multiprocess: Add interfaces::Node::broadCastTransaction method

This fixes a null pointer crash in the bitcoin-gui PSBT dialog. The
bitcoin-gui interfaces::Node object has a null NodeContext pointer, and
can't broadcast transactions directly. It needs to broadcast
transactions through the bitcoin-node process instead.

Author: ryanofsky

src/interfaces/node.h

@@ -31,6 +31,7 @@ class RPCTimerInterface;
 class UniValue;
 class proxyType;
 enum class SynchronizationState;
+enum class TransactionError;
 struct CNodeStateStats;
 struct NodeContext;
 struct bilingual_str;
@@ -174,6 +175,9 @@ class Node
     //! Get unspent outputs associated with a transaction.
     virtual bool getUnspentOutput(const COutPoint& output, Coin& coin) = 0;
 
+    //! Broadcast transaction.
+    virtual TransactionError broadcastTransaction(CTransactionRef tx, CAmount max_tx_fee, std::string& err_string) = 0;
+
     //! Get wallet client.
     virtual WalletClient& walletClient() = 0;
 

src/node/interfaces.cpp

@@ -261,6 +261,10 @@ class NodeImpl : public Node
         LOCK(::cs_main);
         return chainman().ActiveChainstate().CoinsTip().GetCoin(output, coin);
     }
+    TransactionError broadcastTransaction(CTransactionRef tx, CAmount max_tx_fee, std::string& err_string) override
+    {
+        return BroadcastTransaction(*m_context, std::move(tx), err_string, max_tx_fee, /*relay=*/ true, /*wait_callback=*/ false);
+    }
     WalletClient& walletClient() override
     {
         return *Assert(m_context->wallet_client);

src/qt/psbtoperationsdialog.cpp

@@ -110,8 +110,8 @@ void PSBTOperationsDialog::broadcastTransaction()
 
     CTransactionRef tx = MakeTransactionRef(mtx);
     std::string err_string;
-    TransactionError error = BroadcastTransaction(
-        *m_client_model->node().context(), tx, err_string, DEFAULT_MAX_RAW_TX_FEE_RATE.GetFeePerK(), /* relay */ true, /* await_callback */ false);
+    TransactionError error =
+        m_client_model->node().broadcastTransaction(tx, DEFAULT_MAX_RAW_TX_FEE_RATE.GetFeePerK(), err_string);
 
     if (error == TransactionError::OK) {
         showStatus(tr("Transaction broadcast successfully! Transaction ID: %1")