fix(cloudflare): Bind a default re-export with var in the Vite transform

packages/cloudflare/src/vite/transform.ts

@@ -604,6 +604,11 @@ function wrapDefaultSpecifier(localName: string, state: TransformState): string
 /**
  * `export { default } from './handler'`. The handler may already be wrapped in its own module,
  * which this transform cannot see, so the emitted guard returns such a handler as-is.
+ *
+ * The original re-export is a live binding to the handler module's export, but the wrapper is not
+ * set until this module's body runs. A circular import that reads the default export before then
+ * gets `undefined` instead of the handler. The binding is a `var` so that the read does not throw,
+ * as a `const` would, and stop the Worker from starting. See `wrapDefaultSpecifier`.
  */
 function wrapDefaultReexport(
   localName: string,
@@ -616,7 +621,7 @@ function wrapDefaultReexport(
     localName === DEFAULT_IMPORT
       ? `import ${target} from ${sourceLiteral};`
       : `import { ${localName} as ${target} } from ${sourceLiteral};`,
-    `const __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, ${state.optionsFn}, ${target});`,
+    `var __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, ${state.optionsFn}, ${target});`,
   );
   state.needsImport = true;
   state.autoWrapped.add(DEFAULT_EXPORT);

packages/cloudflare/test/vite/transform.test.ts

@@ -241,7 +241,7 @@ describe('default export wrapping', () => {
       [
         "import * as __SENTRY__ from '@sentry/cloudflare';",
         "import __SENTRY_REEXPORT_default__ from './handler';",
-        'const __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
+        'var __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
         'export { __SENTRY_DEFAULT_EXPORT__ as default };',
       ].join('\n'),
     );
@@ -254,20 +254,47 @@ describe('default export wrapping', () => {
       [
         "import * as __SENTRY__ from '@sentry/cloudflare';",
         "import { handler as __SENTRY_REEXPORT_default__ } from './handler';",
-        'const __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
+        'var __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
         'export { __SENTRY_DEFAULT_EXPORT__ as default };',
       ].join('\n'),
     );
   });
 
+  it('does not throw when a circular import reads a default re-export during module evaluation', () => {
+    // `b` reads the entry's default export after `./handler.mjs` has run but before the entry has.
+    const entry = [
+      "export { default } from './handler.mjs';",
+      "import { readDefault } from './b.mjs';",
+      'console.log(readDefault());',
+    ].join('\n');
+    const transformed = transform(entry, ctx)!.code.replace(
+      "import * as __SENTRY__ from '@sentry/cloudflare';",
+      'const __SENTRY__ = { withSentry: (_options, handler) => handler, _INTERNAL_wrapUnlessInstrumented: (wrap, options, handler) => wrap(options, handler) };',
+    );
+
+    const dir = mkdtempSync(join(tmpdir(), 'sentry-cf-cycle-'));
+    try {
+      writeFileSync(join(dir, 'a.mjs'), transformed);
+      writeFileSync(join(dir, 'handler.mjs'), 'export default { fetch() {} };\n');
+      writeFileSync(
+        join(dir, 'b.mjs'),
+        "import handler from './a.mjs';\nconst kind = typeof handler;\nexport function readDefault() { return kind; }\n",
+      );
+      // Plain Node: Vitest's module runner does not evaluate import cycles like native ESM does.
+      expect(execFileSync(process.execPath, [join(dir, 'a.mjs')], { encoding: 'utf8' }).trim()).toBe('undefined');
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   it('leaves sibling specifiers of a default re-export untouched', () => {
     const code = "export { default, helper } from './handler';";
 
     expect(transform(code, ctx)!.code).toBe(
       [
         "import * as __SENTRY__ from '@sentry/cloudflare';",
         "import __SENTRY_REEXPORT_default__ from './handler';",
-        'const __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
+        'var __SENTRY_DEFAULT_EXPORT__ = __SENTRY__._INTERNAL_wrapUnlessInstrumented(__SENTRY__.withSentry, (env) => ({}), __SENTRY_REEXPORT_default__);',
         'export { __SENTRY_DEFAULT_EXPORT__ as default };',
         "export { helper } from './handler';",
       ].join('\n'),