sweep/sweeper_test: add test for reorderd inputs

This add a test for inputs that gets re-ordered because the inputs with
required TxOuts must be added first.

We add a new step to the test that checks that all inputs were signed at
the correct tx input index.

This test would fail without the previous commit.

sweep/sweeper_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
@@ -1617,6 +1618,60 @@ func (i *testInput) RequiredTxOut() *wire.TxOut {
 	return i.reqTxOut
 }
 
+// CraftInputScript is a custom sign method for the testInput type that will
+// encode the spending outpoint and the tx input index as part of the returned
+// witness.
+func (i *testInput) CraftInputScript(_ input.Signer, txn *wire.MsgTx,
+	hashCache *txscript.TxSigHashes, txinIdx int) (*input.Script, error) {
+
+	// We'll encode the outpoint in the witness, so we can assert that the
+	// expected input was signed at the correct index.
+	op := i.OutPoint()
+	return &input.Script{
+		Witness: [][]byte{
+			// We encode the hash of the outpoint...
+			op.Hash[:],
+			// ..the outpoint index...
+			{byte(op.Index)},
+			// ..and finally the tx input index.
+			{byte(txinIdx)},
+		},
+	}, nil
+}
+
+// assertSignedIndex goes through all inputs to the tx and checks that all
+// testInputs have witnesses corresponding to the outpoints they are spending,
+// and are signed at the correct tx input index. All found testInputs are
+// returned such that we can sum up and sanity check that all testInputs were
+// part of the sweep.
+func assertSignedIndex(t *testing.T, tx *wire.MsgTx,
+	testInputs map[wire.OutPoint]*testInput) map[wire.OutPoint]struct{} {
+
+	found := make(map[wire.OutPoint]struct{})
+	for idx, txIn := range tx.TxIn {
+		op := txIn.PreviousOutPoint
+
+		// Not a testInput, it won't have the test encoding we require
+		// to check outpoint and index.
+		if _, ok := testInputs[op]; !ok {
+			continue
+		}
+
+		if _, ok := found[op]; ok {
+			t.Fatalf("input already used")
+		}
+
+		// Check it was signes spending the correct outpoint, and at
+		// the expected tx input index.
+		require.Equal(t, txIn.Witness[0], op.Hash[:])
+		require.Equal(t, txIn.Witness[1], []byte{byte(op.Index)})
+		require.Equal(t, txIn.Witness[2], []byte{byte(idx)})
+		found[op] = struct{}{}
+	}
+
+	return found
+}
+
 // TestLockTimes checks that the sweeper properly groups inputs requiring the
 // same locktime together into sweep transactions.
 func TestLockTimes(t *testing.T) {
@@ -1824,6 +1879,66 @@ func TestRequiredTxOuts(t *testing.T) {
 				)
 			},
 		},
+		{
+			// Two inputs, where the first one required no tx out.
+			name: "two inputs, one with required tx out",
+			inputs: []*testInput{
+				{
+
+					// We add a normal, non-requiredTxOut
+					// input. We use test input 10, to make
+					// sure this has a higher yield than
+					// the other input, and will be
+					// attempted added first to the sweep
+					// tx.
+					BaseInput: inputs[10],
+				},
+				{
+					// The second input requires a TxOut.
+					BaseInput: inputs[0],
+					reqTxOut: &wire.TxOut{
+						PkScript: []byte("aaa"),
+						Value:    inputs[0].SignDesc().Output.Value,
+					},
+				},
+			},
+
+			// We expect the inputs to have been reordered.
+			assertSweeps: func(t *testing.T,
+				_ map[wire.OutPoint]*testInput,
+				txs []*wire.MsgTx) {
+
+				require.Equal(t, 1, len(txs))
+
+				tx := txs[0]
+				require.Equal(t, 2, len(tx.TxIn))
+				require.Equal(t, 2, len(tx.TxOut))
+
+				// The required TxOut should be the first one.
+				out := tx.TxOut[0]
+				require.Equal(t, []byte("aaa"), out.PkScript)
+				require.Equal(
+					t, inputs[0].SignDesc().Output.Value,
+					out.Value,
+				)
+
+				// The first input should be the one having the
+				// required TxOut.
+				require.Len(t, tx.TxIn, 2)
+				require.Equal(
+					t, inputs[0].OutPoint(),
+					&tx.TxIn[0].PreviousOutPoint,
+				)
+
+				// Second one is the one without a required tx
+				// out.
+				require.Equal(
+					t, inputs[10].OutPoint(),
+					&tx.TxIn[1].PreviousOutPoint,
+				)
+			},
+		},
+
 		{
 			// An input committing to an output of equal value, just
 			// add input to pay fees.
@@ -2076,6 +2191,30 @@ func TestRequiredTxOuts(t *testing.T) {
 
 			// Assert the transactions are what we expect.
 			testCase.assertSweeps(t, inputs, sweeps)
+
+			// Finally we assert that all our test inputs were part
+			// of the sweeps, and that they were signed correctly.
+			sweptInputs := make(map[wire.OutPoint]struct{})
+			for _, sweep := range sweeps {
+				swept := assertSignedIndex(t, sweep, inputs)
+				for op := range swept {
+					if _, ok := sweptInputs[op]; ok {
+						t.Fatalf("outpoint %v part of "+
+							"previous sweep", op)
+					}
+
+					sweptInputs[op] = struct{}{}
+				}
+			}
+
+			require.Equal(t, len(inputs), len(sweptInputs))
+			for op := range sweptInputs {
+				_, ok := inputs[op]
+				if !ok {
+					t.Fatalf("swept input %v not part of "+
+						"test inputs", op)
+				}
+			}
 		})
 	}
 }