txscript: New function IsUnspendable

IsUnspendable takes a public key script and returns whether it is
spendable.

Additionally, hook this into the mempool isDust function, since
unspendable outputs can't be spent.

This mimics Bitcoin Core commit 0aad1f13b2430165062bf9436036c1222a8724da

mempool.go

@@ -107,6 +107,11 @@ type txMemPool struct {
 // relay fee.  In particular, if the cost to the network to spend coins is more
 // than 1/3 of the minimum transaction relay fee, it is considered dust.
 func isDust(txOut *wire.TxOut) bool {
+	// Unspendable outputs are considered dust.
+	if txscript.IsUnspendable(txOut.PkScript) {
+		return true
+	}
+
 	// The total serialized size consists of the output and the associated
 	// input script to redeem it.  Since there is no input script
 	// to redeem it yet, use the minimum size of a typical input script.

txscript/script.go

@@ -458,3 +458,15 @@ func GetPreciseSigOpCount(scriptSig, scriptPubKey []byte, bip16 bool) int {
 	shPops, _ := parseScript(shScript)
 	return getSigOpCount(shPops, true)
 }
+
+// IsUnspendable returns whether the passed public key script is unspendable, or
+// guaranteed to fail at execution.  This allows inputs to be pruned instantly
+// when entering the UTXO set.
+func IsUnspendable(pkScript []byte) bool {
+	pops, err := parseScript(pkScript)
+	if err != nil {
+		return true
+	}
+
+	return len(pops) > 0 && pops[0].opcode.value == OP_RETURN
+}

txscript/script_test.go

@@ -486,3 +486,38 @@ func TestIsPushOnlyScript(t *testing.T) {
 			"%v", test.name, true, test.expected)
 	}
 }
+
+// TestIsUnspendable ensures the IsUnspendable function returns the expected
+// results.
+func TestIsUnspendable(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		pkScript []byte
+		expected bool
+	}{
+		{
+			// Unspendable
+			pkScript: []byte{0x6a, 0x04, 0x74, 0x65, 0x73, 0x74},
+			expected: true,
+		},
+		{
+			// Spendable
+			pkScript: []byte{0x76, 0xa9, 0x14, 0x29, 0x95, 0xa0,
+				0xfe, 0x68, 0x43, 0xfa, 0x9b, 0x95, 0x45,
+				0x97, 0xf0, 0xdc, 0xa7, 0xa4, 0x4d, 0xf6,
+				0xfa, 0x0b, 0x5c, 0x88, 0xac},
+			expected: false,
+		},
+	}
+
+	for i, test := range tests {
+		res := txscript.IsUnspendable(test.pkScript)
+		if res != test.expected {
+			t.Errorf("TestIsUnspendable #%d failed: got %v want %v",
+				i, res, test.expected)
+			continue
+		}
+	}
+}