RPC: Delete cookie file before replacing it

Unclear if this is the best thing to do, but due to v26.1.knots20240325 creating it read-only, it is somewhat necessary for now

Author: luke-jr

src/rpc/request.cpp

@@ -126,6 +126,11 @@ bool GenerateAuthCookie(std::string* cookie_out, std::optional<fs::perms> cookie
     file.close();
 
     fs::path filepath = GetAuthCookieFile(false);
+    try {
+        fs::remove(filepath);
+    } catch (const fs::filesystem_error& e) {
+        // ignore
+    }
     if (!RenameOver(filepath_tmp, filepath)) {
         LogWarning("Unable to rename cookie authentication file %s to %s", fs::PathToString(filepath_tmp), fs::PathToString(filepath));
         return false;

test/functional/rpc_users.py

@@ -210,6 +210,18 @@ def run_test(self):
         assert cookie_path.exists()
         self.stop_node(0)
 
+        cookie_path.mkdir()
+        cookie_path_subdir = cookie_path / "subdir"
+        cookie_path_subdir.mkdir()
+        self.nodes[0].assert_start_raises_init_error(expected_msg=init_error)
+        cookie_path_subdir.rmdir()
+        cookie_path.rmdir()
+
+        self.log.info('Check that a non-writable cookie file will get replaced gracefully')
+        cookie_path.mkdir(mode=1)
+        self.restart_node(0)
+        self.stop_node(0)
+
         self.test_rpccookieperms()
 
         self.test_norpccookiefile(cookie_path)