go/proxyd: Add debug cache status header to HTTP responses

.changeset/slimy-sheep-smell.md

+---
+'@eth-optimism/proxyd': minor
+---
+
+Add debug cache status header to proxyd responses

go/proxyd/server.go

@@ -159,6 +159,7 @@ func (s *Server) HandleRPC(w http.ResponseWriter, r *http.Request) {
 		}
 
 		batchRes := make([]*RPCRes, len(reqs), len(reqs))
+		var cached bool
 		for i := 0; i < len(reqs); i++ {
 			req, err := ParseRPCReq(reqs[i])
 			if err != nil {
@@ -167,9 +168,10 @@ func (s *Server) HandleRPC(w http.ResponseWriter, r *http.Request) {
 				continue
 			}
 
-			batchRes[i] = s.handleSingleRPC(ctx, req)
+			batchRes[i], cached = s.handleSingleRPC(ctx, req)
 		}
 
+		setCacheHeader(w, cached)
 		writeBatchRPCRes(ctx, w, batchRes)
 		return
 	}
@@ -181,14 +183,15 @@ func (s *Server) HandleRPC(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	backendRes := s.handleSingleRPC(ctx, req)
+	backendRes, cached := s.handleSingleRPC(ctx, req)
+	setCacheHeader(w, cached)
 	writeRPCRes(ctx, w, backendRes)
 }
 
-func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) *RPCRes {
+func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) (*RPCRes, bool) {
 	if err := ValidateRPCReq(req); err != nil {
 		RecordRPCError(ctx, BackendProxyd, MethodUnknown, err)
-		return NewRPCErrorRes(nil, err)
+		return NewRPCErrorRes(nil, err), false
 	}
 
 	group := s.rpcMethodMappings[req.Method]
@@ -202,7 +205,7 @@ func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) *RPCRes {
 			"method", req.Method,
 		)
 		RecordRPCError(ctx, BackendProxyd, MethodUnknown, ErrMethodNotWhitelisted)
-		return NewRPCErrorRes(req.ID, ErrMethodNotWhitelisted)
+		return NewRPCErrorRes(req.ID, ErrMethodNotWhitelisted), false
 	}
 
 	var backendRes *RPCRes
@@ -215,7 +218,7 @@ func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) *RPCRes {
 		)
 	}
 	if backendRes != nil {
-		return backendRes
+		return backendRes, true
 	}
 
 	backendRes, err = s.backendGroups[group].Forward(ctx, req)
@@ -226,7 +229,7 @@ func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) *RPCRes {
 			"req_id", GetReqID(ctx),
 			"err", err,
 		)
-		return NewRPCErrorRes(req.ID, err)
+		return NewRPCErrorRes(req.ID, err), false
 	}
 
 	if backendRes.Error == nil {
@@ -239,7 +242,7 @@ func (s *Server) handleSingleRPC(ctx context.Context, req *RPCReq) *RPCRes {
 		}
 	}
 
-	return backendRes
+	return backendRes, false
 }
 
 func (s *Server) HandleWS(w http.ResponseWriter, r *http.Request) {
@@ -322,6 +325,15 @@ func (s *Server) populateContext(w http.ResponseWriter, r *http.Request) context
 	)
 }
 
+func setCacheHeader(w http.ResponseWriter, cached bool) {
+	const cacheStatusHdr = "X-Proxyd-Cache-Status"
+	if cached {
+		w.Header().Set(cacheStatusHdr, "HIT")
+	} else {
+		w.Header().Set(cacheStatusHdr, "MISS")
+	}
+}
+
 func writeRPCError(ctx context.Context, w http.ResponseWriter, id json.RawMessage, err error) {
 	var res *RPCRes
 	if r, ok := err.(*RPCErr); ok {