Report configs that extend each other

.changeset/cyclic-extends.md

@@ -0,0 +1,5 @@
+---
+'@shopify/theme-check-node': patch
+---
+
+Report configs that extend each other instead of hanging while resolving them.

packages/theme-check-node/src/config/resolve/resolve-config.spec.ts

@@ -125,6 +125,15 @@ SomeCheck:
     expect(config.context).to.equal('app');
   });
 
+  it('rejects configs that extend each other', async () => {
+    const filePath = await createMockYamlFile('extends: ./base.yml');
+    await createMockConfigFile(tempDir, 'extends: ./.theme-check.yml', 'base.yml');
+
+    await expect(resolveConfig(filePath)).rejects.toThrow(
+      `Cyclic extends in theme-check config: ${filePath} -> ${path.join(tempDir, 'base.yml')} -> ${filePath}`,
+    );
+  });
+
   async function createMockYamlFile(content: string): Promise<string> {
     return createMockConfigFile(tempDir, content);
   }

packages/theme-check-node/src/config/resolve/resolve-config.ts

@@ -20,18 +20,27 @@ const modernConfigsPath = () => {
 export async function resolveConfig(
   configPath: AbsolutePath | ModernIdentifier,
   isRootConfig: boolean = false,
+  /** the configs that extend this one, from the root config down */
+  extendedBy: AbsolutePath[] = [],
 ): Promise<ConfigDescription> {
   if (isModernIdentifier(configPath)) {
     const modernConfigPath = path.join(
       modernConfigsPath(),
       configPath.replace(/^theme-check:/, '') + '.yml',
     );
-    return resolveConfig(modernConfigPath);
+    return resolveConfig(modernConfigPath, false, extendedBy);
+  }
+
+  const chain = [...extendedBy, configPath];
+  if (extendedBy.includes(configPath)) {
+    throw new Error(`Cyclic extends in theme-check config: ${chain.join(' -> ')}`);
   }
 
   // TODO: Add support for more file formats.
   const current = await readYamlConfigDescription(configPath, isRootConfig);
-  const baseConfigs = await Promise.all(current.extends.map((extend) => resolveConfig(extend)));
+  const baseConfigs = await Promise.all(
+    current.extends.map((extend) => resolveConfig(extend, false, chain)),
+  );
   return mergeFragments(baseConfigs, current);
 }