install: remove the same-filesystem probe from $TMPDIR when the rename into the cache fails

src/install/PackageManager/PackageManagerDirectories.rs

@@ -242,6 +242,8 @@ fn get_temporary_directory_run(manager: &mut PackageManager) -> TemporaryDirecto
         match sys::renameat_z(tempdir.fd(), tmpname, cache_directory.fd(), tmpname) {
             Ok(()) => {}
             Err(err) => {
+                // The rename failed, so the probe is still sitting in `tempdir`.
+                let _ = tempdir.delete_file_z(tmpname);
                 if !tried_dot_tmp {
                     tried_dot_tmp = true;
                     tempdir = match cache_directory.make_open_path(b".tmp", Default::default()) {

test/cli/install/bun-install-tmpdir-exdev.test.ts

@@ -0,0 +1,67 @@
+// `bun install` checks whether $TMPDIR and the install cache are on the same
+// filesystem by creating an empty `.<hex>-N.hm` probe in $TMPDIR and
+// renaming it into the cache. When the rename fails with EXDEV, bun falls
+// back to `<cache>/.tmp`; the probe it created in $TMPDIR must not be left
+// behind. Forcing EXDEV needs a writable mount on a different device from
+// the harness temp dir (/dev/shm on Linux).
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, isPosix, tempDir } from "harness";
+import { accessSync, constants, existsSync, mkdtempSync, readdirSync, rmSync, statSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+function findCrossDeviceDir(): string | undefined {
+  if (!isPosix) return undefined;
+  const refDev = statSync(tmpdir()).dev;
+  for (const candidate of ["/dev/shm", "/tmp"]) {
+    try {
+      if (statSync(candidate).dev === refDev) continue;
+      accessSync(candidate, constants.W_OK | constants.X_OK);
+      return candidate;
+    } catch {}
+  }
+  return undefined;
+}
+
+const crossDeviceDir = findCrossDeviceDir();
+
+test.skipIf(!crossDeviceDir)(
+  "leaves nothing in $TMPDIR when it is on a different filesystem than the cache",
+  async () => {
+    using dir = tempDir("install-tmpdir-exdev", {
+      "package.json": JSON.stringify({
+        name: "app",
+        dependencies: { localdep: "file:./localdep" },
+      }),
+      "localdep/package.json": JSON.stringify({ name: "localdep", version: "1.0.0" }),
+    });
+    const cache = join(String(dir), "cache");
+    const tmp = mkdtempSync(join(crossDeviceDir!, "bun-install-exdev-"));
+
+    try {
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), "install"],
+        cwd: String(dir),
+        env: {
+          ...bunEnv,
+          BUN_INSTALL_CACHE_DIR: cache,
+          BUN_TMPDIR: tmp,
+          TMPDIR: tmp,
+        },
+        stdout: "pipe",
+        stderr: "pipe",
+      });
+      const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+      expect(stderr).not.toContain("error:");
+      expect(stdout).toContain("1 package installed");
+      expect(exitCode).toBe(0);
+
+      // The cross-device rename failed, so bun switched to <cache>/.tmp ...
+      expect(existsSync(join(cache, ".tmp"))).toBe(true);
+      // ... and must have removed the probe it had created in $TMPDIR first.
+      expect(readdirSync(tmp)).toEqual([]);
+    } finally {
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  },
+);