Wallet: null out the candidates list after selection so selectors can…

… edit the list if they want.

core/src/main/java/com/google/bitcoin/core/Wallet.java

@@ -1923,6 +1923,7 @@ public void completeTx(SendRequest req) throws InsufficientMoneyException {
                 checkState(req.tx.getOutputs().size() == 1, "Empty wallet TX must have a single output only.");
                 CoinSelector selector = req.coinSelector == null ? coinSelector : req.coinSelector;
                 bestCoinSelection = selector.select(NetworkParameters.MAX_MONEY, candidates);
+                candidates = null;  // Selector took ownership and might have changed candidates. Don't access again.
                 req.tx.getOutput(0).setValue(bestCoinSelection.valueGathered);
                 totalOutput = bestCoinSelection.valueGathered;
             }
@@ -3450,6 +3451,7 @@ public FeeCalculation(SendRequest req, BigInteger value, List<TransactionInput>
                 // Of the coins we could spend, pick some that we actually will spend.
                 CoinSelector selector = req.coinSelector == null ? coinSelector : req.coinSelector;
                 CoinSelection selection = selector.select(valueNeeded, candidates);
+                candidates = null;  // Selector took ownership and might have changed candidates. Don't access again.
                 // Can we afford this?
                 if (selection.valueGathered.compareTo(valueNeeded) < 0) {
                     valueMissing = valueNeeded.subtract(selection.valueGathered);

core/src/main/java/com/google/bitcoin/wallet/CoinSelector.java

@@ -12,5 +12,10 @@
  * enough money in the wallet.
  */
 public interface CoinSelector {
+    /**
+     * Creates a CoinSelection that tries to meet the target amount of value. The candidates list is given to
+     * this call and can be edited freely. See the docs for CoinSelection to learn more, or look a the implementation
+     * of {@link com.google.bitcoin.wallet.DefaultCoinSelector}.
+     */
     public CoinSelection select(BigInteger target, LinkedList<TransactionOutput> candidates);
 }