server: fix NAR fallback; distinguish 404 vs 502; add /metrics endpoint

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ib86ef7efb3cefc34cf5642fce9c7d0ef6a6a6964

internal/server/server.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -8,7 +9,9 @@ import (
 	"strings"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"notashelf.dev/ncro/internal/config"
+	"notashelf.dev/ncro/internal/metrics"
 	"notashelf.dev/ncro/internal/prober"
 	"notashelf.dev/ncro/internal/router"
 )
@@ -38,6 +41,8 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		s.handleCacheInfo(w, r)
 	case path == "/health":
 		s.handleHealth(w, r)
+	case path == "/metrics":
+		promhttp.Handler().ServeHTTP(w, r)
 	case strings.HasSuffix(path, ".narinfo"):
 		s.handleNarinfo(w, r)
 	case strings.HasPrefix(path, "/nar/"):
@@ -64,23 +69,65 @@ func (s *Server) handleNarinfo(w http.ResponseWriter, r *http.Request) {
 
 	result, err := s.router.Resolve(hash, s.upstreamURLs())
 	if err != nil {
-		slog.Warn("narinfo not found", "hash", hash, "error", err)
-		http.NotFound(w, r)
+		slog.Warn("narinfo resolve failed", "hash", hash, "error", err)
+		metrics.NarinfoRequests.WithLabelValues("error").Inc()
+		switch {
+		case errors.Is(err, router.ErrNotFound):
+			http.NotFound(w, r)
+		default:
+			http.Error(w, "upstream unavailable", http.StatusBadGateway)
+		}
 		return
 	}
 
 	slog.Info("narinfo routed", "hash", hash, "upstream", result.URL, "cache_hit", result.CacheHit)
+	metrics.NarinfoRequests.WithLabelValues("200").Inc()
+
+	if len(result.NarInfoBytes) > 0 {
+		w.Header().Set("Content-Type", "text/x-nix-narinfo")
+		w.WriteHeader(http.StatusOK)
+		w.Write(result.NarInfoBytes)
+		return
+	}
 	s.proxyRequest(w, r, result.URL+r.URL.Path)
 }
 
 func (s *Server) handleNAR(w http.ResponseWriter, r *http.Request) {
+	metrics.NARRequests.Inc()
 	sorted := s.prober.SortedByLatency()
 	if len(sorted) == 0 {
 		http.Error(w, "no upstreams available", http.StatusServiceUnavailable)
 		return
 	}
-	slog.Debug("proxying NAR", "path", r.URL.Path, "upstream", sorted[0].URL)
-	s.proxyRequest(w, r, sorted[0].URL+r.URL.Path)
+	for _, h := range sorted {
+		if h.Status == prober.StatusDown {
+			continue
+		}
+		targetURL := h.URL + r.URL.Path
+		req, err := http.NewRequestWithContext(r.Context(), r.Method, targetURL, r.Body)
+		if err != nil {
+			continue
+		}
+		for _, hdr := range []string{"Accept", "Accept-Encoding", "Range"} {
+			if v := r.Header.Get(hdr); v != "" {
+				req.Header.Set(hdr, v)
+			}
+		}
+		resp, err := s.client.Do(req)
+		if err != nil {
+			slog.Warn("NAR upstream failed", "upstream", h.URL, "error", err)
+			continue
+		}
+		if resp.StatusCode == http.StatusNotFound {
+			resp.Body.Close()
+			continue
+		}
+		defer resp.Body.Close()
+		slog.Debug("proxying NAR", "path", r.URL.Path, "upstream", h.URL)
+		s.copyResponse(w, resp)
+		return
+	}
+	http.NotFound(w, r)
 }
 
 // Forwards r to targetURL and streams the response zero-copy.
@@ -95,7 +142,6 @@ func (s *Server) proxyRequest(w http.ResponseWriter, r *http.Request, targetURL
 			req.Header.Set(h, v)
 		}
 	}
-
 	resp, err := s.client.Do(req)
 	if err != nil {
 		slog.Error("upstream request failed", "url", targetURL, "error", err)
@@ -103,7 +149,11 @@ func (s *Server) proxyRequest(w http.ResponseWriter, r *http.Request, targetURL
 		return
 	}
 	defer resp.Body.Close()
+	s.copyResponse(w, resp)
+}
 
+// Copies response headers and body from resp to w.
+func (s *Server) copyResponse(w http.ResponseWriter, resp *http.Response) {
 	for _, h := range []string{
 		"Content-Type", "Content-Length", "Content-Encoding",
 		"X-Nix-Signature", "Cache-Control", "Last-Modified",
@@ -114,7 +164,7 @@ func (s *Server) proxyRequest(w http.ResponseWriter, r *http.Request, targetURL
 	}
 	w.WriteHeader(resp.StatusCode)
 	if _, err := io.Copy(w, resp.Body); err != nil {
-		slog.Warn("stream interrupted", "url", targetURL, "error", err)
+		slog.Warn("stream interrupted", "error", err)
 	}
 }