process: drain microtasks after emitting 'exit' on natural termination

src/jsc/VirtualMachine.rs

@@ -368,7 +368,7 @@
     safe fn Bun__emitHandledPromiseEvent(global: &JSGlobalObject, promise: JSValue) -> bool;
 
     safe fn Process__dispatchOnBeforeExit(global: &JSGlobalObject, code: u8);
-    safe fn Process__dispatchOnExit(global: &JSGlobalObject, code: u8);
+    safe fn Process__dispatchOnExit(global: &JSGlobalObject, code: u8, drain_microtasks: bool);
     safe fn Bun__closeAllSQLiteDatabasesForTermination();
     safe fn Bun__closeAllNodeSqliteDatabasesForTermination(global: &JSGlobalObject);
     safe fn Bun__WebView__closeAllForTermination();
@@ -503,9 +503,9 @@
     /// parent via `container_of` would escape the provenance of `&mut self`
     /// (which only covers the `ExitHandler` field). Callers pass the VM
     /// reference instead; the body re-enters JS so no `&mut` is held.
-    pub fn dispatch_on_exit(vm: &VirtualMachine) {
+    pub fn dispatch_on_exit(vm: &VirtualMachine, drain_microtasks: bool) {
         let exit_code = vm.exit_handler.exit_code;
-        Process__dispatchOnExit(vm.global(), exit_code);
+        Process__dispatchOnExit(vm.global(), exit_code, drain_microtasks);
         if vm.worker.is_none() {
             Bun__closeAllSQLiteDatabasesForTermination();
             Bun__closeAllNodeSqliteDatabasesForTermination(vm.global());
@@ -1513,7 +1513,15 @@
             }
         }
 
-        ExitHandler::dispatch_on_exit(self);
+        // Drain microtasks queued by the 'exit' listener only on a natural
+        // event-loop drain: not after a fatal error, and not when a worker
+        // reached here via terminate()/process.exit() (both set
+        // requested_terminate before shutdown() calls us).
+        let natural = self.unhandled_error_counter == 0
+            && !self
+                .worker_ref()
+                .is_some_and(|w| w.has_requested_terminate());
+        ExitHandler::dispatch_on_exit(self, natural);
         self.is_shutting_down = true;
 
         // Make sure we run new cleanup hooks introduced by running cleanup

src/jsc/bindings/BunProcess.cpp

@@ -281,7 +281,7 @@
     return release;
 }
 
-static void dispatchExitInternal(JSC::JSGlobalObject* globalObject, Process* process, int exitCode)
+static void dispatchExitInternal(JSC::JSGlobalObject* globalObject, Process* process, int exitCode, bool drainMicrotasks)
 {
     if (process->m_isExiting)
         return;
@@ -300,7 +300,15 @@
 
     MarkedArgumentBuffer arguments;
     arguments.append(jsNumber(exitCode));
     emitter.emit(event, arguments);
+
+    // Node performs a final microtask checkpoint after emitting 'exit' on a
+    // natural drain (not process.exit() or fatal exception), so promise
+    // reactions and queueMicrotask callbacks queued by the listener run
+    // before termination. process.nextTick does not.
+    if (drainMicrotasks && !vm.hasTerminationRequest()) {
+        vm.drainMicrotasks();
+    }
 }
 
 JSC_DEFINE_CUSTOM_SETTER(Process_defaultSetter, (JSC::JSGlobalObject * globalObject, JSC::EncodedJSValue thisValue, JSC::EncodedJSValue value, JSC::PropertyName propertyName))
@@ -843,7 +851,7 @@
     }
 }
 
-extern "C" void Process__dispatchOnExit(Zig::GlobalObject* globalObject, uint8_t exitCode)
+extern "C" void Process__dispatchOnExit(Zig::GlobalObject* globalObject, uint8_t exitCode, bool drainMicrotasks)
 {
     if (!globalObject->hasProcessObject()) {
         return;
@@ -852,7 +860,7 @@
     auto* process = globalObject->processObject();
     if (exitCode > 0)
         process->m_isExitCodeObservable = true;
-    dispatchExitInternal(globalObject, process, exitCode);
+    dispatchExitInternal(globalObject, process, exitCode, drainMicrotasks);
 }
 
 JSC_DEFINE_HOST_FUNCTION(Process_functionUptime, (JSC::JSGlobalObject * lexicalGlobalObject, JSC::CallFrame* callFrame))
@@ -875,7 +883,7 @@
     RETURN_IF_EXCEPTION(throwScope, {});
 
     auto exitCode = Bun__getExitCode(bunVM(zigGlobal));
-    Process__dispatchOnExit(zigGlobal, exitCode);
+    Process__dispatchOnExit(zigGlobal, exitCode, false);
 
