rate: Avoid precision loss with edge cases

When burst is 1, there would be edge cases we get tokens of 0.9999999999997222
due to float64 multiplication/division rounding out value. So avoid those
calculation when 'last' is old enough

Fixes golang/go#46579

rate/rate.go

@@ -370,8 +370,8 @@ func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time,
 	// Avoid making delta overflow below when last is very old.
 	maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
 	elapsed := now.Sub(last)
-	if elapsed > maxElapsed {
-		elapsed = maxElapsed
+	if elapsed >= maxElapsed {
+		return now, last, float64(lim.burst)
 	}
 
 	// Calculate the new number of tokens, due to time that passed.

rate/rate_test.go

@@ -476,3 +476,11 @@ func BenchmarkWaitNNoDelay(b *testing.B) {
 		lim.WaitN(ctx, 1)
 	}
 }
+
+func TestPreciseAllow(t *testing.T) {
+	lim := NewLimiter(0.00027777777777777778, 1)
+	lim.tokens = 0.54990492004805558
+	if !lim.Allow() {
+		t.Errorf("want ok, got false")
+	}
+}