Change time Driver contract to never fire the alarm synchronously

embassy-time/src/driver.rs

@@ -105,20 +105,21 @@ pub trait Driver: Send + Sync + 'static {
     /// Sets an alarm at the given timestamp. When the current timestamp reaches the alarm
     /// timestamp, the provided callback function will be called.
     ///
-    /// If `timestamp` is already in the past, the alarm callback must be immediately fired.
-    /// In this case, it is allowed (but not mandatory) to call the alarm callback synchronously from `set_alarm`.
+    /// The `Driver` implementation should guarantee that the alarm callback is never called synchronously from `set_alarm`.
+    /// Rather - if `timestamp` is already in the past - `false` should be returned and alarm should not be set,
+    /// or alternatively, the driver should return `true` and arrange to call the alarm callback as soon as possible, but not synchronously.
     ///
     /// When callback is called, it is guaranteed that now() will return a value greater or equal than timestamp.
     ///
     /// Only one alarm can be active at a time for each AlarmHandle. This overwrites any previously-set alarm if any.
-    fn set_alarm(&self, alarm: AlarmHandle, timestamp: u64);
+    fn set_alarm(&self, alarm: AlarmHandle, timestamp: u64) -> bool;
 }
 
 extern "Rust" {
     fn _embassy_time_now() -> u64;
     fn _embassy_time_allocate_alarm() -> Option<AlarmHandle>;
     fn _embassy_time_set_alarm_callback(alarm: AlarmHandle, callback: fn(*mut ()), ctx: *mut ());
-    fn _embassy_time_set_alarm(alarm: AlarmHandle, timestamp: u64);
+    fn _embassy_time_set_alarm(alarm: AlarmHandle, timestamp: u64) -> bool;
 }
 
 /// See [`Driver::now`]
@@ -139,7 +140,7 @@ pub fn set_alarm_callback(alarm: AlarmHandle, callback: fn(*mut ()), ctx: *mut (
 }
 
 /// See [`Driver::set_alarm`]
-pub fn set_alarm(alarm: AlarmHandle, timestamp: u64) {
+pub fn set_alarm(alarm: AlarmHandle, timestamp: u64) -> bool {
     unsafe { _embassy_time_set_alarm(alarm, timestamp) }
 }
 
@@ -167,7 +168,7 @@ macro_rules! time_driver_impl {
         }
 
         #[no_mangle]
-        fn _embassy_time_set_alarm(alarm: $crate::driver::AlarmHandle, timestamp: u64) {
+        fn _embassy_time_set_alarm(alarm: $crate::driver::AlarmHandle, timestamp: u64) -> bool {
             <$t as $crate::driver::Driver>::set_alarm(&$name, alarm, timestamp)
         }
     };