module loader: fix false "require() async module" for a module racing a concurrent import

src/jsc/bindings/ZigGlobalObject.cpp

@@ -766,6 +766,35 @@ JSC_DEFINE_HOST_FUNCTION(functionEsmLoadSync, (JSC::JSGlobalObject * lexicalGlob
             RETURN_IF_EXCEPTION(scope, {});
             return JSValue::encode(ns);
         }
+
+        // A concurrent import may leave this entry Fetching with no record; drive
+        // the fetch synchronously so a sync-completable module reaches Fetched
+        // before loadModuleSync links it. Async fetches (TLA, plugin) stay pending.
+        if (entry->status() == JSC::ModuleRegistryEntry::Status::Fetching && !entry->record()) {
+            JSPromise* fetchPromise = entry->ensureFetchPromise(globalObject);
+            JSPromise* modulePromise = entry->ensureModulePromise(globalObject);
+            RETURN_IF_EXCEPTION(scope, {});
+            if (modulePromise->status() == JSPromise::Status::Pending && fetchPromise->status() == JSPromise::Status::Pending) {
+                JSC::VM::SynchronousModuleQueue queue;
+                queue.prev = vm.m_synchronousModuleQueue;
+                vm.m_synchronousModuleQueue = &queue;
+                JSPromise* src = loader->fetch(globalObject, keyValue, nullptr, nullptr);
+                bool settled = false;
+                if (!scope.exception()) {
+                    if (src->status() == JSPromise::Status::Fulfilled) {
+                        fetchPromise->fulfillPromise(vm, src->result());
+                        settled = true;
+                    } else if (src->status() == JSPromise::Status::Rejected) {
+                        fetchPromise->rejectPromise(vm, src->result());
+                        settled = true;
+                    }
+                    if (settled && !scope.exception())
+                        JSC::JSModuleLoader::drainSynchronousModuleQueue(globalObject);
+                }
+                vm.m_synchronousModuleQueue = queue.prev;
+                RETURN_IF_EXCEPTION(scope, {});
+            }
+        }
     }
 
     JSPromise* promise = loader->loadModuleSync(globalObject, key, nullptr, nullptr);

test/regression/issue/33180.test.ts

@@ -0,0 +1,56 @@
+// https://github.com/oven-sh/bun/issues/33180
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+// A plain (no top-level await) ESM module must stay require()-able while a
+// concurrent static import is still fetching it off-thread. The target is large
+// so its transpile reliably overlaps the tiny CommonJS sibling's require().
+function makeTarget() {
+  let src = "";
+  for (let i = 0; i < 1000; i++) src += `export const v${i} = ${i};\n`;
+  src += `export const val = 42;\n`;
+  src += `export function getVal() { return val; }\n`;
+  return src;
+}
+
+const files = {
+  "target.mjs": makeTarget(),
+  "cjs-req.cjs": `
+    const t = require("./target.mjs");
+    if (t.val !== 42 || t.getVal() !== 42) {
+      console.error("half-initialized namespace: val=" + t.val);
+      process.exit(3);
+    }
+    console.log("required-ok");
+    module.exports = {};
+  `,
+  "entry.mjs": `
+    import "./target.mjs";
+    import "./cjs-req.cjs";
+    console.log("entry-done");
+  `,
+};
+
+// Independent attempts (separate temp dirs + child processes, nothing shared);
+// without the fix the race throws the false "async module" error on essentially
+// every debug run, so any attempt suffices.
+for (let attempt = 0; attempt < 4; attempt++) {
+  test.concurrent(
+    `require(esm) racing a concurrent static import loads synchronously (attempt ${attempt})`,
+    async () => {
+      using dir = tempDir("issue-33180", files);
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), "entry.mjs"],
+        env: bunEnv,
+        cwd: String(dir),
+        stderr: "pipe",
+        stdout: "pipe",
+      });
+      const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+      expect(stderr).not.toContain("async module");
+      expect(stdout).toContain("required-ok");
+      expect(stdout).toContain("entry-done");
+      expect(exitCode).toBe(0);
+    },
+  );
+}