payout: say what actually failed instead of blaming transaction size

SendMoney's "oversized transaction that requires a transaction fee of
0.00" is not an oversized transaction and there is no fee. That branch is
taken when nValue + nFeeRequired > GetBalance() -- an empty spendable
balance -- and the fee reads 0.00 because CreateTransaction failed before
setting one. 3,267 of these in debug.log sent me looking for a
transaction-size problem that does not exist. Report what is short, by how
much, and against what balance.

Name COINBASE_SPENDABLE_DEPTH in main.h and derive GetBlocksToMaturity
from it, because the distinction it captures is genuinely surprising:
CMerkleTx::GetCredit values a mined coinbase at zero until
COINBASE_MATURITY+20 deep, not COINBASE_MATURITY.

I first changed the payout gate to that deeper value, on the theory that
each payout was firing 20 blocks before its own coinbase could be spent.
Measuring it refuted that: across 934 payments, 96% succeeded at depth
100, the moment they were queued, which is only possible because a payout
is funded from whatever coin is already mature rather than from its own
block. The gate stays at COINBASE_MATURITY, with a comment recording the
measurement so the next person does not re-make the same change.

The real shortfall is structural and not addressed here: the pool pays out
~99% of each block's subsidy while only matured coinbases are spendable,
so it runs with almost no buffer, and any orphaned block it queued a
payout for drains that buffer permanently. The 24h retry loop absorbs this
-- 0 payouts forfeited -- but the buffer is what needs fixing.

(cherry picked from commit e28df01f90d621b0cd818f28d70a20b8ab5d1a8d)

1 files changed, 13 insertions, 2 deletions

diff --git a/src/main.cpp b/src/main.cpp
index f84f209..382a008 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1513,7 +1513,7 @@ int CMerkleTx::GetBlocksToMaturity() const
 {
     if (!IsCoinBase())
         return 0;
-    return max(0, (COINBASE_MATURITY+20) - GetDepthInMainChain());
+    return max(0, COINBASE_SPENDABLE_DEPTH - GetDepthInMainChain());
 }
 
 
@@ -4812,7 +4812,18 @@ bool SendMoney(CScript scriptPubKey, int64 nValue, CWalletTx& wtxNew)
         {
             string strError;
             if (nValue + nFeeRequired > GetBalance())
-                strError = strprintf("Error: This is an oversized transaction that requires a transaction fee of %s ", FormatMoney(nFeeRequired).c_str());
+                // Not an oversized transaction -- this branch is simply "the
+                // spendable balance is too small". Saying otherwise (with a fee
+                // of 0.00, because CreateTransaction failed before setting one)
+                // sends anyone reading the log after a failed payout hunting a
+                // transaction-size problem that does not exist.
+                strError = strprintf("Error: insufficient spendable balance:"
+                                     " need %s (plus fee %s) but only %s is spendable"
+                                     " -- mined coin counts as zero until it is %d deep",
+                                     FormatMoney(nValue).c_str(),
+                                     FormatMoney(nFeeRequired).c_str(),
+                                     FormatMoney(GetBalance()).c_str(),
+                                     COINBASE_SPENDABLE_DEPTH);
             else
                 strError = "Error: Transaction creation failed ";
             return error("SendMoney() : %s\n", strError.c_str());