Merge #16944: gui: create PSBT with watch-only wallet

c6dd565c8820aa8a98b190621e10c6e2821a9ecc [gui] watch-only wallet: copy PSBT to clipboard (Sjors Provoost)
39465d545d521e66bb3accfa788aa94bffaf47eb [wallet] add fillPSBT to interface (Sjors Provoost)
848f88920853724511387ca0b7ef652fa14ced71 [gui] send: include watch-only (Sjors Provoost)
40537f090907f81ba885edb7dff1558382976912 [wallet] ListCoins: include watch-only for wallets without private keys (Sjors Provoost)

Pull request description:

  For wallets with `WALLET_FLAG_DISABLE_PRIVATE_KEYS` this makes the watch-only balance available on the send screen (including coin selection). Instead of sending a transaction it generates a PSBT.

  The user can take this PSBT and process it with [HWI](https://github.com/bitcoin-core/HWI) or put it an SD card for hardware wallets that support that.

  The PSBT is copied to the clipboard. This was the easiest approach; we can add a dialog later to display it, as well as an option to save to disk.

ACKs for top commit:
  instagibbs:
    test and code review ACK https://github.com/bitcoin/bitcoin/pull/16944/commits/c6dd565c8820aa8a98b190621e10c6e2821a9ecc
  meshcollider:
    re-ACK c6dd565c8820aa8a98b190621e10c6e2821a9ecc

Tree-SHA512: ebc3da0737e33b255ed926191b84569aedb6097d14868662bd5dce726ce3048e86e9a31eba987b10dffe1482b35c21ae1cd595c2caa4634bc4cf78a826a83852

6 files changed, 85 insertions, 23 deletions

diff --git a/src/interfaces/wallet.cpp b/src/interfaces/wallet.cpp
index 079df17431..94c6e8c7b7 100644
--- a/src/interfaces/wallet.cpp
+++ b/src/interfaces/wallet.cpp
@@ -18,8 +18,9 @@
 #include <wallet/feebumper.h>
 #include <wallet/fees.h>
 #include <wallet/ismine.h>
-#include <wallet/rpcwallet.h>
 #include <wallet/load.h>
+#include <wallet/psbtwallet.h>
+#include <wallet/rpcwallet.h>
 #include <wallet/wallet.h>
 
 #include <memory>
@@ -357,6 +358,14 @@ public:
         }
         return {};
     }
+    TransactionError fillPSBT(PartiallySignedTransaction& psbtx,
+        bool& complete,
+        int sighash_type = 1 /* SIGHASH_ALL */,
+        bool sign = true,
+        bool bip32derivs = false) override
+    {
+        return FillPSBT(m_wallet.get(), psbtx, complete, sighash_type, sign, bip32derivs);
+    }
     WalletBalances getBalances() override
     {
         const auto bal = m_wallet->GetBalance();
diff --git a/src/interfaces/wallet.h b/src/interfaces/wallet.h
index b7cf9f2111..8d2b8a2eca 100644
--- a/src/interfaces/wallet.h
+++ b/src/interfaces/wallet.h
@@ -14,6 +14,7 @@
 #include <functional>
 #include <map>
 #include <memory>
+#include <psbt.h>
 #include <stdint.h>
 #include <string>
 #include <tuple>
@@ -194,6 +195,13 @@ public:
         bool& in_mempool,
         int& num_blocks) = 0;
 
+    //! Fill PSBT.
+    virtual TransactionError fillPSBT(PartiallySignedTransaction& psbtx,
+        bool& complete,
+        int sighash_type = 1 /* SIGHASH_ALL */,
+        bool sign = true,
+        bool bip32derivs = false) = 0;
+
     //! Get balances.
     virtual WalletBalances getBalances() = 0;
 
diff --git a/src/qt/sendcoinsdialog.cpp b/src/qt/sendcoinsdialog.cpp
index 8edcca684d..c3d00fa38e 100644
--- a/src/qt/sendcoinsdialog.cpp
+++ b/src/qt/sendcoinsdialog.cpp
@@ -21,11 +21,12 @@
 #include <chainparams.h>
 #include <interfaces/node.h>
 #include <key_io.h>
-#include <wallet/coincontrol.h>
-#include <ui_interface.h>
-#include <txmempool.h>
 #include <policy/fees.h>
+#include <txmempool.h>
+#include <ui_interface.h>
+#include <wallet/coincontrol.h>
 #include <wallet/fees.h>
+#include <wallet/psbtwallet.h>
 
 #include <QFontMetrics>
 #include <QScrollBar>
@@ -186,6 +187,11 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         // set default rbf checkbox state
         ui->optInRBF->setCheckState(Qt::Checked);
 
