Merges code from #2076

Thank you @AleksandrHovhannisyan!

src/TemplateLayout.js

@@ -73,9 +73,25 @@ class TemplateLayout extends TemplateContent {
     let mapEntry = await this.getTemplateLayoutMapEntry();
     map.push(mapEntry);
 
+    // Keep track of every layout we see so we can detect cyclical layout chains (e.g., a => b => c => a).
+    const seenLayoutKeys = new Set();
+    seenLayoutKeys.add(mapEntry.key);
+
     while (mapEntry.frontMatterData && cfgKey in mapEntry.frontMatterData) {
+      // Layout of the current layout
+      const parentLayoutKey = mapEntry.frontMatterData[cfgKey];
+      // Abort if a circular layout chain is detected. Otherwise, we'll time out and run out of memory.
+      if (seenLayoutKeys.has(parentLayoutKey)) {
+        throw new Error(
+          `Your layouts have a circular reference, starting at ${map[0].key}! The layout ${parentLayoutKey} was specified twice in this layout chain.`
+        );
+      }
+
+      // Keep track of this layout so we can detect duplicates in subsequent iterations
+      seenLayoutKeys.add(parentLayoutKey);
+
       let layout = TemplateLayout.getTemplate(
-        mapEntry.frontMatterData[cfgKey],
+        parentLayoutKey,
         mapEntry.inputDir,
         this.config,
         this.extensionMap

test/TemplateLayoutTest.js

@@ -208,3 +208,26 @@ test("Cache Duplicates (use full key for cache)", async (t) => {
 
   t.is((await tla.render({})).trim(), "Hello A");
 });
+
+test("Throw an error if a layout references itself as the layout", async (t) => {
+  await t.throwsAsync(async () => {
+    const tl = getTemplateLayoutInstance(
+      "layout-cycle-self.njk",
+      "./test/stubs-circular-layout"
+    );
+
+    const layoutChain = await tl._testGetLayoutChain();
+    return layoutChain;
+  });
+});
+
+test("Throw an error if a circular layout chain is detected", async (t) => {
+  await t.throwsAsync(async () => {
+    const tl = getTemplateLayoutInstance(
+      "layout-cycle-a.njk",
+      "./test/stubsstubs-circular-layout"
+    );
+    const layoutChain = await tl._testGetLayoutChain();
+    return layoutChain;
+  });
+});

test/stubs-circular-layout/_includes/layout-cycle-a.njk

@@ -0,0 +1,3 @@
+---
+layout: layouts/layout-cycle-b.njk
+---

test/stubs-circular-layout/_includes/layout-cycle-b.njk

@@ -0,0 +1,3 @@
+---
+layout: layouts/layout-cycle-c.njk
+---

test/stubs-circular-layout/_includes/layout-cycle-c.njk

@@ -0,0 +1,3 @@
+---
+layout: layouts/layout-cycle-a.njk
+---

test/stubs-circular-layout/_includes/layout-cycle-self.njk

@@ -0,0 +1,3 @@
+---
+layout: layouts/layout-cycle-b.njk
+---