Cache puppet virtual module misses; drop remote query forwarding

internal/virtual/puppet_merger.go

@@ -34,11 +34,13 @@ type PuppetMember struct {
 }
 
 // PuppetDoc is a module's merged releases, newest version first, with
-// file_uri routed via the virtual; Owners maps each version to its member.
+// file_uri routed via the virtual; Owners maps each version to its member;
+// missing caches that no member has the module.
 type PuppetDoc struct {
 	Releases []map[string]any
 	Owners   map[string]string
 	body     []byte
+	missing  bool
 }
 
 func (d *PuppetDoc) index() []byte            { return d.body }
@@ -114,7 +116,11 @@ func (e *Engine) PuppetReleases(ctx context.Context, virt models.Virtual, slug s
 		if err != nil {
 			return nil, err
 		}
-		return MergePuppetReleases(slug, members)
+		d, err := MergePuppetReleases(slug, members)
+		if errors.Is(err, ErrNotFound) {
+			return &PuppetDoc{missing: true}, nil
+		}
+		return d, err
 	})
 	if err != nil {
 		return nil, err
@@ -122,7 +128,10 @@ func (e *Engine) PuppetReleases(ctx context.Context, virt models.Virtual, slug s
 	if inserted {
 		e.evictOldest(prefix, maxPuppetModules)
 	}
-	return repo.(*PuppetDoc), nil
+	if d := repo.(*PuppetDoc); !d.missing {
+		return d, nil
+	}
+	return nil, ErrNotFound
 }
 
 // PuppetRedirect maps v3/releases/<slug>-<version> or v3/files/<file> to the

internal/virtual/puppet_merger_test.go

@@ -180,3 +180,26 @@ func TestPuppetRedirect(t *testing.T) {
 		}
 	}
 }
+
+func TestPuppetReleasesCachesMissingModule(t *testing.T) {
+	var mu sync.Mutex
+	asked := map[string]int{}
+	e := puppetEngine(func(r models.Remote, p string) ([]byte, error) {
+		mu.Lock()
+		asked[r.Name]++
+		mu.Unlock()
+		if r.Name == "internal" {
+			return nil, &proxy.ProxyError{Status: 404}
+		}
+		return forgePages("unkin-nope", nil, p)
+	})
+	virt := models.Virtual{Name: "puppet", PackageType: models.PackagePuppet, Members: []string{"internal", "forge"}}
+	for range 2 {
+		if _, err := e.PuppetReleases(context.Background(), virt, "unkin-nope"); !errors.Is(err, ErrNotFound) {
+			t.Fatalf("want ErrNotFound, got %v", err)
+		}
+	}
+	if asked["internal"] != 1 || asked["forge"] != 1 {
+		t.Fatalf("a module no member has must be cached, asked %v", asked)
+	}
+}