+        if (model->privateKeysDisabled()) {
+            ui->sendButton->setText(tr("Cr&eate Unsigned"));
+            ui->sendButton->setToolTip(tr("Creates a Partially Signed Bitcoin Transaction (PSBT) for use with e.g. an offline %1 wallet, or a PSBT-compatible hardware wallet.").arg(PACKAGE_NAME));
+        }
+
         // set the smartfee-sliders default value (wallets default conf.target or last stored value)
         QSettings settings;
         if (settings.value("nSmartFeeSliderPosition").toInt() != 0) {
@@ -305,9 +311,19 @@ void SendCoinsDialog::on_sendButton_clicked()
         formatted.append(recipientElement);
     }
 
-    QString questionString = tr("Are you sure you want to send?");
+    QString questionString;
+    if (model->privateKeysDisabled()) {
+        questionString.append(tr("Do you want to draft this transaction?"));
+    } else {
+        questionString.append(tr("Are you sure you want to send?"));
+    }
+
     questionString.append("<br /><span style='font-size:10pt;'>");
-    questionString.append(tr("Please, review your transaction."));
+    if (model->privateKeysDisabled()) {
+        questionString.append(tr("Please, review your transaction proposal. This will produce a Partially Signed Bitcoin Transaction (PSBT) which you can copy and then sign with e.g. an offline %1 wallet, or a PSBT-compatible hardware wallet.").arg(PACKAGE_NAME));
+    } else {
+        questionString.append(tr("Please, review your transaction."));
+    }
     questionString.append("</span>%1");
 
     if(txFee > 0)
@@ -358,8 +374,9 @@ void SendCoinsDialog::on_sendButton_clicked()
     } else {
         questionString = questionString.arg("<br /><br />" + formatted.at(0));
     }
-
-    SendConfirmationDialog confirmationDialog(tr("Confirm send coins"), questionString, informative_text, detailed_text, SEND_CONFIRM_DELAY, this);
+    const QString confirmation = model->privateKeysDisabled() ? tr("Confirm transaction proposal") : tr("Confirm send coins");
+    const QString confirmButtonText = model->privateKeysDisabled() ? tr("Copy PSBT to clipboard") : tr("Send");
+    SendConfirmationDialog confirmationDialog(confirmation, questionString, informative_text, detailed_text, SEND_CONFIRM_DELAY, confirmButtonText, this);
     confirmationDialog.exec();
     QMessageBox::StandardButton retval = static_cast<QMessageBox::StandardButton>(confirmationDialog.result());
 
@@ -369,17 +386,35 @@ void SendCoinsDialog::on_sendButton_clicked()
         return;
     }
 
-    // now send the prepared transaction
-    WalletModel::SendCoinsReturn sendStatus = model->sendCoins(currentTransaction);
-    // process sendStatus and on error generate message shown to user
-    processSendCoinsReturn(sendStatus);
+    bool send_failure = false;
+    if (model->privateKeysDisabled()) {
+        CMutableTransaction mtx = CMutableTransaction{*(currentTransaction.getWtx())};
+        PartiallySignedTransaction psbtx(mtx);
+        bool complete = false;
+        const TransactionError err = model->wallet().fillPSBT(psbtx, complete, SIGHASH_ALL, false /* sign */, true /* bip32derivs */);
+        assert(!complete);
+        assert(err == TransactionError::OK);
+        // Serialize the PSBT
+        CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
+        ssTx << psbtx;
+        GUIUtil::setClipboard(EncodeBase64(ssTx.str()).c_str());
+        Q_EMIT message(tr("PSBT copied"), "Copied to clipboard", CClientUIInterface::MSG_INFORMATION);
+    } else {
+        // now send the prepared transaction
+        WalletModel::SendCoinsReturn sendStatus = model->sendCoins(currentTransaction);
+        // process sendStatus and on error generate message shown to user
+        processSendCoinsReturn(sendStatus);
 
-    if (sendStatus.status == WalletModel::OK)
-    {
+        if (sendStatus.status == WalletModel::OK) {
+            Q_EMIT coinsSent(currentTransaction.getWtx()->GetHash());
+        } else {
+            send_failure = true;
+        }
+    }
+    if (!send_failure) {
         accept();
         CoinControlDialog::coinControl()->UnSelectAll();
         coinControlUpdateLabels();
-        Q_EMIT coinsSent(currentTransaction.getWtx()->GetHash());
     }
     fNewRecipientAllowed = true;
 }
@@ -611,6 +646,9 @@ void SendCoinsDialog::useAvailableBalance(SendCoinsEntry* entry)
         coin_control = *CoinControlDialog::coinControl();
     }
 
+    // Include watch-only for wallets without private key
+    coin_control.fAllowWatchOnly = model->privateKeysDisabled();
+
     // Calculate available amount to send.
     CAmount amount = model->wallet().getAvailableBalance(coin_control);
     for (int i = 0; i < ui->entries->count(); ++i) {
@@ -663,6 +701,8 @@ void SendCoinsDialog::updateCoinControlState(CCoinControl& ctrl)
     // Either custom fee will be used or if not selected, the confirmation target from dropdown box
     ctrl.m_confirm_target = getConfTargetForIndex(ui->confTargetSelector->currentIndex());
     ctrl.m_signal_bip125_rbf = ui->optInRBF->isChecked();
+    // Include watch-only for wallets without private key
+    ctrl.fAllowWatchOnly = model->privateKeysDisabled();
 }
 
 void SendCoinsDialog::updateSmartFeeLabel()
@@ -870,8 +910,8 @@ void SendCoinsDialog::coinControlUpdateLabels()
     }
 }
 
