Please describe the problem:
When started without session related command line parameters (e.g. from gnome-session), metacity picks up client_id from the DESKTOP_AUTOSTART_ID environment variable. Unfortunately, there is no way to distinguish if this was passed from a config file, representing old saved session, or generated by gnome-session, therefore load_state is attempted in each case. If the client_id is generated, there will be no session file, and metacity will report a warning. Since use case of starting new fresh session is much more common than loading existing saved session, most users will find a warning in their session log file (e.g. .xsession-errors), which can be fairly confusing. Warning should be either removed or changed to a debug message. Alternatively, there should be a method to distinguish between generated and restored IDs, but that might require changes to the autostart specs.

Steps to reproduce:
Log into gnome desktop without any saved sessions.

Actual results:
metacity reports warning:
Window manager warning: Failed to read saved session file <HOME>/.config/metacity/sessions/<ID>.ms: Failed to open file '<HOME>/.config/metacity/se
ssions/<ID>.ms': No such file or directory

Expected results:
No warning.

Does this happen every time?
Yes

Other information: