ethclient, mobile: add TransactionSender (#15127)

* core/types: make Signer derive address instead of public key

There are two reasons to do this now: The upcoming ethclient signer
doesn't know the public key, just the address. EIP 208 will introduce a
new signer which derives the 'entry point' address for transactions with
zero signature. The entry point has no public key.

Other changes to the interface ease the path make to moving signature
crypto out of core/types later.

* ethclient, mobile: add TransactionSender

The new method can get the right signer without any crypto, and without
knowledge of the signature scheme that was used when the transaction was
included.

2 files changed, 124 insertions, 20 deletions

diff --git a/ethclient/ethclient.go b/ethclient/ethclient.go
index 48639d949..7f73ab113 100644
--- a/ethclient/ethclient.go
+++ b/ethclient/ethclient.go
@@ -20,6 +20,7 @@ package ethclient
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 
@@ -70,9 +71,9 @@ func (ec *Client) BlockByNumber(ctx context.Context, number *big.Int) (*types.Bl
 }
 
 type rpcBlock struct {
-	Hash         common.Hash          `json:"hash"`
-	Transactions []*types.Transaction `json:"transactions"`
-	UncleHashes  []common.Hash        `json:"uncles"`
+	Hash         common.Hash      `json:"hash"`
+	Transactions []rpcTransaction `json:"transactions"`
+	UncleHashes  []common.Hash    `json:"uncles"`
 }
 
 func (ec *Client) getBlock(ctx context.Context, method string, args ...interface{}) (*types.Block, error) {
@@ -129,7 +130,13 @@ func (ec *Client) getBlock(ctx context.Context, method string, args ...interface
 			}
 		}
 	}
-	return types.NewBlockWithHeader(head).WithBody(body.Transactions, uncles), nil
+	// Fill the sender cache of transactions in the block.
+	txs := make([]*types.Transaction, len(body.Transactions))
+	for i, tx := range body.Transactions {
+		setSenderFromServer(tx.tx, tx.From, body.Hash)
+		txs[i] = tx.tx
+	}
+	return types.NewBlockWithHeader(head).WithBody(txs, uncles), nil
 }
 
 // HeaderByHash returns the block header with the given hash.
@@ -153,25 +160,62 @@ func (ec *Client) HeaderByNumber(ctx context.Context, number *big.Int) (*types.H
 	return head, err
 }
 
+type rpcTransaction struct {
+	tx *types.Transaction
+	txExtraInfo
+}
+
+type txExtraInfo struct {
+	BlockNumber *string
+	BlockHash   common.Hash
+	From        common.Address
+}
+
+func (tx *rpcTransaction) UnmarshalJSON(msg []byte) error {
+	if err := json.Unmarshal(msg, &tx.tx); err != nil {
+		return err
+	}
+	return json.Unmarshal(msg, &tx.txExtraInfo)
+}
+
 // TransactionByHash returns the transaction with the given hash.
 func (ec *Client) TransactionByHash(ctx context.Context, hash common.Hash) (tx *types.Transaction, isPending bool, err error) {
-	var raw json.RawMessage
-	err = ec.c.CallContext(ctx, &raw, "eth_getTransactionByHash", hash)
+	var json *rpcTransaction
+	err = ec.c.CallContext(ctx, &json, "eth_getTransactionByHash", hash)
 	if err != nil {
 		return nil, false, err
-	} else if len(raw) == 0 {
+	} else if json == nil {
 		return nil, false, ethereum.NotFound
-	}
-	if err := json.Unmarshal(raw, &tx); err != nil {
-		return nil, false, err
-	} else if _, r, _ := tx.RawSignatureValues(); r == nil {
+	} else if _, r, _ := json.tx.RawSignatureValues(); r == nil {
 		return nil, false, fmt.Errorf("server returned transaction without signature")
 	}
-	var block struct{ BlockNumber *string }
-	if err := json.Unmarshal(raw, &block); err != nil {
-		return nil, false, err
+	setSenderFromServer(json.tx, json.From, json.BlockHash)
+	return json.tx, json.BlockNumber == nil, nil
+}
+
+// TransactionSender returns the sender address of the given transaction. The transaction
+// must be known to the remote node and included in the blockchain at the given block and
+// index. The sender is the one derived by the protocol at the time of inclusion.
+//
+// There is a fast-path for transactions retrieved by TransactionByHash and
+// TransactionInBlock. Getting their sender address can be done without an RPC interaction.
+func (ec *Client) TransactionSender(ctx context.Context, tx *types.Transaction, block common.Hash, index uint) (common.Address, error) {
+	// Try to load the address from the cache.
+	sender, err := types.Sender(&senderFromServer{blockhash: block}, tx)
+	if err == nil {
+		return sender, nil
+	}
+	var meta struct {
+		Hash common.Hash
+		From common.Address
+	}
+	if err = ec.c.CallContext(ctx, &meta, "eth_getTransactionByBlockHashAndIndex", block, hexutil.Uint64(index)); err != nil {
+		return common.Address{}, err
+	}
+	if meta.Hash == (common.Hash{}) || meta.Hash != tx.Hash() {
+		return common.Address{}, errors.New("wrong inclusion block/index")
 	}
-	return tx, block.BlockNumber == nil, nil
+	return meta.From, nil
 }
 
