Struct ConfigCell 

pub struct ConfigCell<T, const WAITERS: usize = { crate::DEFAULT_WAITERS }> { /* private fields */ }

Process-wide storage for one configuration type.

Lives in a static, which is the only place a device has to put anything that outlives a function:

static SETTINGS: ConfigCell<Settings> = ConfigCell::new();

Why a critical section

A reader has to see either the old configuration or the new one, never a mixture. On a host that is an ArcSwap; on a device without an allocator it is a few instructions with interrupts masked, which is what critical_section provides and what every embedded HAL implements.

The section is held for a clone of the value and nothing else. Keep the configuration struct small — which a device’s configuration is — and that is a memcpy with interrupts off, measured in microseconds.

Implementations

impl<T, const WAITERS: usize> ConfigCell<T, WAITERS>

pub const fn new() -> Self

An empty cell.

pub fn waiter_evictions(&self) -> u32

Available on crate feature async only.

How many times a waiter has had to displace another one, saturating.

A ConfigCell parks WAITERS tasks and no more. Past that, a registration evicts an existing waiter and wakes it — no wake-up is lost, but the two tasks then wake each other for as long as both are waiting, and a device that is doing that is not asleep. There is no fifth slot to find: a fixed array cannot park what does not fit, and the alternatives (drop the waker, refuse the registration) are both a task that nobody polls again.

So this is the report. Non-zero means WAITERS is too small for this firmware — raise the second type parameter to the number of tasks that genuinely await this configuration. Zero on a device that has run its real workload means the budget fits, which is the only proof of that worth having.

static SETTINGS: ConfigCell<Settings, 4> = ConfigCell::new();

// On a bench, after the firmware has run everything it does:
assert_eq!(SETTINGS.waiter_evictions(), 0, "raise WAITERS");

impl<T: Clone, const WAITERS: usize> ConfigCell<T, WAITERS>

pub fn store(&self, value: T)

Installs value, replacing whatever was there.

For compiled-in defaults at start-up, and for anything that builds a configuration without parsing one.

pub fn get(&self) -> Option<T>

The current configuration, or None before anything is stored.

Cloned out: there is no allocator, so there is no Arc to hand back. Call it once and reuse the value — two calls could straddle a store and let one piece of work observe two configurations.

pub fn is_set(&self) -> bool

Whether anything has been stored.

pub fn changes(&'static self) -> Changes<T, WAITERS>

Available on crate feature async only.

A handle that resolves each time the configuration is replaced.

impl<T: Clone + DeserializeOwned + Validate, const WAITERS: usize> ConfigCell<T, WAITERS>

pub fn apply(&self, document: &[u8], format: Format) -> Result<(), Error>

Parses document and installs it, if it is usable.

Everything that can fail happens before anything is installed: a document that does not parse, does not fit, or does not validate leaves the previous configuration serving. That is the whole reason this is one call rather than parse-then-store.

Errors

If the bytes are not valid in format, do not fit T, or T rejects itself.

Trait Implementations

impl<T> Debug for ConfigCell<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T, const WAITERS: usize> Default for ConfigCell<T, WAITERS>

fn default() -> Self

Returns the “default value” for a type.