Remove factor of 3 from definition of dust.

>>> backports bitcoin/bitcoin@b138585

src/init.cpp

@@ -561,7 +561,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-minrelaytxfee=<amt>", strprintf(_("Fees (in %s/Kb) smaller than this are considered zero fee for relaying, mining and transaction creation (default: %s)"), CURRENCY_UNIT, FormatMoney(::minRelayTxFee.GetFeePerK())));
     strUsage += HelpMessageOpt("-printtoconsole", strprintf(_("Send trace/debug info to console instead of debug.log file (default: %u)"), 0));
     if (showDebug) {
-        strUsage += HelpMessageOpt("-dustrelayfee=<amt>", strprintf("Fee rate (in %s/kB) used to define 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 define 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("-printpriority", strprintf(_("Log transaction priority and fee per kB when mining blocks (default: %u)"), DEFAULT_PRINTPRIORITY));
     }
     strUsage += HelpMessageOpt("-shrinkdebugfile", _("Shrink debug.log file on client startup (default: 1 when no -debug)"));

src/policy/policy.cpp

@@ -22,7 +22,7 @@ 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 txout is 34 bytes big, and will
     // need a CTxIn of at least 148 bytes to spend:
@@ -33,13 +33,13 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
 
     size_t nSize = GetSerializeSize(txout, SER_DISK, 0);
     nSize += (32 + 4 + 1 + 107 + 4); // the 148 mentioned above
-    return 3 * dustRelayFeeIn.GetFee(nSize);
+    return dustRelayFeeIn.GetFee(nSize);
 }
 
 CAmount GetDustThreshold(const CFeeRate& dustRelayFeeIn)
 {
     // return the dust threshold for a typical 34 bytes output
-    return 3 * dustRelayFeeIn.GetFee(182);
+    return dustRelayFeeIn.GetFee(182);
 }
 
 bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
@@ -50,9 +50,9 @@ bool IsDust(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
 CAmount GetShieldedDustThreshold(const CFeeRate& dustRelayFeeIn)
 {
     unsigned int K = DEFAULT_SHIELDEDTXFEE_K;   // Fixed (100) for now
-    return 3 * K * dustRelayFeeIn.GetFee(SPENDDESCRIPTION_SIZE +
-                                         CTXOUT_REGULAR_SIZE +
-                                         BINDINGSIG_SIZE);
+    return K * dustRelayFeeIn.GetFee(SPENDDESCRIPTION_SIZE +
+                                     CTXOUT_REGULAR_SIZE +
+                                     BINDINGSIG_SIZE);
 }
 
 /**

src/policy/policy.h

@@ -29,12 +29,12 @@ static const unsigned int DEFAULT_MAX_MEMPOOL_SIZE = 300;
 /** Default for -permitbaremultisig */
 static const bool DEFAULT_PERMIT_BAREMULTISIG = true;
 extern bool fIsBareMultisigStd;
-/** 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 = 10000;
+static const unsigned int DUST_RELAY_TX_FEE = 30000;
 
 /**
  * Standard script verification flags that standard transactions will comply

src/test/transaction_tests.cpp

@@ -425,11 +425,26 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
     std::string reason;
     BOOST_CHECK(IsStandardTx(t, 0, reason));
 
-    t.vout[0].nValue = 5011; // dust
+    // Check dust with default relay fee:
+    CAmount nDustThreshold = GetDustThreshold(dustRelayFee);
+    BOOST_CHECK_EQUAL(nDustThreshold, 5460);
+    // dust:
+    t.vout[0].nValue = nDustThreshold - 1;
     BOOST_CHECK(!IsStandardTx(t, 0, reason));
+    // not dust:
+    t.vout[0].nValue = nDustThreshold;
+    BOOST_CHECK(IsStandardTx(t, 0, reason));
 
-    t.vout[0].nValue = 6011; // not dust
+    // Check dust with odd relay fee to verify rounding:
+    // nDustThreshold = 182 * 3702 / 1000
+    dustRelayFee = CFeeRate(3702);
+    // dust:
+    t.vout[0].nValue = 673 - 1;
+    BOOST_CHECK(!IsStandardTx(t, 0, reason));
+    // not dust:
+    t.vout[0].nValue = 673;
     BOOST_CHECK(IsStandardTx(t, 0, reason));
+    dustRelayFee = CFeeRate(DUST_RELAY_TX_FEE);
 
     t.vout[0].scriptPubKey = CScript() << OP_1;
     BOOST_CHECK(!IsStandardTx(t, 0, reason));