-SendConfirmationDialog::SendConfirmationDialog(const QString& title, const QString& text, const QString& informative_text, const QString& detailed_text, int _secDelay, QWidget* parent)
-    : QMessageBox(parent), secDelay(_secDelay)
+SendConfirmationDialog::SendConfirmationDialog(const QString& title, const QString& text, const QString& informative_text, const QString& detailed_text, int _secDelay, const QString& _confirmButtonText, QWidget* parent)
+    : QMessageBox(parent), secDelay(_secDelay), confirmButtonText(_confirmButtonText)
 {
     setIcon(QMessageBox::Question);
     setWindowTitle(title); // On macOS, the window title is ignored (as required by the macOS Guidelines).
@@ -908,11 +948,11 @@ void SendConfirmationDialog::updateYesButton()
     if(secDelay > 0)
     {
         yesButton->setEnabled(false);
-        yesButton->setText(tr("Send") + " (" + QString::number(secDelay) + ")");
+        yesButton->setText(confirmButtonText + " (" + QString::number(secDelay) + ")");
     }
     else
     {
         yesButton->setEnabled(true);
-        yesButton->setText(tr("Send"));
+        yesButton->setText(confirmButtonText);
     }
 }
diff --git a/src/qt/sendcoinsdialog.h b/src/qt/sendcoinsdialog.h
index c6c1816877..ccd8494613 100644
--- a/src/qt/sendcoinsdialog.h
+++ b/src/qt/sendcoinsdialog.h
@@ -108,7 +108,7 @@ class SendConfirmationDialog : public QMessageBox
     Q_OBJECT
 
 public:
-    SendConfirmationDialog(const QString& title, const QString& text, const QString& informative_text = "", const QString& detailed_text = "", int secDelay = SEND_CONFIRM_DELAY, QWidget* parent = nullptr);
+    SendConfirmationDialog(const QString& title, const QString& text, const QString& informative_text = "", const QString& detailed_text = "", int secDelay = SEND_CONFIRM_DELAY, const QString& confirmText = "Send", QWidget* parent = nullptr);
     int exec();
 
 private Q_SLOTS:
@@ -119,6 +119,7 @@ private:
     QAbstractButton *yesButton;
     QTimer countDownTimer;
     int secDelay;
+    QString confirmButtonText;
 };
 
 #endif // BITCOIN_QT_SENDCOINSDIALOG_H
diff --git a/src/qt/walletmodel.cpp b/src/qt/walletmodel.cpp
index 33801d3907..fb92e29f21 100644
--- a/src/qt/walletmodel.cpp
+++ b/src/qt/walletmodel.cpp
@@ -183,7 +183,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         std::string strFailReason;
 
         auto& newTx = transaction.getWtx();
-        newTx = m_wallet->createTransaction(vecSend, coinControl, true /* sign */, nChangePosRet, nFeeRequired, strFailReason);
+        newTx = m_wallet->createTransaction(vecSend, coinControl, !privateKeysDisabled() /* sign */, nChangePosRet, nFeeRequired, strFailReason);
         transaction.setTransactionFee(nFeeRequired);
         if (fSubtractFeeFromAmount && newTx)
             transaction.reassignAmounts(nChangePosRet);
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 3e368985e0..92139bbad8 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2164,7 +2164,7 @@ std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins(interfaces::Ch
 
     for (const COutput& coin : availableCoins) {
         CTxDestination address;
-        if (coin.fSpendable &&
+        if ((coin.fSpendable || (IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS) && coin.fSolvable)) &&
             ExtractDestination(FindNonChangeParentOutput(*coin.tx->tx, coin.i).scriptPubKey, address)) {
             result[address].emplace_back(std::move(coin));
         }
@@ -2172,12 +2172,16 @@ std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins(interfaces::Ch
 
     std::vector<COutPoint> lockedCoins;
     ListLockedCoins(lockedCoins);
+    // Include watch-only for wallets without private keys
+    const bool include_watch_only = IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
+    const isminetype is_mine_filter = include_watch_only ? ISMINE_WATCH_ONLY : ISMINE_SPENDABLE;
     for (const COutPoint& output : lockedCoins) {
         auto it = mapWallet.find(output.hash);
         if (it != mapWallet.end()) {
             int depth = it->second.GetDepthInMainChain();
             if (depth >= 0 && output.n < it->second.tx->vout.size() &&
-                IsMine(it->second.tx->vout[output.n]) == ISMINE_SPENDABLE) {
+                IsMine(it->second.tx->vout[output.n]) == is_mine_filter
+            ) {
                 CTxDestination address;
                 if (ExtractDestination(FindNonChangeParentOutput(*it->second.tx, output.n).scriptPubKey, address)) {
                     result[address].emplace_back(