cosmos-sdk/types/context.go

package types

import (
	"context"
	"github.com/golang/protobuf/proto"

	abci "github.com/tendermint/abci/types"
)

/*

A note on Context security:

The intent of Context is for it to be an immutable object that can be
cloned and updated cheaply with WithValue() and passed forward to the
next decorator or handler. For example,

```golang
func Decorator(ctx Context, tx Tx, next Handler) Result {

	// Clone and update context with new kv pair.
	ctx2 := ctx.WithValueSDK(key, value)

	// Call the next decorator/handler.
	res := next(ctx2, ms, tx)

	...
}
```

While `ctx` and `ctx2`'s shallow values haven't changed, it's
possible that slices or addressable struct fields have been modified
by the call to `next(...)`.

This is generally undesirable because it prevents a decorator from
rolling back all side effects--which is the intent of immutable
Context's and store cache-wraps.

While well-written decorators wouldn't mutate any mutable context
values, a malicious or buggy plugin can create unwanted side-effects,
so it is highly advised for users of Context to only set immutable
values.  To help enforce this contract, we require values to be
certain primitive types, a cloner, or a CacheWrapper.

If an outer (higher) decorator wants to know what an inner decorator
had set on the context, it can consult `context.GetOp(ver int64) Op`,
which retrieves the ver'th opertion to the context, globally since `NewContext()`.

TODO: Add a default logger.
*/

type Context struct {
	context.Context
	pst *thePast
	gen int
	// Don't add any other fields here,
	// it's probably not what you want to do.
}

func NewContext(header abci.Header, isCheckTx bool, txBytes []byte) Context {
	c := Context{
		Context: context.Background(),
		pst:     newThePast(),
		gen:     0,
	}
	c = c.withBlockHeader(header)
	c = c.withBlockHeight(header.Height)
	c = c.withChainID(header.ChainID)
	c = c.withIsCheckTx(isCheckTx)
	c = c.withTxBytes(txBytes)
	return c
}

//----------------------------------------
// Get a value

func (c Context) Value(key interface{}) interface{} {
	value := c.Context.Value(key)
	if cloner, ok := value.(cloner); ok {
		return cloner.Clone()
	}
	if message, ok := value.(proto.Message); ok {
		return proto.Clone(message)
	}
	return value
}

//----------------------------------------
// Set a value

func (c Context) WithValueUnsafe(key interface{}, value interface{}) Context {
	return c.withValue(key, value)
}

func (c Context) WithCloner(key interface{}, value cloner) Context {
	return c.withValue(key, value)
}

func (c Context) WithCacheWrapper(key interface{}, value CacheWrapper) Context {
	return c.withValue(key, value)
}

func (c Context) WithProtoMsg(key interface{}, value proto.Message) Context {
	return c.withValue(key, value)
}

func (c Context) WithString(key interface{}, value string) Context {
	return c.withValue(key, value)
}

func (c Context) WithInt32(key interface{}, value int32) Context {
	return c.withValue(key, value)
}

func (c Context) WithUint32(key interface{}, value uint32) Context {
	return c.withValue(key, value)
}

func (c Context) WithUint64(key interface{}, value uint64) Context {
	return c.withValue(key, value)
}

func (c Context) withValue(key interface{}, value interface{}) Context {
	c.pst.bump(Op{
		gen:   c.gen + 1,
		key:   key,
		value: value,
	}) // increment version for all relatives.

	return Context{
		Context: context.WithValue(c.Context, key, value),
		pst:     c.pst,
		gen:     c.gen + 1,
	}
}

//----------------------------------------
// Our extensions

type contextKey int // local to the context module

const (
	contextKeyBlockHeader contextKey = iota
	contextKeyBlockHeight
	contextKeyChainID
	contextKeyIsCheckTx
	contextKeyTxBytes
)

func (c Context) BlockHeader() abci.Header {
	return c.Value(contextKeyBlockHeader).(abci.Header)
}

func (c Context) BlockHeight() int64 {
	return c.Value(contextKeyBlockHeight).(int64)
}

func (c Context) ChainID() string {
	return c.Value(contextKeyChainID).(string)
}

func (c Context) IsCheckTx() bool {
	return c.Value(contextKeyIsCheckTx).(bool)
}

func (c Context) TxBytes() []byte {
	return c.Value(contextKeyTxBytes).([]byte)
}

func (c Context) KVStore(key interface{}) KVStore {
	return c.Value(key).(KVStore)
}

// Unexposed to prevent overriding.
func (c Context) withBlockHeader(header abci.Header) Context {
	var _ proto.Message = &header // for cloning.
	return c.withValue(contextKeyBlockHeader, header)
}

// Unexposed to prevent overriding.
func (c Context) withBlockHeight(height int64) Context {
	return c.withValue(contextKeyBlockHeight, height)
}

// Unexposed to prevent overriding.
func (c Context) withChainID(chainID string) Context {
	return c.withValue(contextKeyChainID, chainID)
}

// Unexposed to prevent overriding.
func (c Context) withIsCheckTx(isCheckTx bool) Context {
	return c.withValue(contextKeyIsCheckTx, isCheckTx)
}

// Unexposed to prevent overriding.
func (c Context) withTxBytes(txBytes []byte) Context {
	return c.withValue(contextKeyTxBytes, txBytes)
}

//----------------------------------------
// thePast

// Returns false if ver > 0.
// The first operation is version 1.
func (c Context) GetOp(ver int64) (Op, bool) {
	return c.pst.getOp(ver)
}

//----------------------------------------
// Misc.

type cloner interface {
	Clone() interface{} // deep copy
}

type Op struct {
	// type is always 'with'
	gen   int
	key   interface{}
	value interface{}
}

type thePast struct {
	mtx sync.RWMutex
	ver int
	ops []Op
}

func newThePast() *thePast {
	return &thePast{
		val: 0,
		ops: nil,
	}
}

func (pst *thePast) bump(op Op) {
	pst.mtx.Lock()
	pst.ver += 1
	pst.ops = append(pst.ops, op)
	pst.mtx.Unlock()
}

func (pst *thePast) version() int {
	pst.mtx.RLock()
	defer pst.mtx.RUnlock()
	return pst.ver
}

// Returns false if ver > 0.
// The first operation is version 1.
func (pst *thePast) getOp(ver int) (Op, bool) {
	pst.mtx.RLock()
	defer pst.mtx.RUnlock()
	if len(pst.ops) < ver {
		return Op{}, false
	} else {
		return pst.ops[ver-1], true
	}
}