core: adjust System.Storage.PutEx interop

Part of #1055.

Added System.Storage.PutEx to compiler. Added StorageFlag in order to
denote whether item is constant or not.

pkg/compiler/syscall.go

@@ -77,5 +77,6 @@ var syscalls = map[string]map[string]Syscall{
 		"GetContext":               {"System.Storage.GetContext", false},
 		"GetReadOnlyContext":       {"System.Storage.GetReadOnlyContext", false},
 		"Put":                      {"System.Storage.Put", false},
+		"PutEx":                    {"System.Storage.PutEx", false},
 	},
 }

pkg/core/interop_system.go

@@ -41,6 +41,17 @@ type StorageContext struct {
 	ReadOnly bool
 }
 
+// StorageFlag represents storage flag which denotes whether the stored value is
+// a constant.
+type StorageFlag byte
+
+const (
+	// None is a storage flag for non-constant items.
+	None StorageFlag = 0
+	// Constant is a storage flag for constant items.
+	Constant StorageFlag = 0x01
+)
+
 // getBlockHashFromElement converts given vm.Element to block hash using given
 // Blockchainer if needed. Interop functions accept both block numbers and
 // block hashes as parameters, thus this function is needed.
@@ -404,7 +415,7 @@ func storagePutInternal(ic *interop.Context, v *vm.VM, getFlag bool) error {
 	if getFlag {
 		flag = int(v.Estack().Pop().BigInt().Int64())
 	}
-	return putWithContextAndFlags(ic, v, stc, key, value, flag == 1)
+	return putWithContextAndFlags(ic, v, stc, key, value, int(Constant)&flag != 0)
 }
 
 // storagePut puts key-value pair into the storage.

pkg/interop/storage/storage.go

@@ -37,6 +37,13 @@ func GetReadOnlyContext() Context { return Context{} }
 // runtime.Serialize. This function uses `System.Storage.Put` syscall.
 func Put(ctx Context, key interface{}, value interface{}) {}
 
+// PutEx is an advanced version of Put which saves given value with given key
+// and given ReadOnly flag in the storage using given Context. `flag` argument
+// can either be odd for constant storage items or even for variable storage items.
+// Refer to Put function description for details on how to pass the remaining
+// arguments. This function uses `System.Storage.PutEx` syscall.
+func PutEx(ctx Context, key interface{}, value interface{}, flag int64) {}
+
 // Get retrieves value stored for the given key using given Context. See Put
 // documentation on possible key and value types. If the value is not present in
 // the database it returns nil. This function uses `System.Storage.Get` syscall.