feat: add config read-modify-write update and harden file watcher

Config.update_config() applies submitted options to the main
configuration file via configparser read-modify-write: unknown
options are preserved, null removes an option, the merged view
of all files is validated as ServerConfig before anything is
written (a bad file would kill the FileWatcher polling loop),
and the write is atomic (.tmp + os.replace, mode 0600). Options
whose effective value is owned by a later configuration file
raise ConfigConflictError instead of writing a no-op. The
reload logic is factored into reload_and_notify() and the file
watcher callback is exception-guarded so polling never dies.

tests/test_config.py

@@ -17,9 +17,11 @@
 
 
 import configparser
+import os
 import pytest
 
 from gns3server.config import Config
+from gns3server.config import ConfigConflictError
 from gns3server.config import ServerConfig
 from pydantic import ValidationError
 
@@ -158,3 +160,120 @@ def test_vmware_settings(settings: dict, exception_expected: bool):
             ServerConfig(**vmware_settings)
     else:
         ServerConfig(**vmware_settings)
+
+
+def test_update_config_writes_ini_types(tmpdir):
+
+    path = str(tmpdir / "server.conf")
+    with open(path, "w+") as f:
+        f.write("# a comment\n[Server]\nhost = 127.0.0.1\nfrobnicate = 42\n")
+
+    config = Config(files=[path])
+    changed = config.update_config({
+        "Server": {
+            "port": 3081,
+            "report_errors": False,
+            "allowed_interfaces": ["eth0", "eth1"],
+            "default_symbol_theme": "Classic",
+            "additional_images_paths": ["/path/to/dir1", "/path/to/dir2"],
+        }
+    })
+
+    parsed = configparser.ConfigParser()
+    parsed.read(path)
+    assert parsed["Server"]["port"] == "3081"
+    assert parsed["Server"]["report_errors"] == "False"
+    assert parsed["Server"]["allowed_interfaces"] == "eth0,eth1"
+    assert parsed["Server"]["default_symbol_theme"] == "Classic"
+    assert parsed["Server"]["additional_images_paths"] == "/path/to/dir1;/path/to/dir2"
+    # options not submitted are left untouched, including unknown ones
+    assert parsed["Server"]["host"] == "127.0.0.1"
+    assert parsed["Server"]["frobnicate"] == "42"
+
+    # in-memory settings have been reloaded
+    assert config.settings.Server.port == 3081
+    assert config.settings.Server.report_errors is False
+    assert config.settings.Server.allowed_interfaces == ["eth0", "eth1"]
+    assert changed == [
+        "Server.additional_images_paths",
+        "Server.allowed_interfaces",
+        "Server.default_symbol_theme",
+        "Server.port",
+        "Server.report_errors",
+    ]
+
+
+def test_update_config_null_removes_option(tmpdir):
+
+    path = write_config(tmpdir, {"Server": {"host": "127.0.0.1"}})
+    config = Config(files=[path])
+
+    config.update_config({"Server": {"host": None}})
+
+    parsed = configparser.ConfigParser()
+    parsed.read(path)
+    assert not parsed.has_option("Server", "host")
+    assert config.settings.Server.host == "0.0.0.0"  # default restored
+
+
+def test_update_config_validation_failure_leaves_file_unchanged(tmpdir):
+
+    path = write_config(tmpdir, {"Server": {"console_start_port_range": "5000"}})
+    config = Config(files=[path])
+    with open(path) as f:
+        file_content_before = f.read()
+
+    # cross-field violation (console_end_port_range must be > console_start_port_range)
+    with pytest.raises(ValidationError):
+        config.update_config({"Server": {"console_start_port_range": 10000, "console_end_port_range": 5000}})
+
+    with open(path) as f:
+        assert f.read() == file_content_before
+
+
+def test_update_config_conflict(tmpdir):
+
+    main_path = write_config(tmpdir, {"Server": {"host": "127.0.0.1"}})
+    override_path = str(tmpdir / "override.conf")
+    with open(override_path, "w+") as f:
+        f.write("[Server]\nhost = 10.0.0.1\n")
+
+    config = Config(files=[main_path, override_path])
+    assert config.settings.Server.host == "10.0.0.1"  # later file takes precedence
+
+    with pytest.raises(ConfigConflictError):
+        config.update_config({"Server": {"host": "192.168.1.1"}})
+    with pytest.raises(ConfigConflictError):
+        config.update_config({"Server": {"host": None}})
+
+    with open(main_path) as f:
+        assert "host = 127.0.0.1" in f.read()  # main file untouched
+
+
+def test_update_config_creates_missing_main_file(tmpdir):
+
+    path = write_config(tmpdir, {"Server": {"host": "127.0.0.1"}})
+    config = Config(files=[path])
+    os.remove(path)
+
+    config.update_config({"Server": {"port": 3081}})
+
+    parsed = configparser.ConfigParser()
+    parsed.read(path)
+    assert parsed["Server"]["port"] == "3081"
+    assert not parsed.has_option("Server", "host")  # removed file means defaults
+
+
+def test_reload_and_notify(tmpdir):
+
+    path = write_config(tmpdir, {"Server": {"host": "127.0.0.1"}})
+    config = Config(files=[path])
+
+    notified = []
+    config.listen_for_config_changes(lambda: notified.append(True))
+
+    write_config(tmpdir, {"Server": {"host": "192.168.1.2"}})
+    config.reload_and_notify()
+
+    assert config.settings.Server.host == "192.168.1.2"
+    assert notified == [True]

tests/utils/test_file_watcher.py

@@ -66,3 +66,20 @@ async def test_file_watcher_list(tmpdir, strategy):
     file2.write("b")
     await asyncio.sleep(0.5)
     callback.assert_called_with(str(file2))
+
+
+@pytest.mark.asyncio
+async def test_file_watcher_callback_exception_does_not_stop_polling(tmpdir):
+
+    file = tmpdir / "test"
+    file.write("a")
+    callback = MagicMock(side_effect=ValueError("callback error"))
+    FileWatcher(file, callback, delay=0.1)
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 0
+    file.write("b")
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 1  # raised, but must not kill the polling loop
+    file.write("c")
+    await asyncio.sleep(0.5)
+    assert callback.call_count == 2  # polling survived the exception