deploy: Fix NEO transfer deadlock caused by rounding integer division

Previously, auto-deploy/update procedure could stuck when committee
multi-sig account had less than N amount of NEO, where N is a number of
the NeoFS Alphabet accounts in the deployed/update NeoFS network.
This was caused by rounding integer division of fund amounts: zero funds
were transferred due to which the balance did not change and each
iteration did not change the network state.

Fix zero transfers and also distribute remainder as evenly as possible
to decrease total number of transactions.

Fixes #2681.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

pkg/morph/deploy/funds.go

@@ -414,16 +414,13 @@ func distributeNEOToAlphabetContracts(ctx context.Context, prm distributeNEOToAl
 		prm.logger.Info("have available NEO on the committee multi-sig account, going to transfer to the Alphabet contracts",
 			zap.Stringer("balance", bal))
 
-		singleAmount := new(big.Int).Div(bal, big.NewInt(int64(len(prm.alphabetContracts))))
-
 		scriptBuilder.Reset()
 
-		for i := range prm.alphabetContracts {
+		divideFundsEvenly(bal, len(prm.alphabetContracts), func(i int, amount *big.Int) {
 			prm.logger.Info("going to transfer NEO from the committee multi-sig account to the Alphabet contract",
-				zap.Stringer("contract", prm.alphabetContracts[i]), zap.Stringer("amount", singleAmount))
-			transfer(prm.alphabetContracts[i], singleAmount)
-			bal.Sub(bal, singleAmount)
-		}
+				zap.Stringer("contract", prm.alphabetContracts[i]), zap.Stringer("amount", amount))
+			transfer(prm.alphabetContracts[i], amount)
+		})
 
 		script, err := scriptBuilder.Script()
 		if err != nil {
@@ -448,3 +445,44 @@ func distributeNEOToAlphabetContracts(ctx context.Context, prm distributeNEOToAl
 		txMonitor.trackPendingTransactionsAsync(ctx, vub, mainTxID, fallbackTxID)
 	}
 }
+
+func divideFundsEvenly(fullAmount *big.Int, n int, f func(ind int, amount *big.Int)) {
+	if fullAmount.IsUint64() {
+		divideFundsEvenlyU64(fullAmount.Uint64(), n, func(ind int, amount uint64) {
+			f(ind, new(big.Int).SetUint64(amount))
+		})
+		return
+	}
+
+	quot := new(big.Int).Div(fullAmount, big.NewInt(int64(n)))
+	rem := new(big.Int).Mod(fullAmount, big.NewInt(int64(n)))
+
+	for i := 0; i < n; i++ {
+		amount := new(big.Int).Set(quot)
+		if rem.Sign() > 0 {
+			amount.Add(amount, big.NewInt(1))
+			rem.Sub(rem, big.NewInt(1))
+		} else if amount.Sign() == 0 {
+			return
+		}
+
+		f(i, amount)
+	}
+}
+
+func divideFundsEvenlyU64(fullAmount uint64, n int, f func(ind int, amount uint64)) {
+	quot := fullAmount / uint64(n)
+	rem := fullAmount % uint64(n)
+
+	for i := 0; i < n; i++ {
+		amount := quot
+		if rem > 0 {
+			amount++
+			rem--
+		} else if amount == 0 {
+			return
+		}
+
+		f(i, amount)
+	}
+}

pkg/morph/deploy/funds_test.go

@@ -0,0 +1,83 @@
+package deploy
+
+import (
+	"math"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDivideFundsEvenly(t *testing.T) {
+	t.Run("zero", func(t *testing.T) {
+		var vals []*big.Int
+
+		divideFundsEvenly(big.NewInt(0), 5, func(ind int, amount *big.Int) {
+			vals = append(vals, amount)
+		})
+		require.Empty(t, vals)
+	})
+
+	t.Run("less than N", func(t *testing.T) {
+		var vals []*big.Int
+
+		divideFundsEvenly(big.NewInt(4), 5, func(ind int, amount *big.Int) {
+			vals = append(vals, amount)
+		})
+		require.Len(t, vals, 4)
+		for i := range vals {
+			require.Equal(t, big.NewInt(1), vals[i])
+		}
+	})
+
+	t.Run("multiple", func(t *testing.T) {
+		var vals []*big.Int
+
+		divideFundsEvenly(big.NewInt(15), 3, func(ind int, amount *big.Int) {
+			vals = append(vals, amount)
+		})
+		require.Len(t, vals, 3)
+		for i := range vals {
+			require.Equal(t, big.NewInt(5), vals[i])
+		}
+	})
+
+	t.Run("with remainder", func(t *testing.T) {
+		var vals []*big.Int
+
+		divideFundsEvenly(big.NewInt(16), 3, func(ind int, amount *big.Int) {
+			vals = append(vals, amount)
+		})
+		require.Len(t, vals, 3)
+		require.Equal(t, big.NewInt(6), vals[0])
+		require.Equal(t, big.NewInt(5), vals[1])
+		require.Equal(t, big.NewInt(5), vals[2])
+	})
+
+	t.Run("bigger than uint64", func(t *testing.T) {
+		bigU64 := new(big.Int).SetUint64(math.MaxUint64)
+		fullAmount := new(big.Int).Mul(bigU64, big.NewInt(3))
+
+		var vals []*big.Int
+
+		divideFundsEvenly(fullAmount, 3, func(ind int, amount *big.Int) {
+			vals = append(vals, amount)
+		})
+		require.Len(t, vals, 3)
+		for i := range vals {
+			require.Equal(t, bigU64, vals[i])
+		}
+	})
+}
+
+func BenchmarkDivideFundsEvenly(b *testing.B) {
+	const n = 7
+	amount := big.NewInt(100*n + n/2)
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		divideFundsEvenly(amount, n, func(ind int, amount *big.Int) {})
+	}
+}