router: fix silent error in race; add sentinel errors; populate NarHash/NarSize

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I9fa56ed75c609f557ab601883ca899a16a6a6964

internal/router/router.go

@@ -1,21 +1,34 @@
 package router
 
 import (
+	"bytes"
 	"context"
+	"errors"
 	"fmt"
+	"io"
+	"log/slog"
 	"net/http"
 	"sync"
 	"time"
 
 	"notashelf.dev/ncro/internal/cache"
+	"notashelf.dev/ncro/internal/metrics"
+	"notashelf.dev/ncro/internal/narinfo"
 	"notashelf.dev/ncro/internal/prober"
 )
 
+// Returned when all upstreams were reached but none had the path.
+var ErrNotFound = errors.New("not found in any upstream")
+
+// Returned when all upstreams failed with network errors.
+var ErrUpstreamUnavailable = errors.New("all upstreams unavailable")
+
 // Result of a Resolve call.
 type Result struct {
-	URL       string
-	LatencyMs float64
-	CacheHit  bool
+	URL          string
+	LatencyMs    float64
+	CacheHit     bool
+	NarInfoBytes []byte // raw narinfo response on cache miss; nil on cache hit
 }
 
 // Resolves store paths to the best upstream via cache lookup or parallel racing.
@@ -45,6 +58,7 @@ func (r *Router) Resolve(storeHash string, candidates []string) (*Result, error)
 	if err == nil && entry != nil && entry.IsValid() {
 		h := r.prober.GetHealth(entry.UpstreamURL)
 		if h == nil || h.Status == prober.StatusActive {
+			metrics.NarinfoCacheHits.Inc()
 			return &Result{
 				URL:       entry.UpstreamURL,
 				LatencyMs: entry.LatencyEMA,
@@ -52,6 +66,7 @@ func (r *Router) Resolve(storeHash string, candidates []string) (*Result, error)
 			}, nil
 		}
 	}
+	metrics.NarinfoCacheMisses.Inc()
 	return r.race(storeHash, candidates)
 }
 
@@ -69,21 +84,39 @@ func (r *Router) race(storeHash string, candidates []string) (*Result, error) {
 	defer cancel()
 
 	ch := make(chan raceResult, len(candidates))
-	var wg sync.WaitGroup
+	var (
+		wg        sync.WaitGroup
+		mu        sync.Mutex
+		netErrs   int
+		notFounds int
+	)
 
 	for _, u := range candidates {
 		wg.Add(1)
 		go func(upstream string) {
 			defer wg.Done()
 			start := time.Now()
-			req, _ := http.NewRequestWithContext(ctx, http.MethodHead,
+			req, err := http.NewRequestWithContext(ctx, http.MethodHead,
 				upstream+"/"+storeHash+".narinfo", nil)
+			if err != nil {
+				slog.Warn("bad upstream URL in race", "upstream", upstream, "error", err)
+				mu.Lock()
+				netErrs++
+				mu.Unlock()
+				return
+			}
 			resp, err := r.client.Do(req)
 			if err != nil {
+				mu.Lock()
+				netErrs++
+				mu.Unlock()
 				return
 			}
 			resp.Body.Close()
 			if resp.StatusCode != 200 {
+				mu.Lock()
+				notFounds++
+				mu.Unlock()
 				return
 			}
 			ms := float64(time.Since(start).Nanoseconds()) / 1e6
@@ -101,7 +134,13 @@ func (r *Router) race(storeHash string, candidates []string) (*Result, error) {
 
 	winner, ok := <-ch
 	if !ok {
-		return nil, fmt.Errorf("all upstreams failed for %q", storeHash)
+		mu.Lock()
+		ne, nf := netErrs, notFounds
+		mu.Unlock()
+		if ne > 0 && nf == 0 {
+			return nil, ErrUpstreamUnavailable
+		}
+		return nil, ErrNotFound
 	}
 	cancel()
 
@@ -111,6 +150,12 @@ func (r *Router) race(storeHash string, candidates []string) (*Result, error) {
 		}
 	}
 
+	metrics.UpstreamRaceWins.WithLabelValues(winner.url).Inc()
+	metrics.UpstreamLatency.WithLabelValues(winner.url).Observe(winner.latencyMs / 1000)
+
+	// Fetch narinfo body to parse metadata and forward to caller.
+	narInfoBytes, narHash, narSize := r.fetchNarInfo(winner.url, storeHash)
+
 	health := r.prober.GetHealth(winner.url)
 	ema := winner.latencyMs
 	if health != nil {
@@ -127,7 +172,32 @@ func (r *Router) race(storeHash string, candidates []string) (*Result, error) {
 		LastVerified: now,
 		QueryCount:   1,
 		TTL:          now.Add(r.routeTTL),
+		NarHash:      narHash,
+		NarSize:      narSize,
 	})
 
-	return &Result{URL: winner.url, LatencyMs: winner.latencyMs}, nil
+	return &Result{URL: winner.url, LatencyMs: winner.latencyMs, NarInfoBytes: narInfoBytes}, nil
+}
+
+// Fetches narinfo content from upstream and parses metadata.
+// Returns (body, narHash, narSize); body may be non-nil even on parse error.
+func (r *Router) fetchNarInfo(upstream, storeHash string) ([]byte, string, uint64) {
+	url := upstream + "/" + storeHash + ".narinfo"
+	resp, err := r.client.Get(url)
+	if err != nil {
+		return nil, "", 0
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return nil, "", 0
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, "", 0
+	}
+	ni, err := narinfo.Parse(bytes.NewReader(body))
+	if err != nil {
+		return body, "", 0
+	}
+	return body, ni.NarHash, ni.NarSize
 }