gui: improve user dialog when signing multisig psbts

Prior to this commit, when a psbt requiring multiple signers
is signed via gui the dialog is vague/confusing. Whether the
signer successfully partially signs the psbt or not the same warning
is displayed "Could not sign any more inputs." After signing the tx,
the only way for the user to know if their action had any effect is
to inspect the psbt (unless it was the last signature that completed
the psbt, in which case there is a success dialog).

Now we indicate when a psbt has been partially signed, and show how many
partial signatures the psbt has in the transaction description.

src/qt/psbtoperationsdialog.cpp

@@ -83,6 +83,10 @@ void PSBTOperationsDialog::signTransaction()
 
     WalletModel::UnlockContext ctx(m_wallet_model->requestUnlock());
 
+    // For multisigs we indicate when a participant successfully signs the PSBT
+    // even when additional signatures are required for the PSBT to be complete.
+    // Simply check the number of partial signatures before and after signing.
+    const size_t n_partial_sigs_before{m_transaction_data.inputs.at(0).partial_sigs.size()};
     const auto err{m_wallet_model->wallet().fillPSBT(SIGHASH_ALL, /*sign=*/true, /*bip32derivs=*/true, &n_signed, m_transaction_data, complete)};
 
     if (err) {
@@ -93,13 +97,26 @@ void PSBTOperationsDialog::signTransaction()
 
     updateTransactionDisplay();
 
+    const size_t n_partial_sigs_after{m_transaction_data.inputs.at(0).partial_sigs.size()};
+    // We also indicate when the participant successfully signed every tx input.
+    // To keep things simple we only check if this is not true, in which case
+    // fallback to the old (vague) behavior where it's unclear if the PSBT was
+    // successfully partially signed.
+    const bool all_inputs_n_partial_sigs_equal{std::all_of(m_transaction_data.inputs.begin(), m_transaction_data.inputs.end(),
+        [&](const PSBTInput& input) { return n_partial_sigs_after == input.partial_sigs.size(); })};
+
     if (!complete && !ctx.isValid()) {
         showStatus(tr("Cannot sign inputs while wallet is locked."), StatusLevel::WARN);
-    } else if (!complete && n_signed < 1) {
-        showStatus(tr("Could not sign any more inputs."), StatusLevel::WARN);
     } else if (!complete) {
-        showStatus(tr("Signed %1 inputs, but more signatures are still required.").arg(n_signed),
-            StatusLevel::INFO);
+        if (all_inputs_n_partial_sigs_equal && n_partial_sigs_after - n_partial_sigs_before > 0) {
+            showStatus(tr("Added %1 partial signature to each input, but more signatures are still required.").arg(n_partial_sigs_after - n_partial_sigs_before),
+                StatusLevel::INFO);
+        } else if (n_signed < 1) {
+            showStatus(tr("Could not sign any more inputs."), StatusLevel::WARN);
+        } else {
+            showStatus(tr("Signed %1 inputs, but more signatures are still required.").arg(n_signed),
+                StatusLevel::INFO);
+        }
     } else {
         showStatus(tr("Signed transaction successfully. Transaction is ready to broadcast."),
             StatusLevel::INFO);
@@ -217,9 +234,16 @@ QString PSBTOperationsDialog::renderTransaction(const PartiallySignedTransaction
     }
 
     size_t num_unsigned = CountPSBTUnsignedInputs(psbtx);
+    const size_t num_partial_sigs{psbtx.inputs.at(0).partial_sigs.size()};
+    const bool all_inputs_n_partial_sigs_equal{std::all_of(psbtx.inputs.begin(), psbtx.inputs.end(),
+        [&](const PSBTInput& input) { return num_partial_sigs == input.partial_sigs.size(); })};
+
     if (num_unsigned > 0) {
         tx_description.append("<br><br>");
         tx_description.append(tr("Transaction has %1 unsigned inputs.").arg(QString::number(num_unsigned)));
+        if (all_inputs_n_partial_sigs_equal && num_partial_sigs > 0) {
+            tx_description.append(tr(" Each input has %1 partial signatures.").arg(QString::number(num_partial_sigs)));
+        }
     }
 
     return tx_description;