that serves the explored API.
-func NewServer(cm ChainManager, s Syncer) http.Handler {
+func NewServer(e Explorer, cm ChainManager, s Syncer) http.Handler {
 	srv := server{
 		cm: cm,
+		e:  e,
 		s:  s,
 	}
 	return jape.Mux(map[string]jape.Handler{
@@ -138,5 +151,7 @@ func NewServer(cm ChainManager, s Syncer) http.Handler {
 		"GET    /txpool/transactions": srv.txpoolTransactionsHandler,
 		"GET    /txpool/fee":          srv.txpoolFeeHandler,
 		"POST   /txpool/broadcast":    srv.txpoolBroadcastHandler,
+
+		"GET    /explorer/tip": srv.explorerTipHandler,
 	})
 }

cmd/explored/web.go

@@ -10,7 +10,7 @@ import (
 )
 
 func startWeb(l net.Listener, node *node, password string) error {
-	renter := api.NewServer(node.cm, node.s)
+	renter := api.NewServer(node.e, node.cm, node.s)
 	api := jape.BasicAuth(password)(renter)
 	return http.Serve(l, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if strings.HasPrefix(r.URL.Path, "/api") {

explorer/explorer.go

@@ -22,3 +22,7 @@ type Explorer struct {
 func NewExplorer(s Store) *Explorer {
 	return &Explorer{s: s}
 }
+
+func (e *Explorer) Tip() (types.ChainIndex, error) {
+	return e.s.Tip()
+}