Bun.serve: keep the event loop alive while a request is in flight

src/runtime/server/mod.rs

@@ -288,6 +288,8 @@ pub struct NewServer<const SSL: bool, const DEBUG: bool> {
     pub(crate) all_closed_promise: jsc::JSPromiseStrong,
 
     pub poll_ref: KeepAlive,
+    /// Ref'd while `pending_requests > 0` so `server.unref()` doesn't drop in-flight requests.
+    in_flight_keep_alive: KeepAlive,
 
     pub(crate) flags: ServerFlags,
 
@@ -486,6 +488,11 @@ impl<const SSL: bool, const DEBUG: bool> NewServer<SSL, DEBUG> {
     }
 
     pub(crate) fn on_pending_request(&mut self) {
+        if self.pending_requests == 0 {
+            // SAFETY: `self.vm` is the live per-thread VM singleton backref.
+            self.in_flight_keep_alive
+                .ref_(unsafe { jsc::VirtualMachine::event_loop_ctx(self.vm.as_ptr()) });
+        }
         self.pending_requests += 1;
     }
 
@@ -1486,6 +1493,11 @@ impl<const SSL: bool, const DEBUG: bool> NewServer<SSL, DEBUG> {
     #[inline]
     pub(crate) fn on_request_complete(&mut self) {
         self.pending_requests -= 1;
+        if self.pending_requests == 0 {
+            // SAFETY: `self.vm` is the live per-thread VM singleton backref.
+            self.in_flight_keep_alive
+                .unref(unsafe { jsc::VirtualMachine::event_loop_ctx(self.vm.as_ptr()) });
+        }
         self.deinit_if_we_can();
     }
 
@@ -1596,6 +1608,7 @@ impl<const SSL: bool, const DEBUG: bool> NewServer<SSL, DEBUG> {
                 self.unref();
                 self.notify_inspector_server_stopped();
                 if abrupt {
+                    self.in_flight_keep_alive.disable();
                     self.flags.insert(ServerFlags::TERMINATED);
                 }
             }
@@ -1604,6 +1617,9 @@ impl<const SSL: bool, const DEBUG: bool> NewServer<SSL, DEBUG> {
         if abrupt || (self.pending_requests == 0 && !self.has_active_web_sockets()) {
             self.unref();
         }
+        if abrupt {
+            self.in_flight_keep_alive.disable();
+        }
         // A graceful stop with work in flight keeps the ref (deinit_if_we_can
         // unrefs when the drain completes): on Windows uv_run skips I/O with
         // zero ref'd handles, so unrefing here wedged server.close() teardown.
@@ -2031,6 +2047,7 @@ impl<const SSL: bool, const DEBUG: bool> NewServer<SSL, DEBUG> {
             h3_request_pool: core::ptr::null_mut(),
             all_closed_promise: jsc::JSPromiseStrong::default(),
             poll_ref: KeepAlive::default(),
+            in_flight_keep_alive: KeepAlive::default(),
             flags: ServerFlags::default(),
             plugins: None,
             user_routes: Vec::new(),

src/runtime/server/server_body.rs

@@ -3422,7 +3422,7 @@ where
             resp.end_without_body(true);
             return;
         }
-        this.pending_requests += 1;
+        this.on_pending_request();
         req.set_yield(false);
         // SAFETY: `request_pool` is non-null while the server is alive; `claim()`
         // reserves a fresh slot whose `Drop` releases it on panic before init.

test/js/bun/http/bun-server.test.ts

@@ -497,6 +497,36 @@ test("unref keeps process alive for ongoing connections", async () => {
   expect(await bunRun(path.join(import.meta.dir, "unref-fixture-2.ts"))).toSpawn();
 });
 
+test("unref: an in-flight request keeps the process alive on its own", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), path.join(import.meta.dir, "unref-fixture-3.ts")],
+    env: bunEnv,
+    stdin: "ignore",
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const stderrPromise = proc.stderr.text();
+  const exitedPromise = proc.exited;
+
+  // Read the port line while the subprocess keeps running.
+  const reader = proc.stdout.getReader();
+  const decoder = new TextDecoder();
+  let buf = "";
+  while (!buf.includes("\n")) {
+    const { value, done } = await reader.read();
+    if (done) break;
+    buf += decoder.decode(value, { stream: true });
+  }
+  reader.releaseLock();
+  const port = Number(buf.trim());
+  expect(port).toBeGreaterThan(0);
+
+  const res = await fetch(`http://127.0.0.1:${port}/`);
+  const body = await res.text();
+  const [stderr, exitCode] = await Promise.all([stderrPromise, exitedPromise]);
+  expect({ body, stderr, exitCode }).toEqual({ body: "ok", stderr: "", exitCode: 0 });
+});
+
 test("Bun does not crash when given invalid config", async () => {
   await using server1 = Bun.serve({
     fetch(request, server) {