transaction_tests: Be more strict checking dust

* Don't allow off-by-one or more
* Make clear dust is coupled with minRelayTxFee
* Check rounding for odd values

src/primitives/transaction.h

@@ -143,8 +143,8 @@ public:
         // to spend something, then we consider it dust.
         // A typical spendable txout is 34 bytes big, and will
         // need a CTxIn of at least 148 bytes to spend:
-        // so dust is a spendable txout less than 546 satoshis
-        // with default minRelayTxFee.
+        // so dust is a spendable txout less than
+        // 546*minRelayTxFee/1000 (in satoshis)
         if (scriptPubKey.IsUnspendable())
             return 0;
 

src/test/transaction_tests.cpp

@@ -342,12 +342,27 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
     string reason;
     BOOST_CHECK(IsStandardTx(t, reason));
 
-    t.vout[0].nValue = 501; // dust
+    // Check dust with default relay fee:
+    CAmount nDustThreshold = 182 * minRelayTxFee.GetFeePerK()/1000 * 3;
+    BOOST_CHECK_EQUAL(nDustThreshold, 546);
+    // dust:
+    t.vout[0].nValue = nDustThreshold - 1;
     BOOST_CHECK(!IsStandardTx(t, reason));
-
-    t.vout[0].nValue = 2730; // not dust
+    // not dust:
+    t.vout[0].nValue = nDustThreshold;
     BOOST_CHECK(IsStandardTx(t, reason));
 
+    // Check dust with odd relay fee to verify rounding:
+    // nDustThreshold = 182 * 1234 / 1000 * 3
+    minRelayTxFee = CFeeRate(1234);
+    // dust:
+    t.vout[0].nValue = 672 - 1;
+    BOOST_CHECK(!IsStandardTx(t, reason));
+    // not dust:
+    t.vout[0].nValue = 672;
+    BOOST_CHECK(IsStandardTx(t, reason));
+    minRelayTxFee = CFeeRate(1000);
+
     t.vout[0].scriptPubKey = CScript() << OP_1;
     BOOST_CHECK(!IsStandardTx(t, reason));