fix new_deadline_info_from_offset_and_epoch (#1321)

* fix new_deadline_info_from_offset_and_epoch

* remove useless `% policy.wpost_period_deadlines`

* add comment to wpost_challenge_window

Author: zhiqiangxu

actors/miner/src/deadlines.rs

@@ -138,8 +138,7 @@ pub fn new_deadline_info_from_offset_and_epoch(
 ) -> DeadlineInfo {
     let q = QuantSpec { unit: policy.wpost_proving_period, offset: period_start_seed };
     let current_period_start = q.quantize_down(current_epoch);
-    let current_deadline_idx = ((current_epoch - current_period_start)
-        / policy.wpost_challenge_window) as u64
-        % policy.wpost_period_deadlines;
+    let current_deadline_idx =
+        ((current_epoch - current_period_start) / policy.wpost_challenge_window) as u64;
     new_deadline_info(policy, current_period_start, current_deadline_idx, current_epoch)
 }

runtime/src/runtime/policy.rs

@@ -34,6 +34,7 @@ pub struct Policy {
     /// The period over which all a miner's active sectors will be challenged.
     pub wpost_proving_period: ChainEpoch,
     /// The duration of a deadline's challenge window, the period before a deadline when the challenge is available.
+    /// Notice that the challenge window is assumed to have the same duration as a deadline itself both in FIP and implementation.
     pub wpost_challenge_window: ChainEpoch,
     /// The number of non-overlapping PoSt deadlines in each proving period.
     pub wpost_period_deadlines: u64,