config: allow configuring free gas depending on height

And update default mainnet/testnet settings. Follow neo-project/neo#1888 and
neo-project/neo-node#656.

config/protocol.mainnet.yml

@@ -32,7 +32,7 @@ ProtocolConfiguration:
     RegisterTransaction: 10000
   VerifyBlocks: true
   VerifyTransactions: false
-  FreeGasLimit: 10.0
+  FreeGasLimit: {0: 10.0, 6200000: 50.0}
   MaxTransactionsPerBlock: 500
   MaxFreeTransactionsPerBlock: 20
   MaxFreeTransactionSize: 1024

config/protocol.testnet.yml

@@ -32,7 +32,7 @@ ProtocolConfiguration:
     RegisterTransaction: 100
   VerifyBlocks: true
   VerifyTransactions: false
-  FreeGasLimit: 10.0
+  FreeGasLimit: {0: 10.0, 4818000: 50.0}
   MaxTransactionsPerBlock: 500
   MaxFreeTransactionsPerBlock: 20
   MaxFreeTransactionSize: 1024

pkg/config/protocol_config.go

@@ -26,10 +26,12 @@ type (
 		// transactions exceeding the MaxFreeTransactionSize.
 		FeePerExtraByte float64 `yaml:"FeePerExtraByte"`
 		// FreeGasLimit is an amount of GAS which can be spent for free.
-		FreeGasLimit            util.Fixed8 `yaml:"FreeGasLimit"`
-		LowPriorityThreshold    float64     `yaml:"LowPriorityThreshold"`
-		Magic                   NetMode     `yaml:"Magic"`
-		MaxTransactionsPerBlock int         `yaml:"MaxTransactionsPerBlock"`
+		// It can change over time, thus it's a map of block height to the
+		// respective GAS limit.
+		FreeGasLimit            map[uint32]util.Fixed8 `yaml:"FreeGasLimit"`
+		LowPriorityThreshold    float64                `yaml:"LowPriorityThreshold"`
+		Magic                   NetMode                `yaml:"Magic"`
+		MaxTransactionsPerBlock int                    `yaml:"MaxTransactionsPerBlock"`
 		// Maximum size of low priority transaction in bytes.
 		MaxFreeTransactionSize int `yaml:"MaxFreeTransactionSize"`
 		// Maximum number of low priority transactions accepted into block.
@@ -61,6 +63,20 @@ type (
 	NetMode uint32
 )
 
+// GetFreeGas returns FreeGasLimit value for given block height.
+func (p *ProtocolConfiguration) GetFreeGas(block uint32) util.Fixed8 {
+	var gas util.Fixed8
+	var height uint32
+	for h, g := range p.FreeGasLimit {
+		if h > block || h < height {
+			continue
+		}
+		gas = g
+		height = h
+	}
+	return gas
+}
+
 // String implements the stringer interface.
 func (n NetMode) String() string {
 	switch n {

pkg/config/protocol_config_test.go

@@ -0,0 +1,25 @@
+package config
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetFreeGas(t *testing.T) {
+	fixed10 := util.Fixed8FromInt64(10)
+	fixed50 := util.Fixed8FromInt64(50)
+	p := ProtocolConfiguration{
+		FreeGasLimit: map[uint32]util.Fixed8{
+			0:       fixed10,
+			6200000: fixed50,
+		},
+	}
+	require.Equal(t, fixed10, p.GetFreeGas(0))
+	require.Equal(t, fixed10, p.GetFreeGas(1000))
+	require.Equal(t, fixed10, p.GetFreeGas(1000000))
+	require.Equal(t, fixed10, p.GetFreeGas(6100000))
+	require.Equal(t, fixed50, p.GetFreeGas(6200000))
+	require.Equal(t, fixed50, p.GetFreeGas(7000000))
+}

pkg/core/blockchain.go

@@ -792,8 +792,9 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 			v.SetCheckedHash(tx.VerificationHash().BytesBE())
 			v.LoadScript(t.Script)
 			v.SetPriceGetter(getPrice)
-			if bc.config.FreeGasLimit > 0 {
-				v.SetGasLimit(bc.config.FreeGasLimit + t.Gas)
+			gasLimit := bc.config.GetFreeGas(block.Index)
+			if gasLimit > 0 {
+				v.SetGasLimit(gasLimit + t.Gas)
 			}
 
 			err := v.Run()

pkg/core/interop_neo.go

@@ -605,7 +605,8 @@ func (ic *interopContext) contractMigrate(v *vm.VM) error {
 			v := ic.bc.GetTestVM(nil)
 			w := io.NewBufBinWriter()
 			emit.AppCallWithOperationAndArgs(w.BinWriter, hash, "decimals")
-			v.SetGasLimit(ic.bc.GetConfig().FreeGasLimit)
+			conf := ic.bc.GetConfig()
+			v.SetGasLimit(conf.GetFreeGas(ic.bc.BlockHeight() + 1)) // BlockHeight() is already persisted, so it's either a new block or test invocation.
 			v.Load(w.Bytes())
 			if err := v.Run(); err == nil && v.Estack().Len() == 1 {
 				res := v.Estack().Pop().Item().ToContractParameter(map[vm.StackItem]bool{})