Linux only allow single instance socket use by same user

kitty_tests/single_instance.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python
+# License: GPL v3 Copyright: 2026, Kovid Goyal <kovid at kovidgoyal.net>
+
+import json
+import os
+import socket
+import subprocess
+import sys
+from contextlib import closing
+
+from kitty.constants import kitty_exe
+
+from .base import BaseTest
+
+
+class TestSingleInstance(BaseTest):
+    def test_single_instance_client(self):
+        # Test that a client can talk to a single instance socket belonging to
+        # the same user. The abstract socket namespace used below exists only
+        # on Linux.
+        if not sys.platform.startswith('linux'):
+            self.skipTest('Abstract UNIX sockets are only available on Linux')
+        group = f'test-{os.getpid()}'
+        with closing(socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)) as server:
+            server.bind('\0' + f'kitty-ipc-{os.geteuid()}-{group}')
+            server.listen()
+            env = os.environ.copy()
+            env['KITTY_SINGLE_INSTANCE_TEST'] = 'xyz'
+            p = subprocess.Popen(
+                [kitty_exe(), '--single-instance', f'--instance-group={group}', '--title', 'si-test-title'],
+                env=env,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.STDOUT,
+            )
+            try:
+                with closing(server.accept()[0]) as peer:
+                    data = b''
+                    while True:
+                        d = peer.recv(8192)
+                        if not d:
+                            break
+                        data += d
+                output = p.communicate(timeout=30)[0].decode()
+            finally:
+                if p.poll() is None:
+                    p.kill()
+                    p.wait()
+            self.assertEqual(p.returncode, 0, f'kitty --single-instance failed with output: {output}')
+            msg = json.loads(data.decode('utf-8'))
+            self.assertEqual(msg['cmd'], 'new_instance')
+            self.assertIn('si-test-title', msg['args'])
+            self.assertEqual(msg['cwd'], os.getcwd())
+            self.assertEqual(msg['environ'].get('KITTY_SINGLE_INSTANCE_TEST'), 'xyz')