test(#804): reproduce unknown installer argument

packages/mosaic/src/commands/install-arguments.spec.ts

@@ -0,0 +1,41 @@
+import { spawnSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { chmod, mkdir, mkdtemp, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { describe, expect, it } from 'vitest';
+
+const INSTALLER_PATH = fileURLToPath(new URL('../../../../tools/install.sh', import.meta.url));
+
+describe('installer arguments', () => {
+  it('rejects an unknown argument before installation starts', async () => {
+    const home = await mkdtemp(join(tmpdir(), 'mosaic-installer-args-'));
+    const bin = join(home, 'bin');
+    const npmMarker = join(home, 'npm-called');
+
+    try {
+      await mkdir(bin);
+      const npmShim = join(bin, 'npm');
+      await writeFile(npmShim, `#!/bin/sh\n: > "${npmMarker}"\n`);
+      await chmod(npmShim, 0o755);
+
+      const result = spawnSync('bash', [INSTALLER_PATH, '--cli', '--bogus'], {
+        encoding: 'utf8',
+        env: {
+          ...process.env,
+          HOME: home,
+          MOSAIC_NO_COLOR: '1',
+          PATH: `${bin}:${process.env.PATH ?? ''}`,
+        },
+      });
+
+      expect(result.status).not.toBe(0);
+      expect(result.stderr).toContain('Unknown argument: --bogus');
+      expect(result.stderr).toMatch(/Usage: .*install\.sh/);
+      expect(existsSync(npmMarker)).toBe(false);
+    } finally {
+      await rm(home, { recursive: true, force: true });
+    }
+  });
+});