 // TransactionCount returns the total number of transactions in the given block.
@@ -183,16 +227,17 @@ func (ec *Client) TransactionCount(ctx context.Context, blockHash common.Hash) (
 
 // TransactionInBlock returns a single transaction at index in the given block.
 func (ec *Client) TransactionInBlock(ctx context.Context, blockHash common.Hash, index uint) (*types.Transaction, error) {
-	var tx *types.Transaction
-	err := ec.c.CallContext(ctx, &tx, "eth_getTransactionByBlockHashAndIndex", blockHash, hexutil.Uint64(index))
+	var json *rpcTransaction
+	err := ec.c.CallContext(ctx, &json, "eth_getTransactionByBlockHashAndIndex", blockHash, hexutil.Uint64(index))
 	if err == nil {
-		if tx == nil {
+		if json == nil {
 			return nil, ethereum.NotFound
-		} else if _, r, _ := tx.RawSignatureValues(); r == nil {
+		} else if _, r, _ := json.tx.RawSignatureValues(); r == nil {
 			return nil, fmt.Errorf("server returned transaction without signature")
 		}
 	}
-	return tx, err
+	setSenderFromServer(json.tx, json.From, json.BlockHash)
+	return json.tx, err
 }
 
 // TransactionReceipt returns the receipt of a transaction by transaction hash.
diff --git a/ethclient/signer.go b/ethclient/signer.go
new file mode 100644
index 000000000..74a93f1e2
--- /dev/null
+++ b/ethclient/signer.go
@@ -0,0 +1,59 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package ethclient
+
+import (
+	"errors"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
+// senderFromServer is a types.Signer that remembers the sender address returned by the RPC
+// server. It is stored in the transaction's sender address cache to avoid an additional
+// request in TransactionSender.
+type senderFromServer struct {
+	addr      common.Address
+	blockhash common.Hash
+}
+
+var errNotCached = errors.New("sender not cached")
+
+func setSenderFromServer(tx *types.Transaction, addr common.Address, block common.Hash) {
+	// Use types.Sender for side-effect to store our signer into the cache.
+	types.Sender(&senderFromServer{addr, block}, tx)
+}
+
+func (s *senderFromServer) Equal(other types.Signer) bool {
+	os, ok := other.(*senderFromServer)
+	return ok && os.blockhash == s.blockhash
+}
+
+func (s *senderFromServer) Sender(tx *types.Transaction) (common.Address, error) {
+	if s.blockhash == (common.Hash{}) {
+		return common.Address{}, errNotCached
+	}
+	return s.addr, nil
+}
+
+func (s *senderFromServer) Hash(tx *types.Transaction) common.Hash {
+	panic("can't sign with senderFromServer")
+}
+func (s *senderFromServer) SignatureValues(tx *types.Transaction, sig []byte) (R, S, V *big.Int, err error) {
+	panic("can't sign with senderFromServer")
+}