event loop: run the timers phase before the entry point's queued work

src/jsc/VirtualMachine.rs

@@ -2412,6 +2412,7 @@
         if self.is_watcher_enabled() {
             // accessed here (no overlapping `&mut EventLoop`).
             self.event_loop_mut().perform_gc();
+            self.run_entry_point_body(self.pending_internal_promise.unwrap_or(promise));
             loop {
                 let Some(p) = self.pending_internal_promise else {
                     break;
@@ -2435,12 +2436,43 @@
                 return Ok(promise);
             }
             self.event_loop_mut().perform_gc();
+            self.run_entry_point_body(promise);
             self.wait_for_promise(jsc::AnyPromise::Internal(promise));
         }
 
         Ok(self.pending_internal_promise.unwrap_or(promise))
     }
 
+    /// Run the entry point's synchronous body, then the timers phase.
+    ///
+    /// Module evaluation runs on the microtask queue, so draining microtasks
+    /// reaches the first `await` (or the end of a synchronous entry point).
+    /// Leaving that to the `tick()` in the wait loop would dispatch the *task*
+    /// queue in the same tick, running an I/O completion or port message the
+    /// body queued ahead of a timer that expired while it ran. `uv_run` opens
+    /// with `uv__run_timers`, so Node dispatches that timer before either.
+    fn run_entry_point_body(&mut self, evaluation: *mut JSInternalPromise) {
+        // The body has to evaluate with the loop entered, the way it did inside
+        // `tick()`: a nested callback's `exit()` drains microtasks at count 1, and
+        // draining them mid-body reorders a constructor against its own
+        // continuations. `exit()` drains what the body left behind. Reporting
+        // rejections is left to the wait loop's `tick()`, as on the pre-existing
+        // path, so this stays a pure microtask drain.
+        self.event_loop_mut().enter();
+        let drained = self.event_loop_mut().drain_microtasks();
+        self.event_loop_mut().exit();
+        if drained.is_err() {
+            return;
+        }
+        // SAFETY: `evaluation` is a live JSC heap cell returned by
+        // `reload_entry_point*` and kept alive by the module loader. A body that
+        // threw rejects it; Node reports the error and exits without a loop.
+        if crate::JSPromise::status_ptr(evaluation) == crate::js_promise::Status::Rejected {
+            return;
+        }
+        self.event_loop_mut().drain_expired_timers();
+    }
+
     /// Drain pending tasks/microtasks if the event loop is not currently
     /// re-entered. Convenience used after top-level evaluation on
     /// the `bun -e` path.
@@ -4573,6 +4605,12 @@
     ) -> Result<*mut JSInternalPromise, bun_core::Error> {
         let promise = self.reload_entry_point(entry_path)?;
         self.event_loop_mut().perform_gc();
+        // Deliberately not `run_entry_point_body`: a worker's body has to evaluate
+        // inside the termination-aware wait below, which stops before the next
+        // `tick()` once `terminate()` lands. Evaluating it on the microtask queue
+        // instead lets a `tick()` start with the termination exception already
+        // pending, and its first task dispatch trips `scope.assertNoException()`.
+        // `web_worker.rs` runs the timers phase before it enters the loop.
         self.event_loop_mut()
             .wait_for_promise_with_termination(jsc::AnyPromise::Internal(promise));
         if let Some(worker) = self.worker_ref() {
@@ -4593,6 +4631,7 @@
         // pending_internal_promise can change if hot module reloading is enabled
         if self.is_watcher_enabled() {
             self.event_loop_mut().perform_gc();
+            self.run_entry_point_body(self.pending_internal_promise.unwrap_or(promise));
             loop {
                 let Some(p) = self.pending_internal_promise else {
                     break;
@@ -4616,6 +4655,7 @@
                 return Ok(promise);
             }
             self.event_loop_mut().perform_gc();
+            self.run_entry_point_body(promise);
             self.wait_for_promise(jsc::AnyPromise::Internal(promise));
         }
 

src/jsc/event_loop.rs

@@ -185,6 +185,9 @@ unsafe extern "Rust" {
     /// `WTFTimer::run` — `timer` is an erased `*mut bun_runtime::timer::WTFTimer`.
     /// Defined in `bun_runtime::dispatch`. Link-time resolved.
     fn __bun_run_wtf_timer(timer: *mut (), vm: *mut VirtualMachine);
+    /// `timer::All::drain_timers`: fire every timer whose deadline has passed.
+    /// Defined in `bun_runtime::dispatch`. Link-time resolved.
+    fn __bun_drain_expired_timers(vm: *mut VirtualMachine);
     /// Tag-specific shutdown release for a queued-but-never-run task. Called
     /// from `release_queued_tasks_for_shutdown` (after `shutdown_for_exit`,
     /// before `destructOnExit`) for every entry left in `self.tasks`.
@@ -463,6 +466,23 @@ impl EventLoop {
         }
     }
 
