From 99fb2b0dc5031a530ae26a80851474ce987618af Mon Sep 17 00:00:00 2001
From: Ryan Catherman <ryan.catherman@observeinc.com>
Date: Thu, 8 Aug 2024 08:41:48 -0400
Subject: [PATCH] Avoid rendering cycle in partials

We need to avoid self referential and non-self referential
use cases or things will overflow. This avoids the condition
by ensuring that when a partial renders itself, it removes itself
from the available lookups downstream. If lookup failures are
enabled, then the operation will fail. Otherwise, it will
be a silent error.
---
 mustache.go      | 11 ++++++++++-
 mustache_test.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/mustache.go b/mustache.go
index 542b4e1..5f0cecc 100644
--- a/mustache.go
+++ b/mustache.go
@@ -209,7 +209,16 @@ type partialNode struct {
 func (p *partialNode) render(t *Template, w *writer, c ...interface{}) error {
 	w.tag()
 	if template, ok := t.partials[p.name]; ok {
-		template.partials = t.partials
+
+		// We can avoid cycles by removing this node's template from the lookup
+		// before we render
+		template.partials = make(map[string]*Template)
+		for k, v := range t.partials {
+			if k != p.name {
+				template.partials[k] = v
+			}
+		}
+
 		err := template.render(w, c...)
 		if err != nil {
 			if !t.silentMiss {
diff --git a/mustache_test.go b/mustache_test.go
index a61cb53..c4bb5e9 100644
--- a/mustache_test.go
+++ b/mustache_test.go
@@ -228,3 +228,34 @@ func TestSectionTestValue(t *testing.T) {
 	}
 
 }
+
+func TestPartialsCannotCycle(t *testing.T) {
+	innerTemplate := New(Name("inner"))
+	err := innerTemplate.Parse(strings.NewReader(`I am the inner.{{>outer}}`))
+	if err != nil {
+		t.Error(err)
+	}
+
+	outerTemplate := New(Name("outer"))
+	err = outerTemplate.Parse(strings.NewReader(`I am the outer.{{>inner}}`))
+	if err != nil {
+		t.Error(err)
+	}
+
+	mainTemplate := New(Partial(outerTemplate), Partial(innerTemplate))
+	err = mainTemplate.Parse(strings.NewReader(`{{>outer}}`))
+	if err != nil {
+		t.Error(err)
+	}
+
+	var output bytes.Buffer
+	err = mainTemplate.Render(&output)
+	if err != nil {
+		t.Error(err)
+	}
+
+	expected := `I am the outer.I am the inner.`
+	if output.String() != expected {
+		t.Errorf("expected %q got %q", expected, output.String())
+	}
+}