add content linter rule for carousel recommended articles (#57216)

Author: rsese

data/reusables/contributing/content-linter-rules.md

@@ -71,6 +71,7 @@
 | GHD053 | header-content-requirement | Headers must have content between them, such as an introduction | warning | headers, structure, content |
 | GHD054 | third-party-actions-reusable | Code examples with third-party actions must include disclaimer reusable | warning | actions, reusable, third-party |
 | GHD055 | frontmatter-validation | Frontmatter properties must meet character limits and required property requirements | warning | frontmatter, character-limits, required-properties |
+| GHD056 | frontmatter-landing-recommended | Only landing pages can have recommended articles, there should be no duplicate recommended articles, and all recommended articles must exist | error | frontmatter, landing, recommended |
 | [search-replace](https://github.com/OnkarRuikar/markdownlint-rule-search-replace) | deprecated liquid syntax: octicon-<icon-name> | The octicon liquid syntax used is deprecated. Use this format instead `octicon "<octicon-name>" aria-label="<Octicon aria label>"` | error |  |
 | [search-replace](https://github.com/OnkarRuikar/markdownlint-rule-search-replace) | deprecated liquid syntax: site.data | Catch occurrences of deprecated liquid data syntax. | error |  |
 | [search-replace](https://github.com/OnkarRuikar/markdownlint-rule-search-replace) | developer-domain | Catch occurrences of developer.github.com domain. | error |  |

src/content-linter/lib/linting-rules/frontmatter-landing-recommended.js

@@ -0,0 +1,88 @@
+import fs from 'fs'
+import path from 'path'
+import { addError } from 'markdownlint-rule-helpers'
+
+import { getFrontmatter } from '../helpers/utils'
+
+function isValidArticlePath(articlePath, currentFilePath) {
+  // Article paths in recommended are relative to the current page's directory
+  const relativePath = articlePath.startsWith('/') ? articlePath.substring(1) : articlePath
+  const currentDir = path.dirname(currentFilePath)
+  const fullPath = path.join(currentDir, `${relativePath}.md`)
+
+  try {
+    return fs.existsSync(fullPath) && fs.statSync(fullPath).isFile()
+  } catch {
+    return false
+  }
+}
+
+export const frontmatterLandingRecommended = {
+  names: ['GHD056', 'frontmatter-landing-recommended'],
+  description:
+    'Only landing pages can have recommended articles, there should be no duplicate recommended articles, and all recommended articles must exist',
+  tags: ['frontmatter', 'landing', 'recommended'],
+  function: (params, onError) => {
+    const fm = getFrontmatter(params.lines)
+    if (!fm || !fm.recommended) return
+
+    const recommendedLine = params.lines.find((line) => line.startsWith('recommended:'))
+
+    if (!recommendedLine) return
+
+    const lineNumber = params.lines.indexOf(recommendedLine) + 1
+
+    if (!fm.layout || !fm.layout.includes('landing')) {
+      addError(
+        onError,
+        lineNumber,
+        'recommended frontmatter key is only valid for landing pages',
+        recommendedLine,
+        [1, recommendedLine.length],
+        null,
+      )
+    }
+
+    // Check for duplicate recommended items and invalid paths
+    if (Array.isArray(fm.recommended)) {
+      const seen = new Set()
+      const duplicates = []
+      const invalidPaths = []
+
+      fm.recommended.forEach((item) => {
+        if (seen.has(item)) {
+          duplicates.push(item)
+        } else {
+          seen.add(item)
+        }
+
+        // Validate that the article path exists
+        if (!isValidArticlePath(item, params.name)) {
+          invalidPaths.push(item)
+        }
+      })
+
+      if (duplicates.length > 0) {
+        addError(
+          onError,
+          lineNumber,
+          `Found duplicate recommended articles: ${duplicates.join(', ')}`,
+          recommendedLine,
+          [1, recommendedLine.length],
+          null,
+        )
+      }
+
+      if (invalidPaths.length > 0) {
+        addError(
+          onError,
+          lineNumber,
+          `Found invalid recommended article paths: ${invalidPaths.join(', ')}`,
+          recommendedLine,
+          [1, recommendedLine.length],
+          null,
+        )
+      }
+    }
+  },
+}

src/content-linter/lib/linting-rules/index.js

@@ -54,6 +54,7 @@ import { frontmatterVersionsWhitespace } from '@/content-linter/lib/linting-rule
 import { frontmatterValidation } from '@/content-linter/lib/linting-rules/frontmatter-validation'
 import { headerContentRequirement } from '@/content-linter/lib/linting-rules/header-content-requirement'
 import { thirdPartyActionsReusable } from '@/content-linter/lib/linting-rules/third-party-actions-reusable'
+import { frontmatterLandingRecommended } from '@/content-linter/lib/linting-rules/frontmatter-landing-recommended'
 
 const noDefaultAltText = markdownlintGitHub.find((elem) =>
   elem.names.includes('no-default-alt-text'),
@@ -115,6 +116,7 @@ export const gitHubDocsMarkdownlint = {
     headerContentRequirement, // GHD053
     thirdPartyActionsReusable, // GHD054
     frontmatterValidation, // GHD055
+    frontmatterLandingRecommended, // GHD056
 
     // Search-replace rules
     searchReplace, // Open-source plugin

src/content-linter/style/github-docs.js

@@ -310,6 +310,12 @@ export const githubDocsFrontmatterConfig = {
     'partial-markdown-files': false,
     'yml-files': false,
   },
+  'frontmatter-landing-recommended': {
+    // GHD056
+    severity: 'error',
+    'partial-markdown-files': false,
+    'yml-files': false,
+  },
 }
 
 // Configures rules from the `github/markdownlint-github` repo

src/content-linter/tests/fixtures/landing-recommended/article-one.md

@@ -0,0 +1,14 @@
+---
+title: Article One
+layout: inline
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+---
+
+# Article One
+
+This is a supporting article for testing.

src/content-linter/tests/fixtures/landing-recommended/article-two.md

@@ -0,0 +1,14 @@
+---
+title: Article Two
+layout: inline
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+---
+
+# Article Two
+
+This is another supporting article for testing.

src/content-linter/tests/fixtures/landing-recommended/duplicate-recommended.md

@@ -0,0 +1,19 @@
+---
+title: Landing with Duplicates
+layout: product-landing
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+recommended:
+  - /article-one
+  - /article-two
+  - /article-one
+  - /subdir/article-three
+---
+
+# Landing with Duplicates
+
+This landing page has duplicate recommended articles.

src/content-linter/tests/fixtures/landing-recommended/invalid-non-landing.md

@@ -0,0 +1,18 @@
+---
+title: Not a Landing Page
+layout: inline
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+recommended:
+  - /article-one
+  - /article-two
+  - /subdir/article-three
+---
+
+# Not a Landing Page
+
+This page has a recommended property but is not a landing page.

src/content-linter/tests/fixtures/landing-recommended/invalid-paths.md

@@ -0,0 +1,18 @@
+---
+title: Landing with Invalid Paths
+layout: product-landing
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+recommended:
+  - /article-one
+  - /nonexistent/path
+  - /another/invalid/path
+---
+
+# Landing with Invalid Paths
+
+This landing page has some invalid recommended article paths.

src/content-linter/tests/fixtures/landing-recommended/no-recommended.md

@@ -0,0 +1,14 @@
+---
+title: Landing without Recommended
+layout: product-landing
+versions:
+  fpt: '*'
+  ghec: '*'
+  ghes: '*'
+topics:
+  - Testing
+---
+
+# Landing without Recommended
+
+This is a landing page without any recommended articles.