+    /// Fire every timer whose deadline has already passed: libuv's timers phase.
+    ///
+    /// `auto_tick` runs it at the end of an iteration rather than the start, so
+    /// the cyclic order matches `uv_run`'s for every iteration but the first.
+    /// Callers about to enter the loop for the first time run one themselves,
+    /// so a timer that expired while the entry point was still running
+    /// dispatches before the tasks and `setImmediate` callbacks it queued, as
+    /// it does in Node.
+    pub fn drain_expired_timers(&mut self) {
+        // The real `timer::All` lives in `bun_runtime` (cycle), so the body
+        // dispatches through `__bun_drain_expired_timers` (link-time extern).
+        let vm = self.vm();
+        // SAFETY: `vm` is the live owning VM; the definer no-ops when this
+        // thread has no `RuntimeState` (bun_jsc unit tests).
+        unsafe { __bun_drain_expired_timers(vm) };
+    }
+
     pub fn tick_concurrent_with_count(&mut self) -> usize {
         self.update_counts();
 

src/jsc/web_worker.rs

@@ -1132,6 +1132,19 @@ impl WebWorker {
             let _ = vm.global().vm().run_gc(false);
         }
 
+        // `uv_run` opens with `uv__run_timers`, so a timer that expired while the
+        // worker's entrypoint ran synchronously is due before the `setImmediate`
+        // callbacks it queued alongside it. Only `setImmediate` is reordered
+        // here: a MessagePort or fs completion the body queued dispatches inside
+        // the `tick()` of the wait above, the same as `--preload`. The two guards
+        // match the `is_event_loop_alive()` check below, so the entrypoint never
+        // fires a timer on a path that otherwise exits: skip once `terminate()`
+        // has landed (firing a callback would re-enter JS with the termination
+        // exception pending), and skip when the body left an unhandled error.
+        if !self.has_requested_terminate() && vm.unhandled_error_counter == 0 {
+            vm.event_loop_mut().drain_expired_timers();
+        }
+
         // Always do a first tick so we call CppTask without delay after
         // dispatchOnline.
         vm.as_mut().tick();

src/runtime/dispatch.rs

@@ -894,6 +894,24 @@ pub(crate) unsafe fn __bun_run_wtf_timer(
     unsafe { crate::timer::WTFTimer::run(real, vm) }
 }
 
+/// `__bun_drain_expired_timers` body: run the timers phase over this thread's
+/// `timer::All`. No-op before `init_runtime_state` (bun_jsc unit tests).
+///
+/// # Safety
+/// `vm` is the live per-thread VM and stays live across the JS callbacks
+/// `drain_timers` fires.
+#[unsafe(no_mangle)]
+pub(crate) unsafe fn __bun_drain_expired_timers(vm: *mut bun_jsc::virtual_machine::VirtualMachine) {
+    let all = crate::jsc_hooks::timer_all();
+    if all.is_null() {
+        return;
+    }
+    // SAFETY: `all` is the live per-thread `All`; `drain_timers` forms
+    // short-lived `&mut` only around heap pop/peek, so no `&mut All` is held
+    // across `fire()` (which re-enters `All` via `runtime_state()`).
+    unsafe { (*all).drain_timers(vm.cast::<()>()) };
+}
+
 // ════════════════════════════════════════════════════════════════════════════
 // EventLoopTimer dispatch
 // ════════════════════════════════════════════════════════════════════════════

