Default omitted mutable_ttl and keep the ETag past TTL expiry

- default mutable_ttl to 3600 on create and keep the stored value on update
- store mutable ETags without a TTL so expiry revalidates with If-None-Match
- drop the stored ETag when a refetch returns none

internal/proxy/engine.go

@@ -160,7 +160,6 @@ func (e *Engine) Fetch(ctx context.Context, remote models.Remote, path string, p
 			notModified, err := e.checkUpstream(ctx, remote, path, etag, prov)
 			if err == nil && notModified {
 				_ = e.cache.SetTTL(ctx, remote.Name, path, ttl)
-				_ = e.cache.SetETag(ctx, remote.Name, path, etag, ttl)
 				result, err := e.serveFromStore(ctx, remote, path)
 				if err == nil {
 					result.Source = "cache"
@@ -491,9 +490,8 @@ func (e *Engine) fetchFromUpstreamOnce(ctx context.Context, remote models.Remote
 		}
 
 		_ = e.cache.SetTTL(ctx, remote.Name, path, ttl)
-		if etag := resp.Header.Get("ETag"); etag != "" {
-			_ = e.cache.SetETag(ctx, remote.Name, path, etag, ttl)
-		}
+		// The ETag outlives the freshness key so an expired path is revalidated.
+		_ = e.cache.SetETag(ctx, remote.Name, path, resp.Header.Get("ETag"), 0)
 
 		return &FetchResult{
 			Reader:      io.NopCloser(bytesReader(body)),

internal/proxy/engine_test.go

@@ -262,28 +262,37 @@ func TestCircuitOpenServesStale(t *testing.T) {
 
 func TestMutableRevalidation(t *testing.T) {
 	requireStack(t)
-	ctx := context.Background()
-	r := seed(t, models.Remote{Name: "eng-npm", PackageType: models.PackageNPM, RepoType: models.RepoTypeRemote, BaseURL: upstream.URL, CheckMutable: true, MutableTTL: 3600, StaleOnError: true})
-	p := prov(t, models.PackageNPM)
+	srv, rev, conditional := changingUpstream(t)
+	r := seed(t, models.Remote{Name: "eng-reval", PackageType: models.PackageRPM, RepoType: models.RepoTypeRemote, BaseURL: srv.URL, CheckMutable: true, MutableTTL: 1})
+	const path = "8/Everything/x86_64/repodata/repomd.xml"
+	fetch := func() (string, string) {
+		t.Helper()
+		res, err := testEngine.Fetch(context.Background(), r, path, prov(t, models.PackageRPM))
+		if err != nil {
+			t.Fatalf("fetch: %v", err)
+		}
+		return readAll(t, res), res.Source
+	}
 
-	res, err := testEngine.Fetch(ctx, r, "pkg", p)
-	if err != nil {
-		t.Fatalf("initial mutable fetch: %v", err)
+	if body, _ := fetch(); body != "rev1:/"+path {
+		t.Fatalf("initial fetch = %q", body)
+	}
+	time.Sleep(1100 * time.Millisecond)
+	if body, src := fetch(); body != "rev1:/"+path || src != "cache" {
+		t.Errorf("after expiry = %q from %s, want cached rev1", body, src)
+	}
+	if n := conditional.Load(); n != 1 {
+		t.Errorf("conditional requests = %d, want 1 revalidation", n)
+	}
+	if fresh, _ := testCache.CheckTTL(context.Background(), r.Name, path); !fresh {
+		t.Error("304 should re-arm the freshness key")
 	}
-	res.Reader.Close()
 
-	// Expire only the freshness marker; the ETag persists, forcing a
-	// conditional revalidation that the upstream answers with 304.
-	testCache.SetTTL(ctx, "eng-npm", "pkg", time.Millisecond)
-	time.Sleep(10 * time.Millisecond)
-	res, err = testEngine.Fetch(ctx, r, "pkg", p)
-	if err != nil {
-		t.Fatalf("revalidation fetch: %v", err)
+	rev.Store("rev2")
+	time.Sleep(1100 * time.Millisecond)
+	if body, src := fetch(); body != "rev2:/"+path || src != "remote" {
+		t.Errorf("after upstream change = %q from %s, want remote rev2", body, src)
 	}
-	if res.Source != "cache" {
-		t.Errorf("revalidated response should come from cache, got %s", res.Source)
-	}
-	res.Reader.Close()
 }
 
 func TestBearerTokenFlow(t *testing.T) {