Merge #10817: Redefine Dust and add a discard_rate

f4d00e6 Add a discard_rate (Alex Morcos)
b138585 Remove factor of 3 from definition of dust. (Alex Morcos)

Pull request description:

  The definition of dust is redefined to remove the factor of 3.

  Dust is redefined 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.

  A discard_rate is added which defaults to 10,000 sat/kB

  Any change output which would be dust at the discard_rate you are
  willing to discard completely and add to fee (as well as continuing to
  pay the fee that would have been needed for creating the change)

  This would be a nice addition for 0.15 and I think will remain useful for 0.16 with the new coin selection algorithms in discussion, but its not crucial.

  It does add translation strings, but we could (should?) avoid that by hiding the option

Tree-SHA512: 5b6f655354d0ab6b8b6cac1e8d1fe3136d10beb15c6d948fb15bfb105155a9d03684c6240624039b3eed6428b7e60e54216cc8b2f90c4600701e39f646284a9b

2 files changed, 8 insertions, 6 deletions

diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
index 9f2d623e76..605e3e0696 100644
--- a/src/policy/policy.cpp
+++ b/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)
diff --git a/src/policy/policy.h b/src/policy/policy.h
index 2c2ea9d5b8..c06820f84e 100644
--- a/src/policy/policy.h
+++ b/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