Write app glance in wakeup_schedule_simple_robust to ensure time is accurate

src/c/scheduler.c

@@ -0,0 +1,44 @@
+#include <pebble.h>
+
+static void glance_reload_callback(AppGlanceReloadSession *session, size_t limit, void *target) {
+	static char s_glance_buffer[22];
+	time_t alarm_time = (time_t)(uintptr_t)target;
+	struct tm wakeup_tm = *localtime(&alarm_time);
+	snprintf(s_glance_buffer, sizeof(s_glance_buffer), "%02u:%02u:%02u, %02u/%02u", wakeup_tm.tm_hour, wakeup_tm.tm_min, wakeup_tm.tm_sec, wakeup_tm.tm_mon, wakeup_tm.tm_mday);
+	AppGlanceSlice slice = {
+	    .expiration_time = (time_t)(uintptr_t)target,
+	    .layout.icon = APP_GLANCE_SLICE_DEFAULT_ICON,
+	    .layout.subtitle_template_string = s_glance_buffer,
+	};
+	app_glance_add_slice(session, slice);
+}
+
+// Mostly stolen from https://github.com/SeaPea/GentleWake;
+// A slightly more robust wakeup scheduler that retries on certain errors and
+// can retry for a different wakeup time offset by retry_diff up to retry_max times
+WakeupId wakeup_schedule_simple_robust(time_t wakeup_time, int8_t retry_diff, bool is_fallback) {
+	WakeupId result = 0;
+	uint8_t try_count = 0;
+
+	result = wakeup_schedule(wakeup_time, is_fallback, true);
+
+	if (result < 0) {
+		// If result is negative, something went wrong, so make sure all wakeups for this app are cancelled and try again
+		wakeup_cancel_all();
+
+		result = wakeup_schedule(wakeup_time, is_fallback, true);
+
+		if (result == E_RANGE) {
+			// E_RANGE means some other app has the same wakeup time +/- 1 minute, so try to set
+			// the wakeup for a time up to (retry_diff * 8) minutes before/after.
+			while (result == E_RANGE && try_count++ < 8) {
+				wakeup_time += retry_diff;
+				result = wakeup_schedule(wakeup_time, is_fallback, true);
+			}
+		}
+	} else {
+		app_glance_reload(glance_reload_callback, (void *)(uintptr_t)wakeup_time);
+	}
+
+	return result;
+}