extract `nextLightClientSyncTask` helper (#5108)

The `nextLightClientSyncTask` helper is generic enough to be useful for
contexts besides libp2p based light client sync. Extract it.

beacon_chain/sync/light_client_manager.nim

@@ -393,39 +393,29 @@ proc loop(self: LightClientManager) {.async.} =
       continue
 
     # Fetch updates
-    var allowWaitNextPeriod = false
     let
-      finalized = self.getFinalizedPeriod()
-      optimistic = self.getOptimisticPeriod()
       current = wallTime.slotOrZero().sync_committee_period
-      isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown()
+
+      syncTask = nextLightClientSyncTask(
+        finalized = self.getFinalizedPeriod(),
+        optimistic = self.getOptimisticPeriod(),
+        current = current,
+        isNextSyncCommitteeKnown = self.isNextSyncCommitteeKnown())
 
       didProgress =
-        if finalized == optimistic and not isNextSyncCommitteeKnown:
-          if finalized >= current:
-            await self.query(UpdatesByRange, (
-              startPeriod: finalized,
-              count: 1'u64))
-          else:
-            await self.query(UpdatesByRange, (
-              startPeriod: finalized,
-              count: min(current - finalized,
-                MAX_REQUEST_LIGHT_CLIENT_UPDATES)))
-        elif finalized + 1 < current:
-          await self.query(UpdatesByRange, (
-            startPeriod: finalized + 1,
-            count: min(current - (finalized + 1),
-              MAX_REQUEST_LIGHT_CLIENT_UPDATES)))
-        elif finalized != optimistic:
+        case syncTask.kind
+        of LcSyncKind.UpdatesByRange:
+          await self.query(UpdatesByRange,
+            (startPeriod: syncTask.startPeriod, count: syncTask.count))
+        of LcSyncKind.FinalityUpdate:
           await self.query(FinalityUpdate)
-        else:
-          allowWaitNextPeriod = true
+        of LcSyncKind.OptimisticUpdate:
           await self.query(OptimisticUpdate)
 
       schedulingMode =
         if not didProgress or not self.isGossipSupported(current):
           Soon
-        elif not allowWaitNextPeriod:
+        elif syncTask.kind != LcSyncKind.OptimisticUpdate:
           CurrentPeriod
         else:
           NextPeriod

beacon_chain/sync/light_client_sync_helpers.nim

@@ -48,3 +48,43 @@ func checkLightClientUpdates*(
       else:
         return err("Invalid context bytes")
   ok()
+
+type
+  LcSyncKind* {.pure.} = enum
+    UpdatesByRange
+    FinalityUpdate
+    OptimisticUpdate
+
+  LcSyncTask* = object
+    case kind*: LcSyncKind
+    of LcSyncKind.UpdatesByRange:
+      startPeriod*: SyncCommitteePeriod
+      count*: uint64
+    of LcSyncKind.FinalityUpdate, LcSyncKind.OptimisticUpdate:
+      discard
+
+func nextLightClientSyncTask*(
+    finalized: SyncCommitteePeriod,
+    optimistic: SyncCommitteePeriod,
+    current: SyncCommitteePeriod,
+    isNextSyncCommitteeKnown: bool): LcSyncTask =
+  if finalized == optimistic and not isNextSyncCommitteeKnown:
+    if finalized >= current:
+      LcSyncTask(
+        kind: LcSyncKind.UpdatesByRange,
+        startPeriod: finalized,
+        count: 1)
+    else:
+      LcSyncTask(
+        kind: LcSyncKind.UpdatesByRange,
+        startPeriod: finalized,
+        count: min(current - finalized, MAX_REQUEST_LIGHT_CLIENT_UPDATES))
+  elif finalized + 1 < current:
+    LcSyncTask(
+      kind: LcSyncKind.UpdatesByRange,
+      startPeriod: finalized + 1,
+      count: min(current - (finalized + 1), MAX_REQUEST_LIGHT_CLIENT_UPDATES))
+  elif finalized != optimistic:
+    LcSyncTask(kind: LcSyncKind.FinalityUpdate)
+  else:
+    LcSyncTask(kind: LcSyncKind.OptimisticUpdate)