fix: avoid shared error variable in cycle tag closure

CHANGELOG.md

@@ -3,6 +3,12 @@
 
 ## Unreleased
 
+### Fixed
+
+- The `{% cycle %}` tag no longer assigns to an error variable shared by every
+  render of a parsed template, so a template containing `{% cycle %}` can be
+  rendered concurrently from multiple goroutines.
+
 ## 1.9.2 (2026-08-16)
 
 ### Performance

tags/iteration_tags.go

@@ -62,7 +62,7 @@ func cycleTag(args string) (func(io.Writer, render.Context) error, error) {
 		n := cycleMap[group]
 		cycleMap[group] = n + 1
 		// The parser guarantees that there will be at least one item.
-		_, err = io.WriteString(w, values[n%len(values)])
+		_, err := io.WriteString(w, values[n%len(values)])
 
 		return err
 	}, nil

template_test.go

@@ -115,6 +115,35 @@ func TestTemplate_Render_race(t *testing.T) {
 	wg2.Wait()
 }
 
+func TestTemplate_Render_cycle_race(t *testing.T) {
+	engine := NewEngine()
+	tpl, err := engine.ParseString(`{% for a in (1..4) %}{% cycle 'a', 'b' %}{% endfor %}`)
+	require.NoError(t, err)
+
+	const (
+		goroutines = 16
+		renders    = 50
+	)
+
+	var wg sync.WaitGroup
+
+	for range goroutines {
+		wg.Add(1)
+
+		go func() {
+			defer wg.Done()
+
+			for range renders {
+				out, err := tpl.RenderString(Bindings{})
+				assert.NoError(t, err)
+				assert.Equal(t, "abab", out)
+			}
+		}()
+	}
+
+	wg.Wait()
+}
+
 func BenchmarkTemplate_Render(b *testing.B) {
 	engine := NewEngine()
 	bindings := Bindings{"a": "string value"}