fix: startup with disabled index provider (#1604)

Author: dirkmc

node/modules/graphsync.go

@@ -70,8 +70,10 @@ func (p *LinkSystemProv) LinkSys() *ipld.LinkSystem {
 }
 
 func SetLinkSystem(proxy *LinkSystemProv, prov provider.Interface) {
-	e := prov.(*engine.Engine)
-	proxy.LinkSystem = e.LinkSystem()
+	e, ok := prov.(*engine.Engine)
+	if ok {
+		proxy.LinkSystem = e.LinkSystem()
+	}
 }
 
 // RetrievalGraphsync creates a graphsync instance used to serve retrievals.

retrievalmarket/server/gsunpaidretrieval.go

@@ -120,7 +120,7 @@ func (g *GraphsyncUnpaidRetrieval) Start(ctx context.Context) error {
 	g.ctx = ctx
 	g.validator.ctx = ctx
 
-	if g.linkSystem != nil {
+	if g.linkSystem != nil && g.linkSystem.LinkSys() != nil {
 		// The index provider uses graphsync to fetch advertisements.
 		// We need to tell graphsync to use a different IPLD Link System to provide
 		// the advertisements (instead of using the blockstore).