wallet: add -signer argument for external signer command

Create basic ExternalSigner class with contructor. A Signer(<cmd>)
is added to CWallet on load if -signer=<cmd> is set.

src/Makefile.am

@@ -265,6 +265,7 @@ BITCOIN_CORE_H = \
   wallet/crypter.h \
   wallet/db.h \
   wallet/dump.h \
+  wallet/external_signer.h \
   wallet/feebumper.h \
   wallet/fees.h \
   wallet/ismine.h \
@@ -379,6 +380,7 @@ libbitcoin_wallet_a_SOURCES = \
   wallet/crypter.cpp \
   wallet/db.cpp \
   wallet/dump.cpp \
+  wallet/external_signer.cpp \
   wallet/feebumper.cpp \
   wallet/fees.cpp \
   wallet/interfaces.cpp \

src/dummywallet.cpp

@@ -38,6 +38,7 @@ void DummyWalletInit::AddWalletOptions(ArgsManager& argsman) const
         "-paytxfee=<amt>",
         "-rescan",
         "-salvagewallet",
+        "-signer=<cmd>",
         "-spendzeroconfchange",
         "-txconfirmtarget=<n>",
         "-wallet=<path>",

src/wallet/external_signer.cpp

@@ -0,0 +1,8 @@
+// Copyright (c) 2018-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/external_signer.h>
+#include <util/system.h>
+
+ExternalSigner::ExternalSigner(const std::string& command, const std::string& fingerprint): m_command(command), m_fingerprint(fingerprint) {}

src/wallet/external_signer.h

@@ -0,0 +1,34 @@
+// Copyright (c) 2018-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_WALLET_EXTERNAL_SIGNER_H
+#define BITCOIN_WALLET_EXTERNAL_SIGNER_H
+
+#include <stdexcept>
+#include <string>
+#include <univalue.h>
+
+class ExternalSignerException : public std::runtime_error {
+public:
+    using std::runtime_error::runtime_error;
+};
+
+//! Enables interaction with an external signing device or service, such as
+//! a hardware wallet. See doc/external-signer.md
+class ExternalSigner
+{
+private:
+    //! The command which handles interaction with the external signer.
+    std::string m_command;
+
+public:
+    //! @param[in] command      the command which handles interaction with the external signer
+    //! @param[in] fingerprint  master key fingerprint of the signer
+    ExternalSigner(const std::string& command, const std::string& fingerprint);
+
+    //! Master key fingerprint of the signer
+    std::string m_fingerprint;
+};
+
+#endif // BITCOIN_WALLET_EXTERNAL_SIGNER_H

src/wallet/init.cpp

@@ -61,6 +61,9 @@ void WalletInit::AddWalletOptions(ArgsManager& argsman) const
     argsman.AddArg("-paytxfee=<amt>", strprintf("Fee (in %s/kB) to add to transactions you send (default: %s)",
                                                             CURRENCY_UNIT, FormatMoney(CFeeRate{DEFAULT_PAY_TX_FEE}.GetFeePerK())), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     argsman.AddArg("-rescan", "Rescan the block chain for missing wallet transactions on startup", ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
+#ifdef ENABLE_EXTERNAL_SIGNER
+    argsman.AddArg("-signer=<cmd>", "External signing tool, see docs/external-signer.md", ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
+#endif
     argsman.AddArg("-spendzeroconfchange", strprintf("Spend unconfirmed change when sending transactions (default: %u)", DEFAULT_SPEND_ZEROCONF_CHANGE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     argsman.AddArg("-txconfirmtarget=<n>", strprintf("If paytxfee is not set, include enough fee so transactions begin confirmation on average within n blocks (default: %u)", DEFAULT_TX_CONFIRM_TARGET), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
     argsman.AddArg("-wallet=<path>", "Specify wallet path to load at startup. Can be used multiple times to load multiple wallets. Path is to a directory containing wallet data and log files. If the path is not absolute, it is interpreted relative to <walletdir>. This only loads existing wallets and does not create new ones. For backwards compatibility this also accepts names of existing top-level data files in <walletdir>.", ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::WALLET);

src/wallet/wallet.h

@@ -22,6 +22,7 @@
 #include <wallet/coinselection.h>
 #include <wallet/crypter.h>
 #include <wallet/scriptpubkeyman.h>
+#include <wallet/external_signer.h>
 #include <wallet/walletdb.h>
 #include <wallet/walletutil.h>
 
@@ -95,7 +96,6 @@ constexpr CAmount DEFAULT_TRANSACTION_MAXFEE{COIN / 10};
 constexpr CAmount HIGH_TX_FEE_PER_KB{COIN / 100};
 //! -maxtxfee will warn if called with a higher fee than this amount (in satoshis)
 constexpr CAmount HIGH_MAX_TX_FEE{100 * HIGH_TX_FEE_PER_KB};
-
 //! Pre-calculated constants for input size estimation in *virtual size*
 static constexpr size_t DUMMY_NESTED_P2WPKH_INPUT_SIZE = 91;