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A few days ago I was sending my friend a payment of 1.2 BTC.My account had only slightly more than this amount available, not even enough for recommended transaction fee.Instead of borrowing or waiting for a miner to generate more, I decided I would push a transaction with a much smaller fee to the network and hope it gets confirmed in under a day.It did not happen.According to Bitcoin wiki, a transaction needs to pay the recommended fee unless all outputs are more than 0.1 BTC and few other conditions.I did not meet this condition, because I had one 1.2 BTC output and one change which was smaller than transaction fee.It was stuck there for more than a day, my mining pool paid me, so I decided I want to just cancel this transaction and create a new one with a proper fee.Easier said than done :).I use Electrum as a client - it is much better than stock Bitcoin client and there’s a nice console.I was able to extract the transaction and try to modify it to include the fee.

The interface is not so nice, or at least I am too lame.I got the transaction as a JSON structure from Wallet object by transaction hash.I was not able to easily create a different transaction without going through manually finding keys to sign.mktx does not work, because I don’t have enough unspent outputs (same as paying from the GUI).createrawtransaction is something I managed to do, but then for signing, I would need a list of private keys, addresses and scripts.My transaction had more than 10 inputs and I was too lazy to find which keys belonged to the other addresses (and if it’s possible to do it programatically, there should be a function to do it - I guess that’s what mktx does internally too).I guess it should work with less parameters per documentation, but the console call insisted I fill all the parameters.I found out a very lame and easy solution.I thought if Electrum does not see the old transaction, it can spend the inputs again.

So I changed wallet.py.There’s a function called update_tx_outputs that takes a tx_hash and updates a list of spent outputs.I modified it like this: So if my transaction (the hash is from other stuck transaction I found online) is found, it’s skipped, so Electrum thinks the outputs are not spent.how to set up pooled bitcoin mining in windowsI thought I would just pay, but the transaction got refused by Electrum server, because it thought it’s a double spend (which was correct).bitcoins per pscSo I used mktx and used Coinb.in’s wonderful Raw Transaction utilities to broadcast the hash to the network.bitcoin calculator next difficultyIt also returned that the transaction is invalid, but it propagated anyway and a miner included it in a block.c span bitcoin

I found out that blockchain.info reports on attempts to double spend when I look at a transaction or address and recommends you proceed with caution.Takeaways: Pay the transaction fee, really.buy bitcoin with echeckNodes try to refuse double spends (they cache stuck transactions), but the transaction eventually propagates.bitcoin double spend exampleElectrum could have much nicer Python interface for things.I guess Amir’s sx command-line utility would make my life much easier, but I had no time to upgrade my g++ toolchain, it does not compile on any system I own.Installing Ubuntu or ArchLinux just because I want to play with it takes a lot of time.I tried fixing a few of the main problems of libbitcoin not compiling on OS X, but I had no more time doing it.I would love to use sx with OS X or Scientific Linux someday.

Playing with Electrum and it’s internals is fun too though.Why is my transaction 'Pending'?This article is for digital currency transactions sent from your Coinbase wallet.If you're wondering about a pending purchase, you can learn more here.Once a transaction is verified, it will show Complete in green.This lets you know that the transaction cannot be reversed and that the funds can be withdrawn.Coinbase runs its own digital currency nodes Sending a very small amount with insufficient fees All bitcoin transactions require a miner fee to be confirmed by the bitcoin network.When sending bitcoin you will be prompted to add a miner fee to make sure the transaction gets confirmed.If an insufficient fee is included, your transaction may never confirm.A double-spend of the same coins Under normal conditions this should not occur with transactions generated by Coinbase (our backend systems are designed to prevent this), but in general if two transactions try to spend the same coins, one of them will never be confirmed by the network.