Distinguish between DC not existing and not being available (#6399)

agent/consul/rpc.go

@@ -315,7 +315,11 @@ func (s *Server) getLeader() (bool, *metadata.Server) {
 func (s *Server) forwardDC(method, dc string, args interface{}, reply interface{}) error {
 	manager, server, ok := s.router.FindRoute(dc)
 	if !ok {
-		s.logger.Printf("[WARN] consul.rpc: RPC request for DC %q, no path found", dc)
+		if s.router.HasDatacenter(dc) {
+			s.logger.Printf("[WARN] consul.rpc: RPC request to DC %q is currently failing as no server can be reached", dc)
+			return structs.ErrDCNotAvailable
+		}
+		s.logger.Printf("[WARN] consul.rpc: RPC request for unkown DC %q", dc)
 		return structs.ErrNoDCPath
 	}
 

agent/router/router.go

@@ -356,6 +356,14 @@ func (r *Router) GetDatacenters() []string {
 	return dcs
 }
 
+// HasDatacenter checks whether dc is defined in WAN
+func (r *Router) HasDatacenter(dc string) bool {
+	r.RLock()
+	defer r.RUnlock()
+	_, ok := r.managers[dc]
+	return ok
+}
+
 // datacenterSorter takes a list of DC names and a parallel vector of distances
 // and implements sort.Interface, keeping both structures coherent and sorting
 // by distance.

agent/structs/errors.go

@@ -8,6 +8,7 @@ import (
 const (
 	errNoLeader                   = "No cluster leader"
 	errNoDCPath                   = "No path to datacenter"
+	errDCNotAvailable             = "Remote DC has no server currently reachable"
 	errNoServers                  = "No known Consul servers"
 	errNotReadyForConsistentReads = "Not ready to serve consistent reads"
 	errSegmentsNotSupported       = "Network segments are not supported in this version of Consul"
@@ -22,6 +23,7 @@ var (
 	ErrNotReadyForConsistentReads = errors.New(errNotReadyForConsistentReads)
 	ErrSegmentsNotSupported       = errors.New(errSegmentsNotSupported)
 	ErrRPCRateExceeded            = errors.New(errRPCRateExceeded)
+	ErrDCNotAvailable             = errors.New(errDCNotAvailable)
 )
 
 func IsErrNoLeader(err error) bool {