bun test: don't park the event loop after a test file's entry promise settles

src/jsc/VirtualMachine.rs

@@ -4603,6 +4603,8 @@ impl VirtualMachine {
             self.wait_for_promise(jsc::AnyPromise::Internal(promise));
         }
 
+        // Pre-arm the waker so this settled-promise tick cannot park (#36450).
+        self.wakeup();
         self.auto_tick();
         Ok(self.pending_internal_promise.unwrap())
     }

test/regression/issue/36450.test.ts

@@ -0,0 +1,76 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+// https://github.com/oven-sh/bun/issues/36450
+// A pending ref'd timer left behind by one test file made every subsequent
+// test file that loads a module stall until the next JSC housekeeping timer
+// (~100ms): after the file's entry promise settled, the runner's final
+// blocking tick parked in the poller because the ref'd timer kept the loop
+// active. The stall sits between module evaluation and the first test
+// callback, so that gap is what we measure, paired against a control run
+// without the pending timer so machine speed cancels out.
+test("pending ref'd timer does not stall subsequent test files", async () => {
+  const fileCount = 6;
+  const files: Record<string, string> = {
+    "leak.test.ts": `
+      import { test, expect } from "bun:test";
+      test("leaves one pending ref'd timer", () => {
+        setTimeout(() => {}, 300_000);
+        globalThis.__timerArmed36450 = true;
+        expect(1).toBe(1);
+      });
+    `,
+  };
+  for (let i = 1; i <= fileCount; i++) {
+    // The import must be used, otherwise it is elided and no module loads.
+    files[`mod${i}.ts`] = `export const v = ${i};`;
+    files[`plain${i}.test.ts`] = `
+      import { test, expect } from "bun:test";
+      import { v } from "./mod${i}";
+      // All files share one process; prove the leak file ran first so the
+      // measured run really has a pending ref'd timer.
+      if (process.env.ISSUE_36450_EXPECT_TIMER === "1" && !globalThis.__timerArmed36450) {
+        throw new Error("expected leak.test.ts to arm its timer before this file");
+      }
+      const loadedAt = performance.now();
+      test("t${i}", () => {
+        console.log("GAP${i}:" + (performance.now() - loadedAt).toFixed(2));
+        expect(v).toBe(${i});
+      });
+    `;
+  }
+  using dir = tempDir("issue-36450", files);
+  const plainFiles = Array.from({ length: fileCount }, (_, i) => `plain${i + 1}.test.ts`);
+
+  async function medianGap(withLeak: boolean): Promise<number> {
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "test", ...(withLeak ? ["leak.test.ts"] : []), ...plainFiles],
+      env: { ...bunEnv, ISSUE_36450_EXPECT_TIMER: withLeak ? "1" : "0" },
+      cwd: String(dir),
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    const gaps: number[] = [];
+    for (const match of (stdout + stderr).matchAll(/GAP\d+:([\d.]+)/g)) {
+      gaps.push(Number(match[1]));
+    }
+    // Include stderr so a child failure (e.g. the fixture guard) prints its
+    // own error instead of just a short gap count.
+    expect({ gapCount: gaps.length, exitCode, stderr }).toEqual({
+      gapCount: fileCount,
+      exitCode: 0,
+      stderr: expect.any(String),
+    });
+    return gaps.toSorted((a, b) => a - b)[Math.floor(gaps.length / 2)];
+  }
+
+  const withoutTimer = await medianGap(false);
+  const withTimer = await medianGap(true);
+
+  // Unfixed, the pending timer pins every gap to the next JSC timer deadline
+  // (~15ms release, ~90ms debug on an idle machine); fixed, both runs behave
+  // identically.
+  expect(withTimer - withoutTimer).toBeLessThan(10);
+});