This directory is concerned with the management of Lacros's startup parameters.

Startup parameters are passed by Ash's BrowserManager to Lacros via memfds. The fd numbers are passed to Lacros via command line arguments defined in startup_switches.

Startup parameters belong to two classes: BrowserInitParams and BrowserPostLoginParams. That's because Lacros supports being pre-launched at login screen, when user-specific parameters are not known yet. Accessing startup parameters via BrowserPostLoginParams before login blocks, waiting for login.

BrowserInitParams and BrowserPostLoginParams are not to be used directly. Instead, Lacros code should use BrowserParamsProxy which dispatches to either BrowserInitParams and BrowserPostLoginParams based on whether Lacros was prelaunched at login screen or not.

BrowserInitParams in production vs. testing

In production, Lacros is invoked by Ash with the --crosapi-mojo-platform-channel-handle command line switch pointing to a file descriptor. This causes Lacros to populate BrowserInitParams with Ash-provided data read from the file. This happens whenever the BrowserInitParams are queried for the first time.

In testing where Lacros is not invoked by Ash (e.g. Lacros unit and browser tests) there are two scenarios:

1. Lacros is invoked without crosapi command line switch. Then BrowserInitParams is created empty. This is the case for example for unit_tests and browser_tests. Crosapi is thereby effectively disabled.

2. Lacros is invoked with crosapi command line switch. Then BrowserInitParams are read from Ash just as in production unless there's a call to DisableCrosapiForTesting, in which case the behavior follows that in (1). (An assertion ensures that DisableCrosapiForTesting is called before the first query of BrowserInitParams, if at all).

   Detail: For technical reasons, the command line switch here is not --crosapi-mojo-platform-channel-handle but --lacros-mojo-socket-for-testing. The test runner passes a socket through which BrowserTestBase then requests the --crosapi-mojo-platform-channel-handle file descriptor from Ash.

Only lacros_chrome_browsertests and Lacros interactive_ui_tests follow (2), with only very few uses of DisableCrosapiForTesting.