Properly handle execution time<->energy conversion in v8 host (#4884)

# Description of Changes

Fixes a todo.

# Expected complexity level and risk

1

# Testing

- [x] Checked that conversion ratio is sane.

crates/client-api-messages/src/energy.rs

@@ -16,13 +16,17 @@ pub struct EnergyQuanta {
 impl EnergyQuanta {
     pub const ZERO: Self = EnergyQuanta { quanta: 0 };
 
+    // per the comment on [`FunctionBudget::DEFAULT_BUDGET`]: 1 second of wasm runtime is roughtly 2 TeV
+    pub const PER_EXECUTION_SEC: Self = Self::new(2_000_000_000_000);
+    pub const PER_EXECUTION_NANOSEC: Self = Self::new(Self::PER_EXECUTION_SEC.get() / 1_000_000_000);
+
     #[inline]
-    pub fn new(quanta: u128) -> Self {
+    pub const fn new(quanta: u128) -> Self {
         Self { quanta }
     }
 
     #[inline]
-    pub fn get(&self) -> u128 {
+    pub const fn get(&self) -> u128 {
         self.quanta
     }
 

crates/core/src/host/v8/budget.rs

@@ -13,7 +13,7 @@ use core::ptr;
 use core::sync::atomic::Ordering;
 use core::time::Duration;
 use core::{ffi::c_void, sync::atomic::AtomicBool};
-use spacetimedb_client_api_messages::energy::FunctionBudget;
+use spacetimedb_client_api_messages::energy::{EnergyQuanta, FunctionBudget};
 use std::sync::Arc;
 use v8::{Isolate, IsolateHandle};
 
@@ -118,7 +118,12 @@ fn budget_to_duration(_budget: FunctionBudget) -> Duration {
 /// Returns [`EnergyStats`] for a reducer given its `budget`
 /// and the `duration` it took to execute.
 pub(super) fn energy_from_elapsed(budget: FunctionBudget, duration: Duration) -> EnergyStats {
-    let used = duration_to_budget(duration);
+    let used = duration.as_nanos() * EnergyQuanta::PER_EXECUTION_NANOSEC.get();
+    // in order for duration_nanos * ev_per_ns >= u64::MAX:
+    //              duration_nanos >= u64::MAX / ev_per_ns
+    //              duration_nanos >= (9223372036854775 ns = 106.75 days)
+    // so it's unlikely we'll have to worry about it
+    let used = FunctionBudget::new(u64::try_from(used).unwrap_or(u64::MAX));
     let remaining = budget - used;
     EnergyStats { budget, remaining }
 }