cache: Short TTL negative cache for 404/410 depot objects

Stop re-fetching gone depot objects on every miss: store 404/410 in the
existing VFS cache under the same key with a short TTL (default 5m).

steamcache/metrics/metrics.go

@@ -12,14 +12,15 @@ import (
 // Metrics tracks various performance and operational metrics
 type Metrics struct {
 	// Request metrics
-	TotalRequests  int64
-	CacheHits      int64
-	CacheMisses    int64
-	CacheCoalesced int64
-	RangeCache     int64 // Range requests served as 206 from an already-cached object (HIT)
-	RangeUpstream  int64 // Range requests that required an upstream fetch (full object), served as 206
-	Errors         int64
-	RateLimited    int64
+	TotalRequests     int64
+	CacheHits         int64
+	CacheMisses       int64
+	CacheCoalesced    int64
+	NegativeCacheHits int64 // 404/410 served from a still-valid negative cache entry
+	RangeCache        int64 // Range requests served as 206 from an already-cached object (HIT)
+	RangeUpstream     int64 // Range requests that required an upstream fetch (full object), served as 206
+	Errors            int64
+	RateLimited       int64
 
 	// Performance metrics
 	TotalResponseTime int64 // in nanoseconds
@@ -76,6 +77,11 @@ func (m *Metrics) IncrementCacheMisses() {
 	atomic.AddInt64(&m.CacheMisses, 1)
 }
 
+// IncrementNegativeCacheHits increments hits served from a 404/410 negative entry.
+func (m *Metrics) IncrementNegativeCacheHits() {
+	atomic.AddInt64(&m.NegativeCacheHits, 1)
+}
+
 // IncrementCacheCoalesced increments the coalesced request counter
 func (m *Metrics) IncrementCacheCoalesced() {
 	atomic.AddInt64(&m.CacheCoalesced, 1)
@@ -213,6 +219,7 @@ func (m *Metrics) GetStats() *Stats {
 		CacheHits:          cacheHits,
 		CacheMisses:        cacheMisses,
 		CacheCoalesced:     atomic.LoadInt64(&m.CacheCoalesced),
+		NegativeCacheHits:  atomic.LoadInt64(&m.NegativeCacheHits),
 		RangeCache:         atomic.LoadInt64(&m.RangeCache),
 		RangeUpstream:      atomic.LoadInt64(&m.RangeUpstream),
 		Errors:             atomic.LoadInt64(&m.Errors),
@@ -243,6 +250,7 @@ func (m *Metrics) Reset() {
 	atomic.StoreInt64(&m.CacheHits, 0)
 	atomic.StoreInt64(&m.CacheMisses, 0)
 	atomic.StoreInt64(&m.CacheCoalesced, 0)
+	atomic.StoreInt64(&m.NegativeCacheHits, 0)
 	atomic.StoreInt64(&m.RangeCache, 0)
 	atomic.StoreInt64(&m.RangeUpstream, 0)
 	atomic.StoreInt64(&m.Errors, 0)
@@ -270,19 +278,20 @@ func (m *Metrics) Reset() {
 
 // Stats represents a snapshot of metrics
 type Stats struct {
-	TotalRequests    int64
-	CacheHits        int64
-	CacheMisses      int64
-	CacheCoalesced   int64
-	RangeCache       int64
-	RangeUpstream    int64
-	Errors           int64
-	RateLimited      int64
-	HitRate          float64
-	AvgResponseTime  time.Duration
-	TotalBytesServed int64
-	TotalBytesSaved  int64
-	MemoryCacheSize  int64
+	TotalRequests     int64
+	CacheHits         int64
+	CacheMisses       int64
+	CacheCoalesced    int64
+	NegativeCacheHits int64
+	RangeCache        int64
+	RangeUpstream     int64
+	Errors            int64
+	RateLimited       int64
+	HitRate           float64
+	AvgResponseTime   time.Duration
+	TotalBytesServed  int64
+	TotalBytesSaved   int64
+	MemoryCacheSize   int64
 
 	DiskCacheSize      int64
 	DiskTierReady      int64
@@ -308,6 +317,7 @@ func WriteText(w http.ResponseWriter, stats *Stats) {
 	_, _ = fmt.Fprintf(w, "total_requests %d\n", stats.TotalRequests)
 	_, _ = fmt.Fprintf(w, "cache_hits %d\n", stats.CacheHits)
 	_, _ = fmt.Fprintf(w, "cache_misses %d\n", stats.CacheMisses)
+	_, _ = fmt.Fprintf(w, "negative_cache_hits %d\n", stats.NegativeCacheHits)
 	_, _ = fmt.Fprintf(w, "cache_coalesced %d\n", stats.CacheCoalesced)
 	_, _ = fmt.Fprintf(w, "range_cache %d\n", stats.RangeCache)
 	_, _ = fmt.Fprintf(w, "range_upstream %d\n", stats.RangeUpstream)