Answer 504, not 408, when a request outlasts its time limit (closes #70)

The hand-written time limit middleware answered 408 and wrote that
answer from its own goroutine while the handler could still be writing.
It is replaced by chi's middleware.Timeout, placed after
JSONResponseMiddleware so its 504 is not lost behind a 200.

After a database read or other lookup, a handler asks deadlinePassed
whether the request's time limit passed and, if so, answers 504 instead
of its 500, 404 or partial result: the AS and prefix reads, cut off
while reading rows, return part of them, or none, without an error.
JSONValidationMiddleware answers 504 with the JSON error body when the
limit has passed and the handler wrote nothing. status.json and
/api/v1/stats answer 504 when their 4-second database read runs out.

Model: opus-5-5

internal/server/handlers.go

@@ -201,12 +201,16 @@ func (s *Server) handleStatusJSON() http.HandlerFunc {
 		select {
 		case <-ctx.Done():
 			s.logger.Error("Database stats timeout in status.json")
-			writeJSONError(w, http.StatusRequestTimeout, "Database timeout")
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
 
 			return
 		case err := <-errChan:
 			s.logger.Error("Failed to get database stats", "error", err)
-			writeJSONError(w, http.StatusInternalServerError, err.Error())
+			if deadlinePassed(ctx) {
+				writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+			} else {
+				writeJSONError(w, http.StatusInternalServerError, err.Error())
+			}
 
 			return
 		case dbStats = <-statsChan:
@@ -386,7 +390,7 @@ func (s *Server) handleStats() http.HandlerFunc {
 		// Check if context is already cancelled
 		select {
 		case <-ctx.Done():
-			http.Error(w, "Request timeout", http.StatusRequestTimeout)
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
 
 			return
 		default:
@@ -415,11 +419,16 @@ func (s *Server) handleStats() http.HandlerFunc {
 		select {
 		case <-ctx.Done():
 			s.logger.Error("Database stats timeout")
-			// Don't write response here - timeout middleware already handles it
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+
 			return
 		case err := <-errChan:
 			s.logger.Error("Failed to get database stats", "error", err)
-			writeJSONError(w, http.StatusInternalServerError, err.Error())
+			if deadlinePassed(ctx) {
+				writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+			} else {
+				writeJSONError(w, http.StatusInternalServerError, err.Error())
+			}
 
 			return
 		case dbStats = <-statsChan:
@@ -622,6 +631,13 @@ func (s *Server) handleIPInfo() http.HandlerFunc {
 			}
 		}
 
+		// The time limit passed while resolving host names.
+		if deadlinePassed(ctx) {
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+
+			return
+		}
+
 		if len(ipsToLookup) == 0 {
 			writeJSONError(w, http.StatusBadRequest, "No valid IPs or hostnames provided")
 
@@ -664,6 +680,13 @@ func (s *Server) handleIPInfo() http.HandlerFunc {
 			}
 		}
 
+		// The time limit passed while looking up the IPs.
+		if deadlinePassed(ctx) {
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+
+			return
+		}
+
 		// Return response (even if no results, include errors)
 		if len(response.Results) == 0 && len(response.Errors) > 0 {
 			writeJSONError(w, http.StatusNotFound, "No routes found: "+response.Errors[0])
@@ -702,6 +725,13 @@ func (s *Server) handleASDetailJSON() http.HandlerFunc {
 		}
 
 		asInfo, prefixes, err := s.db.GetASDetailsContext(r.Context(), asn)
+		// A read cut off by the time limit can return part of its rows, or
+		// none, without an error.
+		if deadlinePassed(r.Context()) {
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+
+			return
+		}
 		if err != nil {
 			if errors.Is(err, database.ErrNoRoute) {
 				writeJSONError(w, http.StatusNotFound, err.Error())
@@ -760,6 +790,13 @@ func (s *Server) handlePrefixDetailJSON() http.HandlerFunc {
 		prefix := parsed.Masked().String()
 
 		routes, _, err := s.db.GetPrefixDetailsContext(r.Context(), prefix)
+		// A read cut off by the time limit can return part of its rows, or
+		// none, without an error.
+		if deadlinePassed(r.Context()) {
+			writeJSONError(w, http.StatusGatewayTimeout, requestTimeoutMessage)
+
+			return
+		}
 		if err != nil {
 			if errors.Is(err, database.ErrNoRoute) {
 				writeJSONError(w, http.StatusNotFound, err.Error())
@@ -802,6 +839,13 @@ func (s *Server) handleASDetail() http.HandlerFunc {
 		}
 
 		asInfo, prefixes, err := s.db.GetASDetailsContext(r.Context(), asn)
+		// A read cut off by the time limit can return part of its rows, or
+		// none, without an error.
+		if deadlinePassed(r.Context()) {
+			http.Error(w, requestTimeoutMessage, http.StatusGatewayTimeout)
+
+			return
+		}
 		if err != nil {
 			if errors.Is(err, database.ErrNoRoute) {
 				http.Error(w, "AS not found", http.StatusNotFound)
@@ -931,6 +975,13 @@ func (s *Server) handlePrefixDetail() http.HandlerFunc {
 		prefix := parsed.Masked().String()
 
 		routes, originASNs, err := s.db.GetPrefixDetailsContext(r.Context(), prefix)
+		// A read cut off by the time limit can return part of its rows, or
+		// none, without an error.
+		if deadlinePassed(r.Context()) {
+			http.Error(w, requestTimeoutMessage, http.StatusGatewayTimeout)
+
+			return
+		}
 		if err != nil {
 			if errors.Is(err, database.ErrNoRoute) {
 				http.Error(w, "Prefix not found", http.StatusNotFound)
@@ -1080,8 +1131,12 @@ func (s *Server) handlePrefixLength() http.HandlerFunc {
 		const maxPrefixes = 500
 		prefixes, err := s.db.GetRandomPrefixesByLengthContext(r.Context(), maskLength, ipVersion, maxPrefixes)
 		if err != nil {
-			s.logger.Error("Failed to get prefixes by length", "error", err)
-			http.Error(w, "Internal server error", http.StatusInternalServerError)
+			if deadlinePassed(r.Context()) {
+				http.Error(w, requestTimeoutMessage, http.StatusGatewayTimeout)
+			} else {
+				s.logger.Error("Failed to get prefixes by length", "error", err)
+				http.Error(w, "Internal server error", http.StatusInternalServerError)
+			}
 
 			return
 		}
@@ -1171,8 +1226,12 @@ func (s *Server) handlePrefixLength6() http.HandlerFunc {
 		const maxPrefixes = 500
 		prefixes, err := s.db.GetRandomPrefixesByLengthContext(r.Context(), maskLength, ipVersion, maxPrefixes)
 		if err != nil {
-			s.logger.Error("Failed to get prefixes by length", "error", err)
-			http.Error(w, "Internal server error", http.StatusInternalServerError)
+			if deadlinePassed(r.Context()) {
+				http.Error(w, requestTimeoutMessage, http.StatusGatewayTimeout)
+			} else {
+				s.logger.Error("Failed to get prefixes by length", "error", err)
+				http.Error(w, "Internal server error", http.StatusInternalServerError)
+			}
 
 			return
 		}