test(core): add FSEvents stress diagnostic

packages/core/test/filesystem/watcher-stress.test.ts

@@ -0,0 +1,46 @@
+import watcher from "@parcel/watcher"
+import { describe, expect, test } from "bun:test"
+import fs from "node:fs/promises"
+import os from "node:os"
+import path from "node:path"
+
+const backend = process.platform === "darwin" ? "fs-events" : process.platform === "linux" ? "inotify" : undefined
+const describeNative = backend ? describe : describe.skip
+
+async function descriptors() {
+  if (process.platform === "linux") return fs.readdir("/proc/self/fd").then((entries) => entries.length)
+  return Number(Bun.spawnSync(["sh", "-c", `lsof -p ${process.pid} 2>/dev/null | wc -l`]).stdout.toString().trim())
+}
+
+describeNative("native watcher stress diagnostic", () => {
+  test(
+    "releases native descriptors after repeated subscription churn",
+    async () => {
+      const root = await fs.mkdtemp(path.join(os.tmpdir(), "opencode-watcher-stress-"))
+      const targets = Array.from({ length: 20 }, (_, index) => path.join(root, `project-${index}`))
+      await Promise.all(targets.map((target) => fs.mkdir(target)))
+      const before = await descriptors()
+
+      try {
+        for (let round = 0; round < 100; round++) {
+          const subscriptions = await Promise.all(
+            targets.map((target) => watcher.subscribe(target, () => {}, { backend })),
+          )
+          await Promise.all(
+            targets.map((target, index) => fs.writeFile(path.join(target, "event.txt"), `${round}-${index}`)),
+          )
+          await Promise.all(subscriptions.map((subscription) => subscription.unsubscribe()))
+        }
+
+        Bun.gc(true)
+        await Bun.sleep(250)
+        const after = await descriptors()
+        console.log(JSON.stringify({ subscriptions: targets.length * 100, before, after }))
+        expect(after - before).toBeLessThanOrEqual(4)
+      } finally {
+        await fs.rm(root, { recursive: true, force: true })
+      }
+    },
+    60_000,
+  )
+})