     // process.reallyExit(exitCode);
     auto reallyExitVal = process->get(globalObject, Identifier::fromString(vm, "reallyExit"_s));

src/jsc/web_worker.rs

@@ -1587,7 +1587,7 @@ fn on_unhandled_rejection(
     // they may change process.exitCode). Run them before arming termination — a pending
     // termination exception makes dispatchExitInternal skip 'exit' (as terminate() should),
     // and its processIsExiting guard stops shutdown() from running them twice.
-    virtual_machine::ExitHandler::dispatch_on_exit(vm);
+    virtual_machine::ExitHandler::dispatch_on_exit(vm, false);
     let _ = worker.set_requested_terminate();
     // Do NOT call `worker.shutdown()` here —
     // `shutdown()` RETURNS, so calling it here would destroy

test/js/node/process/process.test.js

@@ -712,6 +712,119 @@ describe.concurrent(() => {
   });
 
   describe("process.onExit", () => {
+    it("drains microtasks queued by the listener on natural exit", async () => {
+      await using proc = Bun.spawn({
+        cmd: [
+          bunExe(),
+          "-e",
+          `process.on("exit", async () => {
+             console.log("exit-listener");
+             Promise.resolve().then(() => console.log("pt-in-exit"));
+             queueMicrotask(() => console.log("qm-in-exit"));
+             await Promise.resolve();
+             console.log("after-await-in-exit");
+           });`,
+        ],
+        env: bunEnv,
+        stdio: ["inherit", "pipe", "pipe"],
+      });
+      const [stderr, stdout, exitCode] = await Promise.all([proc.stderr.text(), proc.stdout.text(), proc.exited]);
+      expect({ stdout, stderr, exitCode }).toEqual({
+        stdout: "exit-listener\npt-in-exit\nqm-in-exit\nafter-await-in-exit\n",
+        stderr: "",
+        exitCode: 0,
+      });
+    });
+
+    it("does not drain microtasks queued by the listener on process.exit()", async () => {
+      await using proc = Bun.spawn({
+        cmd: [
+          bunExe(),
+          "-e",
+          `process.on("exit", () => {
+             console.log("exit-listener");
+             queueMicrotask(() => console.log("qm-in-exit"));
+           });
+           process.exit(0);`,
+        ],
+        env: bunEnv,
+        stdio: ["inherit", "pipe", "pipe"],
+      });
+      const [stderr, stdout, exitCode] = await Promise.all([proc.stderr.text(), proc.stdout.text(), proc.exited]);
+      expect({ stdout, stderr, exitCode }).toEqual({
+        stdout: "exit-listener\n",
+        stderr: "",
+        exitCode: 0,
+      });
+    });
+
+    it("does not drain microtasks queued by the listener after a fatal uncaught exception", async () => {
+      await using proc = Bun.spawn({
+        cmd: [
+          bunExe(),
+          "-e",
+          `process.on("exit", () => {
+             console.log("exit-listener");
+             queueMicrotask(() => console.log("qm-in-exit"));
+           });
+           throw new Error("boom");`,
+        ],
+        env: bunEnv,
+        stdio: ["inherit", "pipe", "pipe"],
+      });
+      const [stderr, stdout, exitCode] = await Promise.all([proc.stderr.text(), proc.stdout.text(), proc.exited]);
+      expect(stderr).toInclude("error: boom");
+      expect(stdout).toBe("exit-listener\n");
+      expect(exitCode).toBe(1);
+    });
+
+    it("does not drain process.nextTick queued by the listener on natural exit", async () => {
+      await using proc = Bun.spawn({
+        cmd: [
+          bunExe(),
+          "-e",
+          `process.on("exit", () => {
+             console.log("exit-listener");
+             process.nextTick(() => console.log("nt-in-exit"));
+             queueMicrotask(() => console.log("qm-in-exit"));
+           });`,
+        ],
+        env: bunEnv,
+        stdio: ["inherit", "pipe", "pipe"],
+      });
+      const [stderr, stdout, exitCode] = await Promise.all([proc.stderr.text(), proc.stdout.text(), proc.exited]);
+      expect({ stdout, stderr, exitCode }).toEqual({
+        stdout: "exit-listener\nqm-in-exit\n",
+        stderr: "",
+        exitCode: 0,
+      });
+    });
+
+    it("drains microtasks queued by the listener on a worker's natural exit", async () => {
+      await using proc = Bun.spawn({
+        cmd: [
+          bunExe(),
+          "-e",
+          `const { Worker } = require("worker_threads");
+           const w = new Worker(\`
+             process.on("exit", () => {
+               console.log("exit-listener");
+               queueMicrotask(() => console.log("qm-in-exit"));
+             });
+           \`, { eval: true });
+           w.on("exit", () => console.log("worker-done"));`,
+        ],
+        env: bunEnv,
+        stdio: ["inherit", "pipe", "pipe"],
+      });
+      const [stderr, stdout, exitCode] = await Promise.all([proc.stderr.text(), proc.stdout.text(), proc.exited]);
+      expect({ stdout, stderr, exitCode }).toEqual({
+        stdout: "exit-listener\nqm-in-exit\nworker-done\n",
+        stderr: "",
+        exitCode: 0,
+      });
+    });
+
     it("throwing inside preserves exit code", async () => {
       await using proc = Bun.spawn({
         cmd: [bunExe(), "-e", `process.on("exit", () => {throw new Error("boom")});`],