fix(markdown): skip circular includes

Expanding an include that is its own ancestor recursed forever.
Track the ancestor chain and leave such includes unexpanded; the
same file can still be included repeatedly as a sibling.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

__tests__/unit/node/utils/processIncludes.test.ts

@@ -0,0 +1,54 @@
+import { createMarkdownItAsync } from 'markdown-it-async'
+import { mkdtemp, readFile, rm, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import path from 'node:path'
+import { processIncludes } from 'node/utils/processIncludes'
+
+describe('node/utils/processIncludes', () => {
+  let root: string
+
+  beforeEach(async () => {
+    root = await mkdtemp(path.join(tmpdir(), 'vitepress-includes-'))
+  })
+
+  afterEach(async () => {
+    await rm(root, { recursive: true, force: true })
+  })
+
+  async function write(name: string, src: string) {
+    await writeFile(path.join(root, name), src)
+  }
+
+  async function run(name: string) {
+    const file = path.join(root, name)
+    const src = await readFile(file, 'utf8')
+    return processIncludes(createMarkdownItAsync(), root, src, file, [], false)
+  }
+
+  test('leaves a self-include unexpanded', async () => {
+    await write('a.md', '# A\n\n<!-- @include: ./a.md -->\n')
+
+    expect(await run('a.md')).toContain('<!-- @include: ./a.md -->')
+  })
+
+  test('leaves circular includes unexpanded', async () => {
+    await write('a.md', 'A-content\n\n<!-- @include: ./b.md -->\n')
+    await write('b.md', 'B-content\n\n<!-- @include: ./a.md -->\n')
+
+    const result = await run('a.md')
+    expect(result).toContain('B-content')
+    expect(result).toContain('<!-- @include: ./a.md -->')
+  })
+
+  test('expands repeated includes outside the ancestor chain', async () => {
+    await write(
+      'a.md',
+      '<!-- @include: ./b.md -->\n<!-- @include: ./c.md -->\n'
+    )
+    await write('b.md', 'B-content\n\n<!-- @include: ./d.md -->\n')
+    await write('c.md', 'C-content\n\n<!-- @include: ./d.md -->\n')
+    await write('d.md', 'D-content\n')
+
+    expect((await run('a.md')).match(/D-content/g)).toHaveLength(2)
+  })
+})

src/node/utils/processIncludes.ts

@@ -11,7 +11,8 @@ export function processIncludes(
   src: string,
   file: string,
   includes: string[],
-  cleanUrls: boolean
+  cleanUrls: boolean,
+  ancestors: string[] = []
 ): Promise<string> {
   const includesRE = /<!--\s*@include:\s*(.*?)\s*-->/g
   const regionRE = /(#[^\s\{]+)/
@@ -36,6 +37,10 @@ export function processIncludes(
       ? path.join(srcDir, m1.slice(m1[1] === '/' ? 2 : 1))
       : path.join(path.dirname(file), m1)
 
+    // leave circular includes unexpanded — only repeats along the ancestor
+    // chain are cycles, the same file may still be included by siblings
+    if (includePath === file || ancestors.includes(includePath)) return m
+
     let content = await readFile(includePath, 'utf8')
 
     if (region) {
@@ -102,7 +107,8 @@ export function processIncludes(
       content,
       includePath,
       includes,
-      cleanUrls
+      cleanUrls,
+      [...ancestors, file]
     )
   })
 }