cros_evaluate_seedThis directory is for code to evaluate field trial state early in boot on ChromeOS. All code here will be used in a single program, evaluate_seed. It is not for general use in ash (or lacros).
evaluate_seedThe executable evaluate_seed is a minimal program (budget: 2MiB of disk space for the executable itself) used early in ChromeOS boot to determine which group each early-boot experiment should be in, as well as any parameters for the experiment. It lives here so that it is trivial to keep the code in sync between ChromeOS's platform layer and chrome.
It will be built alongside ash.
It will be executed primarily by featured, which lives in //platform2/featured/.
evaluate_seed will write a serialized version of the computed state to stdout, along with a representation of the seed used for computation (for purposes of determining whether the seed can be marked as “safe”).
When the device starts up, featured will exec evaluate_seed, which will by default use whatever the latest seed in /home/chronos/Local State is. The state computed by evaluate_seed will be cached in a tmpfs until the next reboot, so even if ash later downloads and applies different seeds, evaluate_seed will not re-evaluate the seed until the next device reboot.
For disaster recovery, featured will determine whether to use the safe (or null) seed, and pass that information along with the value of the safe seed to use along to evaluate_seed.