proxyd: avoid banning a backend that is a forced candidate (#7275)

* Add consensus configuration option to never ban a backend

* Fix panic

* Remove additional flag, use `consensus_forced_candidate` instead

proxyd/consensus_poller.go

@@ -261,7 +261,7 @@ func (cp *ConsensusPoller) UpdateBackend(ctx context.Context, be *Backend) {
 	}
 
 	// if backend is not healthy state we'll only resume checking it after ban
-	if !be.IsHealthy() {
+	if !be.IsHealthy() && !be.forcedCandidate {
 		log.Warn("backend banned - not healthy", "backend", be.Name)
 		cp.Ban(be)
 		return
@@ -327,7 +327,7 @@ func (cp *ConsensusPoller) UpdateBackend(ctx context.Context, be *Backend) {
 
 	RecordBackendUnexpectedBlockTags(be, !expectedBlockTags)
 
-	if !expectedBlockTags {
+	if !expectedBlockTags && !be.forcedCandidate {
 		log.Warn("backend banned - unexpected block tags",
 			"backend", be.Name,
 			"oldFinalized", bs.finalizedBlockNumber,
@@ -490,6 +490,10 @@ func (cp *ConsensusPoller) IsBanned(be *Backend) bool {
 
 // Ban bans a specific backend
 func (cp *ConsensusPoller) Ban(be *Backend) {
+	if be.forcedCandidate {
+		return
+	}
+
 	bs := cp.backendState[be]
 	defer bs.backendStateMux.Unlock()
 	bs.backendStateMux.Lock()

proxyd/rewriter.go

@@ -99,7 +99,11 @@ func rewriteParam(rctx RewriteContext, req *RPCReq, res *RPCRes, pos int, requir
 		return RewriteNone, nil
 	}
 
-	val, rw, err := rewriteTag(rctx, p[pos].(string))
+	s, ok := p[pos].(string)
+	if !ok {
+		return RewriteOverrideError, errors.New("expected string")
+	}
+	val, rw, err := rewriteTag(rctx, s)
 	if err != nil {
 		return RewriteOverrideError, err
 	}