fix: prevent infinite loop in base URL with a relative base

src/tests/transformers/base.test.ts

@@ -243,6 +243,22 @@ describe('base URL', () => {
       const result = run(html, { url: { base: 'https://cdn.example.com/' } })
       expect(result).toBe(html)
     })
+
+    it('prepends a relative base to url() exactly once (no infinite re-prepend)', () => {
+      // A relative base never makes the URL absolute; the PostCSS declaration
+      // must not be re-visited, or it re-prepends forever and hangs.
+      const html = '<style>.bg { background-image: url(bg.jpg) }</style>'
+      const result = run(html, { url: { base: '/images/' } })
+      expect(result).toContain('url(/images/bg.jpg)')
+      expect(result).not.toContain('/images//images/')
+    })
+
+    it('prepends a relative base in an inline style exactly once', () => {
+      const html = '<div style="background-image: url(bg.jpg)"></div>'
+      const result = run(html, { url: { base: '/images/' } })
+      expect(result).toContain('url(/images/bg.jpg)')
+      expect(result).not.toContain('/images//images/')
+    })
   })
 
   describe('inlineCss option', () => {

src/transformers/base.ts

@@ -54,31 +54,43 @@ const defaultTagConfig: Record<string, Record<string, string | boolean>> = Objec
 )
 
 const postcssBaseUrl: postcss.PluginCreator<{ url: string }> = (opts) => {
-  return {
-    postcssPlugin: 'postcss-base-url',
-    Declaration(decl) {
-      if (!decl.value.includes('url(')) return
+  const rewriteDecl = (decl: postcss.Declaration) => {
+    if (!decl.value.includes('url(')) return
 
-      const parsed = valueParser(decl.value)
-      let changed = false
+    const parsed = valueParser(decl.value)
+    let changed = false
 
-      parsed.walk(node => {
-        if (node.type !== 'function' || node.value !== 'url') return
+    parsed.walk(node => {
+      if (node.type !== 'function' || node.value !== 'url') return
 
-        const urlNode = node.nodes[0]
-        if (!urlNode) return
+      const urlNode = node.nodes[0]
+      if (!urlNode) return
 
-        if (isAbsoluteUrl(urlNode.value)) return
+      if (isAbsoluteUrl(urlNode.value)) return
 
-        urlNode.value = opts!.url + urlNode.value
-        changed = true
-      })
+      urlNode.value = opts!.url + urlNode.value
+      changed = true
+    })
 
-      if (changed) {
-        decl.value = parsed.toString()
-      }
+    if (changed) {
+      decl.value = parsed.toString()
     }
   }
+
+  return {
+    postcssPlugin: 'postcss-base-url',
+    /**
+     * Walk declarations once, in `OnceExit`, rather than via the visitor
+     * `Declaration` hook. The visitor re-runs on nodes it mutates, and a
+     * relative base (e.g. `/images/`) never makes the URL absolute — so the
+     * guard `isAbsoluteUrl` stays false and it re-prepends the base forever
+     * (`/images//images/…`), hanging the build. A single manual pass rewrites
+     * each declaration exactly once.
+     */
+    OnceExit(root) {
+      root.walkDecls(rewriteDecl)
+    },
+  }
 }
 postcssBaseUrl.postcss = true