extract LC sync task scheduling helper function (#5117)

The helper function to compute delay until next light client sync task
can be useful from more general purpose contexts. Move to helpers, and
change it to return `Duration` instead of `BeaconTime` for flexibility.

beacon_chain/sync/light_client_manager.nim

@@ -9,7 +9,6 @@
 
 import chronos, chronicles
 import
-  eth/p2p/discoveryv5/random2,
   ../spec/network,
   ../networking/eth2_network,
   ../beacon_clock,
@@ -107,13 +106,9 @@ proc isGossipSupported*(
   if not self.isLightClientStoreInitialized():
     return false
 
-  let
-    finalizedPeriod = self.getFinalizedPeriod()
-    isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown()
-  if isNextSyncCommitteeKnown:
-    period <= finalizedPeriod + 1
-  else:
-    period <= finalizedPeriod
+  period.isGossipSupported(
+    finalizedPeriod = self.getFinalizedPeriod(),
+    isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown())
 
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-alpha.3/specs/altair/light-client/p2p-interface.md#getlightclientbootstrap
 proc doRequest(
@@ -340,45 +335,13 @@ template query[E](
 ): Future[bool] =
   self.query(e, Nothing())
 
-type SchedulingMode = enum
-  Now,
-  Soon,
-  CurrentPeriod,
-  NextPeriod
-
-func fetchTime(
-    self: LightClientManager,
-    wallTime: BeaconTime,
-    schedulingMode: SchedulingMode
-): BeaconTime =
-  let
-    remainingTime =
-      case schedulingMode:
-      of Now:
-        return wallTime
-      of Soon:
-        chronos.seconds(0)
-      of CurrentPeriod:
-        let
-          wallPeriod = wallTime.slotOrZero().sync_committee_period
-          deadlineSlot = (wallPeriod + 1).start_slot - 1
-          deadline = deadlineSlot.start_beacon_time()
-        chronos.nanoseconds((deadline - wallTime).nanoseconds)
-      of NextPeriod:
-        chronos.seconds(
-          (SLOTS_PER_SYNC_COMMITTEE_PERIOD * SECONDS_PER_SLOT).int64)
-    minDelay = max(remainingTime div 8, chronos.seconds(10))
-    jitterSeconds = (minDelay * 2).seconds
-    jitterDelay = chronos.seconds(self.rng[].rand(jitterSeconds).int64)
-  return wallTime + minDelay + jitterDelay
-
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-alpha.3/specs/altair/light-client/light-client.md#light-client-sync-process
 proc loop(self: LightClientManager) {.async.} =
-  var nextFetchTime = self.getBeaconTime()
+  var nextSyncTaskTime = self.getBeaconTime()
   while true:
     # Periodically wake and check for changes
     let wallTime = self.getBeaconTime()
-    if wallTime < nextFetchTime or
+    if wallTime < nextSyncTaskTime or
         self.network.peerPool.lenAvailable < 1:
       await sleepAsync(chronos.seconds(2))
       continue
@@ -391,18 +354,21 @@ proc loop(self: LightClientManager) {.async.} =
         continue
 
       let didProgress = await self.query(Bootstrap, trustedBlockRoot.get)
-      if not didProgress:
-        nextFetchTime = self.fetchTime(wallTime, Soon)
+      nextSyncTaskTime =
+        if didProgress:
+          wallTime
+        else:
+          wallTime + self.rng.computeDelayWithJitter(chronos.seconds(0))
       continue
 
     # Fetch updates
     let
       current = wallTime.slotOrZero().sync_committee_period
 
       syncTask = nextLightClientSyncTask(
+        current = current,
         finalized = self.getFinalizedPeriod(),
         optimistic = self.getOptimisticPeriod(),
-        current = current,
         isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown())
 
       didProgress =
@@ -415,18 +381,12 @@ proc loop(self: LightClientManager) {.async.} =
         of LcSyncKind.OptimisticUpdate:
           await self.query(OptimisticUpdate)
 
