test: add coverage for loading a wallet in a non-writable directory

Previously, wallets in non-writable directories were loaded,
leading to crashes on any subsequent write.

test/functional/wallet_startup.py

@@ -6,12 +6,17 @@
 
 Verify that a bitcoind node can maintain list of wallets loading on startup
 """
+import os
 import shutil
+import stat
 import uuid
 
+
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
+    assert_raises_rpc_error,
+    is_dir_writable,
 )
 
 
@@ -38,6 +43,27 @@ class WalletStartupTest(BitcoinTestFramework):
         shutil.move(self.nodes[0].wallets_path / wallet_name / "wallet.dat", self.nodes[0].wallets_path / "wallet.dat")
         (self.nodes[0].wallets_path / wallet_name).rmdir()
 
+    def test_load_unwritable_wallet(self, node):
+        self.log.info("Test wallet load failure due to non-writable directory")
+        wallet_name = "bad_permissions"
+
+        node.createwallet(wallet_name)
+        node.unloadwallet(wallet_name)
+
+        dir_path = node.wallets_path / wallet_name
+        original_dir_perms = dir_path.stat().st_mode
+        os.chmod(dir_path, original_dir_perms & ~(stat.S_IWUSR | stat.S_IWGRP | stat.S_IWOTH))
+
+        if is_dir_writable(dir_path):
+            self.log.warning("Skipping load non-writable directory test: unable to enforce read-only permissions")
+        else:
+            # Ensure we don't load a wallet located in a non-writable directory.
+            # The node will crash later on if we cannot write to disk.
+            assert_raises_rpc_error(-4, f"SQLiteDatabase: Failed to open database in directory '{str(dir_path)}': directory is not writable", node.loadwallet, wallet_name)
+
+        # Reset directory permissions for cleanup
+        dir_path.chmod(original_dir_perms)
+
     def run_test(self):
         self.log.info('Should start without any wallets')
         assert_equal(self.nodes[0].listwallets(), [])
@@ -67,5 +93,7 @@ class WalletStartupTest(BitcoinTestFramework):
         self.restart_node(0)
         assert_equal(set(self.nodes[0].listwallets()), set(('w2', 'w3')))
 
+        self.test_load_unwritable_wallet(self.nodes[0])
+
 if __name__ == '__main__':
     WalletStartupTest(__file__).main()