test: add rpccookieperms test

* tests one perm with and one without special bit set

test/functional/rpc_users.py

@@ -11,6 +11,7 @@
 )
 
 import http.client
+import os
 import urllib.parse
 import subprocess
 from random import SystemRandom
@@ -84,6 +85,17 @@ def test_auth(self, node, user, password):
         self.log.info('Wrong...')
         assert_equal(401, call_with_auth(node, user + 'wrong', password + 'wrong').status)
 
+    def test_rpccookieperms(self, perm: str):
+        self.restart_node(1, extra_args=[f"-rpccookieperms={perm}"])
+
+        # Pad perm to 4 chars for comparison as we always include special bits in test
+        perm = perm.zfill(4)
+
+        cookie_file_path = self.nodes[1].chain_path / '.cookie'
+        file_stat = os.stat(cookie_file_path)
+        actual_perms = f"{(file_stat.st_mode & 0o7777):04o}"
+        assert_equal(perm, actual_perms)
+
     def run_test(self):
         self.conf_setup()
         self.log.info('Check correctness of the rpcauth config option')
@@ -115,6 +127,18 @@ def run_test(self):
         (self.nodes[0].chain_path / ".cookie.tmp").mkdir()
         self.nodes[0].assert_start_raises_init_error(expected_msg=init_error)
 
+        self.log.info('Check that cookie file permissions can be set using -rpccookieperms')
+        # Remove any leftover rpc{user|password} config options
+        conf = self.nodes[1].bitcoinconf
+        with conf.open('r') as file:
+            lines = file.readlines()
+        filtered_lines = [line for line in lines if not line.startswith('rpcuser') and not line.startswith('rpcpassword')]
+        with conf.open('w') as file:
+            file.writelines(filtered_lines)
+
+        custom_cookie_perms = ["0640", "666", "1666"]
+        [self.test_rpccookieperms(perm) for perm in custom_cookie_perms]
+
 
 if __name__ == '__main__':
     HTTPBasicsTest().main()