[RPC] bumpfee

This command allows a user to increase the fee on a wallet transaction T, creating a "bumper" transaction B.
T must signal that it is BIP-125 replaceable.
T's change output is decremented to pay the additional fee.  (B will not add inputs to T.)
T cannot have any descendant transactions.
Once B bumps T, neither T nor B's outputs can be spent until either T or (more likely) B is mined.

Includes code by @jonasschnelli and @ryanofsky

src/wallet/wallet.cpp

@@ -411,6 +411,13 @@ set<uint256> CWallet::GetConflicts(const uint256& txid) const
     return result;
 }
 
+bool CWallet::HasWalletSpend(const uint256& txid) const
+{
+    AssertLockHeld(cs_wallet);
+    auto iter = mapTxSpends.lower_bound(COutPoint(txid, 0));
+    return (iter != mapTxSpends.end() && iter->first.hash == txid);
+}
+
 void CWallet::Flush(bool shutdown)
 {
     bitdb.Flush(shutdown);
@@ -826,6 +833,35 @@ void CWallet::MarkDirty()
     }
 }
 
+bool CWallet::MarkReplaced(const uint256& originalHash, const uint256& newHash)
+{
+    LOCK(cs_wallet);
+
+    auto mi = mapWallet.find(originalHash);
+
+    // There is a bug if MarkReplaced is not called on an existing wallet transaction.
+    assert(mi != mapWallet.end());
+
+    CWalletTx& wtx = (*mi).second;
+
+    // Ensure for now that we're not overwriting data
+    assert(wtx.mapValue.count("replaced_by_txid") == 0);
+
+    wtx.mapValue["replaced_by_txid"] = newHash.ToString();
+
+    CWalletDB walletdb(strWalletFile, "r+");
+
+    bool success = true;
+    if (!walletdb.WriteTx(wtx)) {
+        LogPrintf("%s: Updating walletdb tx %s failed", __func__, wtx.GetHash().ToString());
+        success = false;
+    }
+
+    NotifyTransactionChanged(this, originalHash, CT_UPDATED);
+
+    return success;
+}
+
 bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose)
 {
     LOCK(cs_wallet);
@@ -1981,6 +2017,37 @@ void CWallet::AvailableCoins(vector<COutput>& vCoins, bool fOnlyConfirmed, const
             if (nDepth == 0 && !pcoin->InMempool())
                 continue;
 
+            // We should not consider coins from transactions that are replacing
+            // other transactions.
+            //
+            // Example: There is a transaction A which is replaced by bumpfee
+            // transaction B. In this case, we want to prevent creation of
+            // a transaction B' which spends an output of B.
+            //
+            // Reason: If transaction A were initially confirmed, transactions B
+            // and B' would no longer be valid, so the user would have to create
+            // a new transaction C to replace B'. However, in the case of a
+            // one-block reorg, transactions B' and C might BOTH be accepted,
+            // when the user only wanted one of them. Specifically, there could
+            // be a 1-block reorg away from the chain where transactions A and C
+            // were accepted to another chain where B, B', and C were all
+            // accepted.
+            if (nDepth == 0 && fOnlyConfirmed && pcoin->mapValue.count("replaces_txid")) {
+                continue;
+            }
+
+            // Similarly, we should not consider coins from transactions that
+            // have been replaced. In the example above, we would want to prevent
+            // creation of a transaction A' spending an output of A, because if
+            // transaction B were initially confirmed, conflicting with A and
+            // A', we wouldn't want to the user to create a transaction D
+            // intending to replace A', but potentially resulting in a scenario
+            // where A, A', and D could all be accepted (instead of just B and
+            // D, or just A and A' like the user would want).
+            if (nDepth == 0 && fOnlyConfirmed && pcoin->mapValue.count("replaced_by_txid")) {
+                continue;
+            }
+
             for (unsigned int i = 0; i < pcoin->tx->vout.size(); i++) {
                 isminetype mine = IsMine(pcoin->tx->vout[i]);
                 if (!(IsSpent(wtxid, i)) && mine != ISMINE_NO &&