-      schedulingMode =
-        if not self.isGossipSupported(current):
-          if didProgress:
-            Now
-          else:
-            Soon
-        elif self.getFinalizedPeriod() != self.getOptimisticPeriod():
-          CurrentPeriod
-        else:
-          NextPeriod
-
-    nextFetchTime = self.fetchTime(wallTime, schedulingMode)
+    nextSyncTaskTime = wallTime + self.rng.nextLcSyncTaskDelay(
+      wallTime,
+      finalized = self.getFinalizedPeriod(),
+      optimistic = self.getOptimisticPeriod(),
+      isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown(),
+      didLatestSyncTaskProgress = didProgress)
 
 proc start*(self: var LightClientManager) =
   ## Start light client manager's loop.

beacon_chain/sync/light_client_sync_helpers.nim

@@ -11,7 +11,9 @@ import
   std/typetraits,
   chronos,
   stew/base10,
-  ../spec/[forks_light_client, network]
+  eth/p2p/discoveryv5/random2,
+  ../spec/[forks_light_client, network],
+  ../beacon_clock
 
 func checkLightClientUpdates*(
     updates: openArray[ForkedLightClientUpdate],
@@ -49,6 +51,15 @@ func checkLightClientUpdates*(
         return err("Invalid context bytes")
   ok()
 
+func isGossipSupported*(
+    period: SyncCommitteePeriod,
+    finalizedPeriod: SyncCommitteePeriod,
+    isNextSyncCommitteeKnown: bool): bool =
+  if isNextSyncCommitteeKnown:
+    period <= finalizedPeriod + 1
+  else:
+    period <= finalizedPeriod
+
 type
   LcSyncKind* {.pure.} = enum
     UpdatesByRange
@@ -64,9 +75,9 @@ type
       discard
 
 func nextLightClientSyncTask*(
+    current: SyncCommitteePeriod,
     finalized: SyncCommitteePeriod,
     optimistic: SyncCommitteePeriod,
-    current: SyncCommitteePeriod,
     isNextSyncCommitteeKnown: bool): LcSyncTask =
   if finalized == optimistic and not isNextSyncCommitteeKnown:
     if finalized >= current:
@@ -88,3 +99,41 @@ func nextLightClientSyncTask*(
     LcSyncTask(kind: LcSyncKind.FinalityUpdate)
   else:
     LcSyncTask(kind: LcSyncKind.OptimisticUpdate)
+
+func computeDelayWithJitter*(
+    rng: ref HmacDrbgContext, duration: Duration): Duration =
+  let
+    minDelay = max(duration div 8, chronos.seconds(10))
+    jitterSeconds = (minDelay * 2).seconds
+    jitterDelay = chronos.seconds(rng[].rand(jitterSeconds).int64)
+  minDelay + jitterDelay
+
+func nextLcSyncTaskDelay*(
+    rng: ref HmacDrbgContext,
+    wallTime: BeaconTime,
+    finalized: SyncCommitteePeriod,
+    optimistic: SyncCommitteePeriod,
+    isNextSyncCommitteeKnown: bool,
+    didLatestSyncTaskProgress: bool
+): Duration =
+  let
+    current = wallTime.slotOrZero().sync_committee_period
+    remainingDuration =
+      if not current.isGossipSupported(finalized, isNextSyncCommitteeKnown):
+        if didLatestSyncTaskProgress:
+          # Now
+          return chronos.seconds(0)
+        # Soon
+        chronos.seconds(0)
+      elif finalized != optimistic:
+        # Current sync committee period
+        let
+          wallPeriod = wallTime.slotOrZero().sync_committee_period
+          deadlineSlot = (wallPeriod + 1).start_slot - 1
+          deadline = deadlineSlot.start_beacon_time()
+        chronos.nanoseconds((deadline - wallTime).nanoseconds)
+      else:
+        # Next sync committee period
+        chronos.seconds(
+          (SLOTS_PER_SYNC_COMMITTEE_PERIOD * SECONDS_PER_SLOT).int64)
+  rng.computeDelayWithJitter(remainingDuration)