[effective] Guard median call site against zero length SlotList (#1900)

* [effective] Guard median call site against zero length SlotList

* [committee] Exit early if stakedSlotsCount is 0, committee just made up of hmy

shard/committee/assignment.go

@@ -149,6 +149,13 @@ func eposStakedCommittee(
 			nil,
 		})
 	}
+
+	if stakedSlotsCount == 0 {
+		utils.Logger().Info().Int("slots-for-epos", stakedSlotsCount).
+			Msg("committe composed only of harmony node")
+		return superComm, nil
+	}
+
 	staked := effective.Apply(essentials, stakedSlotsCount)
 	shardBig := big.NewInt(int64(shardCount))
 

staking/effective/calculate.go

@@ -66,6 +66,12 @@ func (s Slots) JSON() string {
 }
 
 func median(stakes []SlotPurchase) numeric.Dec {
+
+	if len(stakes) == 0 {
+		utils.Logger().Error().Int("non-zero", len(stakes)).
+			Msg("Input to median has len 0, check caller")
+	}
+
 	sort.SliceStable(
 		stakes,
 		func(i, j int) bool { return stakes[i].Dec.LTE(stakes[j].Dec) },
@@ -115,6 +121,11 @@ func Apply(shortHand map[common.Address]SlotOrder, pull int) Slots {
 		pull = l
 	}
 	picks := eposedSlots[:pull]
+
+	if len(picks) == 0 {
+		return Slots{}
+	}
+
 	median := median(picks)
 
 	for i := range picks {