Check enough funds on sender on TxProc &TxSel

- TxSelector
	- Add check enough funds on sender at TxSelector and don't
include the tx in the selection
- TxProcessor
	- Add checks that the balance when substracted the
amount/amount+fee never goes below 0

fix #502

txselector/txselector_test.go

@@ -378,3 +378,107 @@ func TestGetL2TxSelectionMinimumFlow0(t *testing.T) {
 	err = txsel.l2db.StartForging(common.TxIDsFromPoolL2Txs(poolL2Txs), txsel.localAccountsDB.CurrentBatch())
 	require.NoError(t, err)
 }
+
+func TestPoolL2TxsWithoutEnoughBalance(t *testing.T) {
+	set := `
+		Type: Blockchain
+
+		CreateAccountDeposit(0) Coord: 0
+		CreateAccountDeposit(0) A: 100
+		CreateAccountDeposit(0) B: 100
+
+		> batchL1 // freeze L1User{1}
+		> batchL1 // forge L1User{1}
+		> block
+	`
+
+	chainID := uint16(0)
+	tc := til.NewContext(chainID, common.RollupConstMaxL1UserTx)
+	// generate test transactions, the L1CoordinatorTxs generated by Til
+	// will be ignored at this test, as will be the TxSelector who
+	// generates them when needed
+	blocks, err := tc.GenerateBlocks(set)
+	assert.NoError(t, err)
+
+	hermezContractAddr := ethCommon.HexToAddress("0xc344E203a046Da13b0B4467EB7B3629D0C99F6E6")
+	txsel := initTest(t, chainID, hermezContractAddr, tc.Users["Coord"])
+
+	// restart nonces of TilContext, as will be set by generating directly
+	// the PoolL2Txs for each specific batch with tc.GeneratePoolL2Txs
+	tc.RestartNonces()
+
+	tpc := txprocessor.Config{
+		NLevels:  16,
+		MaxFeeTx: 10,
+		MaxTx:    20,
+		MaxL1Tx:  10,
+		ChainID:  chainID,
+	}
+	selectionConfig := &SelectionConfig{
+		MaxL1UserTxs:      5,
+		TxProcessorConfig: tpc,
+	}
+	// batch1
+	l1UserTxs := []common.L1Tx{}
+	_, _, _, _, _, err = txsel.GetL1L2TxSelection(selectionConfig, l1UserTxs)
+	require.NoError(t, err)
+
+	// batch2
+	// prepare the PoolL2Txs
+	batchPoolL2 := `
+	Type: PoolL2
+	PoolTransferToEthAddr(0) A-B: 100 (126)
+	PoolExit(0) B: 100 (126)`
+	poolL2Txs, err := tc.GeneratePoolL2Txs(batchPoolL2)
+	require.NoError(t, err)
+	// add the PoolL2Txs to the l2DB
+	addL2Txs(t, txsel, poolL2Txs)
+
+	l1UserTxs = til.L1TxsToCommonL1Txs(tc.Queues[*blocks[0].Rollup.Batches[1].Batch.ForgeL1TxsNum])
+	_, _, oL1UserTxs, oL1CoordTxs, oL2Txs, err := txsel.GetL1L2TxSelection(selectionConfig, l1UserTxs)
+	require.NoError(t, err)
+	assert.Equal(t, 3, len(oL1UserTxs))
+	assert.Equal(t, 0, len(oL1CoordTxs))
+	assert.Equal(t, 0, len(oL2Txs)) // should be 0 as the 2 PoolL2Txs does not have enough funds
+	err = txsel.l2db.StartForging(common.TxIDsFromPoolL2Txs(oL2Txs), txsel.localAccountsDB.CurrentBatch())
+	require.NoError(t, err)
+
+	// as the PoolL2Txs have not been really processed, restart nonces
+	tc.RestartNonces()
+
+	// batch3
+	// NOTE: this batch will result with 1 L2Tx, as the PoolExit tx is not
+	// possible, as the PoolTransferToEthAddr is not processed yet when
+	// checking availability of PoolExit.  This, in a near-future iteration
+	// of the TxSelector will return the 2 transactions as valid and
+	// selected, as the TxSelector will handle this kind of combinations.
+	batchPoolL2 = `
+	Type: PoolL2
+	PoolTransferToEthAddr(0) A-B: 50 (126)`
+	poolL2Txs, err = tc.GeneratePoolL2Txs(batchPoolL2)
+	require.NoError(t, err)
+	addL2Txs(t, txsel, poolL2Txs)
+
+	l1UserTxs = []common.L1Tx{}
+	_, _, oL1UserTxs, oL1CoordTxs, oL2Txs, err = txsel.GetL1L2TxSelection(selectionConfig, l1UserTxs)
+	require.NoError(t, err)
+	assert.Equal(t, 0, len(oL1UserTxs))
+	assert.Equal(t, 0, len(oL1CoordTxs))
+	assert.Equal(t, 1, len(oL2Txs)) // see 'NOTE' at the beginning of 'batch3' of this test
+	assert.Equal(t, common.TxTypeTransferToEthAddr, oL2Txs[0].Type)
+	err = txsel.l2db.StartForging(common.TxIDsFromPoolL2Txs(oL2Txs), txsel.localAccountsDB.CurrentBatch())
+	require.NoError(t, err)
+
+	// batch4
+	// make the selection of another batch, which should include the
+	// initial PoolExit, which now is valid as B has enough Balance
+	l1UserTxs = []common.L1Tx{}
+	_, _, oL1UserTxs, oL1CoordTxs, oL2Txs, err = txsel.GetL1L2TxSelection(selectionConfig, l1UserTxs)
+	require.NoError(t, err)
+	assert.Equal(t, 0, len(oL1UserTxs))
+	assert.Equal(t, 0, len(oL1CoordTxs))
+	assert.Equal(t, 1, len(oL2Txs))
+	assert.Equal(t, common.TxTypeExit, oL2Txs[0].Type)
+	err = txsel.l2db.StartForging(common.TxIDsFromPoolL2Txs(oL2Txs), txsel.localAccountsDB.CurrentBatch())
+	require.NoError(t, err)
+}