fix(frontmatter): improve duplicate frontmatter key error handling (#7679)

* fix(frontmatter): improve duplicate frontmatter key error handling

* fix(frontmatter): improve duplicate key warning with file and line info

* fix(frontmatter): made duplicate detection path awarness using indendation

* fix(frontmatter): removed backslash

* feat: use `yaml` diagnostics

---------

Co-authored-by: Martin Jagodic <jagodicmartin1@gmail.com>
Co-authored-by: Yan <61414485+yanthomasdev@users.noreply.github.com>

Author: 3 people

packages/decap-cms-core/src/formats/__tests__/frontmatter.spec.js

@@ -72,6 +72,27 @@ describe('Frontmatter', () => {
       });
     });
 
+    it('should throw on duplicate frontmatter keys', () => {
+      expect(() =>
+        FrontmatterInfer.fromFile('---\ntitle: Hello\ntitle: World\n---\nContent'),
+      ).toThrow(/Map keys must be unique/);
+    });
+
+    it('should throw on duplicate frontmatter keys with explicit YAML format', () => {
+      expect(() =>
+        frontmatterYAML().fromFile('---\ntitle: Hello\ntitle: World\n---\nContent'),
+      ).toThrow(/Map keys must be unique/);
+    });
+
+    it('should not throw when body contains YAML-like patterns', () => {
+      expect(
+        FrontmatterInfer.fromFile('---\ntitle: Hello\n---\ntitle: this is not a duplicate'),
+      ).toEqual({
+        title: 'Hello',
+        body: 'title: this is not a duplicate',
+      });
+    });
+
     it('should stringify YAML with --- delimiters', () => {
       expect(
         FrontmatterInfer.toFile({

packages/decap-cms-core/src/formats/__tests__/yaml.spec.js

@@ -85,6 +85,33 @@ describe('yaml', () => {
         time: '10:05',
       });
     });
+
+    test('throws on duplicate keys', () => {
+      expect(() => yaml.fromFile('title: Hello\ntitle: World')).toThrow(
+        /Map keys must be unique; "title" is repeated/,
+      );
+    });
+
+    test('throws on duplicate nested keys', () => {
+      expect(() => yaml.fromFile('nested:\n  a: 1\n  a: 2')).toThrow(
+        /Map keys must be unique; "a" is repeated/,
+      );
+    });
+
+    test('does not throw when same key appears in different nested objects', () => {
+      expect(yaml.fromFile('obj1:\n  name: foo\nobj2:\n  name: bar')).toEqual({
+        obj1: { name: 'foo' },
+        obj2: { name: 'bar' },
+      });
+    });
+
+    test('logs warnings to console.warn', () => {
+      const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+      // Valid YAML should produce no warnings
+      yaml.fromFile('title: Hello');
+      expect(warnSpy).not.toHaveBeenCalled();
+      warnSpy.mockRestore();
+    });
   });
   describe('toFile', () => {
     test('outputs valid yaml', () => {

packages/decap-cms-core/src/formats/yaml.ts

@@ -41,7 +41,22 @@ export default {
     if (content && content.trim().endsWith('---')) {
       content = content.trim().slice(0, -3);
     }
-    return yaml.parse(content, { customTags: [timestampTag] });
+
+    const doc = yaml.parseDocument(content, {
+      customTags: [timestampTag],
+      prettyErrors: true,
+    });
+
+    for (const warn of doc.warnings) {
+      console.warn(`YAML warning: ${warn.message}`);
+    }
+
+    if (doc.errors.length > 0) {
+      const messages = doc.errors.map(e => e.message).join('\n');
+      throw new Error(`YAML parsing error:\n${messages}`);
+    }
+
+    return doc.toJSON();
   },
 
   toFile(data: object, sortedKeys: string[] = [], comments: Record<string, string> = {}) {