Remove factor of 3 from definition of dust.

This redefines dust to be the value of an output such that it would
cost that value in fees to (create and) spend the output at the dust
relay rate.  The previous definition was that it would cost 1/3 of the
value.  The default dust relay rate is correspondingly increased to
3000 sat/kB so the actual default dust output value of 546 satoshis
for a non-segwit output remains unchanged.  This commit is a refactor
only unless a dustrelayfee is passed on the commandline in which case
that number now needs to be increased by a factor of 3 to get the same
behavior.  -dustrelayfee is a hidden command line option.

Note: It's not exactly a refactor due to edge case changes in rounding
as evidenced by the required change to the unit test.

src/init.cpp

@@ -479,7 +479,7 @@ std::string HelpMessage(HelpMessageMode mode)
     if (showDebug) {
         strUsage += HelpMessageOpt("-acceptnonstdtxn", strprintf("Relay and mine \"non-standard\" transactions (%sdefault: %u)", "testnet/regtest only; ", defaultChainParams->RequireStandard()));
         strUsage += HelpMessageOpt("-incrementalrelayfee=<amt>", strprintf("Fee rate (in %s/kB) used to define cost of relay, used for mempool limiting and BIP 125 replacement. (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_INCREMENTAL_RELAY_FEE)));
-        strUsage += HelpMessageOpt("-dustrelayfee=<amt>", strprintf("Fee rate (in %s/kB) used to defined dust, the value of an output such that it will cost about 1/3 of its value in fees at this fee rate to spend it. (default: %s)", CURRENCY_UNIT, FormatMoney(DUST_RELAY_TX_FEE)));
+        strUsage += HelpMessageOpt("-dustrelayfee=<amt>", strprintf("Fee rate (in %s/kB) used to defined dust, the value of an output such that it will cost more than its value in fees at this fee rate to spend it. (default: %s)", CURRENCY_UNIT, FormatMoney(DUST_RELAY_TX_FEE)));
     }
     strUsage += HelpMessageOpt("-bytespersigop", strprintf(_("Equivalent bytes per sigop in transactions for relay and mining (default: %u)"), DEFAULT_BYTES_PER_SIGOP));
     strUsage += HelpMessageOpt("-datacarrier", strprintf(_("Relay and mine data carrier transactions (default: %u)"), DEFAULT_ACCEPT_DATACARRIER));

src/policy/policy.cpp

@@ -19,16 +19,18 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
 {
     // "Dust" is defined in terms of dustRelayFee,
     // which has units satoshis-per-kilobyte.
-    // If you'd pay more than 1/3 in fees
+    // If you'd pay more in fees than the value of the output
     // to spend something, then we consider it dust.
     // A typical spendable non-segwit 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*dustRelayFee/1000 (in satoshis).
+    // 182*dustRelayFee/1000 (in satoshis).
+    // 546 satoshis at the default rate of 3000 sat/kB.
     // A typical spendable segwit txout is 31 bytes big, and will
     // need a CTxIn of at least 67 bytes to spend:
     // so dust is a spendable txout less than
-    // 294*dustRelayFee/1000 (in satoshis).
+    // 98*dustRelayFee/1000 (in satoshis).
+    // 294 satoshis at the default rate of 3000 sat/kB.
     if (txout.scriptPubKey.IsUnspendable())
         return 0;
 
@@ -44,7 +46,7 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
         nSize += (32 + 4 + 1 + 107 + 4); // the 148 mentioned above
     }
 
-    return 3 * dustRelayFeeIn.GetFee(nSize);
+    return dustRelayFeeIn.GetFee(nSize);
 }
 
 bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)

src/policy/policy.h

@@ -40,12 +40,12 @@ static const unsigned int MAX_STANDARD_P2WSH_STACK_ITEMS = 100;
 static const unsigned int MAX_STANDARD_P2WSH_STACK_ITEM_SIZE = 80;
 /** The maximum size of a standard witnessScript */
 static const unsigned int MAX_STANDARD_P2WSH_SCRIPT_SIZE = 3600;
-/** Min feerate for defining dust. Historically this has been the same as the
+/** Min feerate for defining dust. Historically this has been based on the
  * minRelayTxFee, however changing the dust limit changes which transactions are
  * standard and should be done with care and ideally rarely. It makes sense to
  * only increase the dust limit after prior releases were already not creating
  * outputs below the new threshold */
-static const unsigned int DUST_RELAY_TX_FEE = 1000;
+static const unsigned int DUST_RELAY_TX_FEE = 3000;
 /**
  * Standard script verification flags that standard transactions will comply
  * with. However scripts violating these flags may still be present in valid

src/test/transaction_tests.cpp

@@ -692,7 +692,7 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
     BOOST_CHECK(IsStandardTx(t, reason));
 
     // Check dust with default relay fee:
-    CAmount nDustThreshold = 182 * dustRelayFee.GetFeePerK()/1000 * 3;
+    CAmount nDustThreshold = 182 * dustRelayFee.GetFeePerK()/1000;
     BOOST_CHECK_EQUAL(nDustThreshold, 546);
     // dust:
     t.vout[0].nValue = nDustThreshold - 1;
@@ -702,13 +702,13 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
     BOOST_CHECK(IsStandardTx(t, reason));
 
     // Check dust with odd relay fee to verify rounding:
-    // nDustThreshold = 182 * 1234 / 1000 * 3
-    dustRelayFee = CFeeRate(1234);
+    // nDustThreshold = 182 * 3702 / 1000
+    dustRelayFee = CFeeRate(3702);
     // dust:
-    t.vout[0].nValue = 672 - 1;
+    t.vout[0].nValue = 673 - 1;
     BOOST_CHECK(!IsStandardTx(t, reason));
     // not dust:
-    t.vout[0].nValue = 672;
+    t.vout[0].nValue = 673;
     BOOST_CHECK(IsStandardTx(t, reason));
     dustRelayFee = CFeeRate(DUST_RELAY_TX_FEE);