Re: 300 BTC Coding Contest: Distributed Exchange (MasterCoin Developer Thread)

Quote from: dexX7 on March 16, 2014, 08:10:25 PM

Quote from: zathras on March 16, 2014, 07:22:50 PM

Additionally because there are a number of them, the size is actually over 1KB. Thus we have a small input value and a (relatively) large byte size for the transaction.

This is the most important part related to this transaction. Over 1 KB size = 0.0002 fee required to be treated as standard transaction. If this was issued via one of the wallets, the wallet creator needs to adjust to take transaction size into consideration asap. Delayed transactions due to insufficient fees can be a game breaker, if MSC are bought via DEX and the time window closes before the transaction confirms.

That's actually a really interesting problem - especially in relation to these fractional transactions. We build the transaction before we can see its total size, but by then we've already selected the inputs. As we're talking such tiny amounts (often inputs of 0.00006) increasing the fee by 0.0001 likely means we have to add another/more inputs to cover the increased fee, but that has now changed the size of our transaction again so we may need to yet again go back and add more inputs & more fees and so on. Fun game

In all seriousness though I do agree with DexX - changes to payments make me nervous but it should be safe. I've pushed up a shortcut fix which grows the fee during transaction build using simple (very conservative 200 bytes per input) guestimation based on total number of inputs at that point in the transaction build. Should take care of any potential risk for now until I can be a bit more classy with input selection

Code:

'sanity check input count is not >24
   If inputcount > 24 Then
   MsgBox("ERROR: Input count >24, temporary restriction applied - library currently will not send transactions with over 24 inputs. Aborting")
   Exit Function
   End If
   'quick fix for avoiding large transactions with small fees - come back & revisit this
   If inputcount <= 4 Then totaltxfee = 17000
   If inputcount > 4 Then totaltxfee = 27000
   If inputcount > 9 Then totaltxfee = 37000
   If inputcount > 14 Then totaltxfee = 47000
   If inputcount > 19 Then totaltxfee = 57000
   'recalculate totalout
   totalout = totaltxfee + paymentamount
   'do we have enough yet?
   If inputsum >= totalout + 6000 Then Exit For

Note this only applies to DEx payment transactions, all other transaction types Masterchest selects a single input only to cover fees.

Thanks
Zathras