test/js/node/timers/node-timers.test.ts

@@ -1,6 +1,6 @@
 import jsc from "bun:jsc";
 import { describe, expect, it, mock, test } from "bun:test";
-import { bunEnv, bunExe, isWindows } from "harness";
+import { bunEnv, bunExe, isWindows, tempDir } from "harness";
 import path from "node:path";
 import { clearInterval, clearTimeout, promises, setImmediate, setInterval, setTimeout } from "node:timers";
 import { promisify } from "util";
@@ -245,3 +245,125 @@ describe.each(["with", "without"])("setImmediate %s timers running", mode => {
 it("should defer microtasks when an exception is thrown in an immediate", async () => {
   expect(["run", path.join(import.meta.dir, "timers-immediate-exception-fixture.js")]).toRun();
 });
+
+describe.concurrent("an already-expired timer runs before the first poll and check phases", () => {
+  // Block well past the 1ms deadline so the timer is unambiguously expired by
+  // the time the event loop is entered. libuv opens every `uv_run` iteration
+  // with the timers phase, so the timer runs before anything queued next to it.
+  const blockPastTheDeadline = `const start = Date.now(); while (Date.now() - start < 10) {}`;
+
+  test.concurrent.each([
+    [
+      "setImmediate",
+      `setTimeout(() => order.push("timeout"), 1);
+       setImmediate(() => order.push("immediate"));`,
+      ["timeout", "immediate"],
+    ],
+    [
+      "a MessagePort self-delivery",
+      `const { port1, port2 } = new MessageChannel();
+       port1.onmessage = () => { order.push("message"); port1.close(); port2.close(); };
+       port2.postMessage(1);
+       setTimeout(() => order.push("timeout"), 1);`,
+      ["timeout", "message"],
+    ],
+    [
+      "completed fs I/O",
+      `require("fs").readFile(__filename, () => order.push("readFile"));
+       setTimeout(() => order.push("timeout"), 1);`,
+      ["timeout", "readFile"],
+    ],
+  ])("before %s", async (_name, scheduled, expected) => {
+    using dir = tempDir("timers-phase-order", {
+      "index.js": `
+        const order = [];
+        process.on("exit", () => console.log(JSON.stringify(order)));
+        ${scheduled}
+        ${blockPastTheDeadline}
+      `,
+    });
+
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "index.js"],
+      cwd: String(dir),
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    if (exitCode !== 0) {
+      expect(stderr).toBe("");
+    }
+    expect(stdout.trim()).toBe(JSON.stringify(expected));
+    expect(exitCode).toBe(0);
+  });
+
+  test("inside a worker", async () => {
+    const worker = new Worker(new URL("timers-phase-order-worker-fixture.js", import.meta.url).href);
+    const { promise, resolve, reject } = Promise.withResolvers<string[]>();
+    worker.onmessage = event => resolve(event.data);
+    worker.onerror = reject;
+
+    try {
+      expect(await promise).toEqual(["timeout", "immediate"]);
+    } finally {
+      await worker.terminate();
+    }
+  });
+
+  test("inside a test file, same as under `bun run`", async () => {
+    using dir = tempDir("timers-phase-order-test", {
+      "order.test.js": `
+        const { test, expect } = require("bun:test");
+        const order = [];
+        setTimeout(() => order.push("timeout"), 1);
+        setImmediate(() => order.push("immediate"));
+        ${blockPastTheDeadline}
+        test("the expired timer ran first", () => {
+          console.log("order=" + JSON.stringify(order));
+          expect(order).toEqual(["timeout", "immediate"]);
+        });
+      `,
+    });
+
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "test", "order.test.js"],
+      cwd: String(dir),
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    if (exitCode !== 0) {
+      expect(stderr).toBe("");
+    }
+    expect(stdout).toContain(`order=${JSON.stringify(["timeout", "immediate"])}`);
+    expect(exitCode).toBe(0);
+  });
+
+  // An entry point that throws rejects the module promise and exits without
+  // entering the loop, as Node does; the timer never fires.
+  test.concurrent("but not when the entry point throws", async () => {
+    using dir = tempDir("timers-phase-order-throw", {
+      "index.js": `
+        setTimeout(() => console.log("timer fired"), 1);
+        ${blockPastTheDeadline}
+        throw new Error("boom");
+      `,
+    });
+
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "index.js"],
+      cwd: String(dir),
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stderr).toContain("boom");
+    expect({ stdout, exitCode }).toEqual({ stdout: "", exitCode: 1 });
+  });
+});