How to check if an output has been spent?

Question

Using an rpc client (e.g. bitcoin-cli), for a given transaction and index of an output of it, how can I tell if the output is currently an utxo, or if it has already been spent?

In the latter case, I'd also want to know the transaction which spent it.

I looked at the gettxout command, but it isn't clear to me how to extract the answer from the data it returns.

I'm running a full node with txindex=1.

Answer 1

If gettxout returns something, the output is unspent. If it returns nothing, the output either never existed or is spent.

You do not need -txindex for this.

Answer 2

As @pieter-wuille already answered the first half of your question:

If gettxout returns something, the output is unspent. If it returns nothing, the output either never existed or is spent.

For that I'd add you can set the unconfirmed flag true, so it factors in unconfirmed transactions, too.

To answer the second part of your question, here's a sub-optimal way to find the transaction which spent your txo:
This assumes txindex=1 and prune=0.

To find out who spent the txout you can figure out the block it was confirmed with: getrawtransaction {txid} 1. From here on you can iterate through all the blocks to the last arrived one (getblock) and the mempool (getrawmempool), while checking every transaction:

foreach(var input in tx.inputs)
{
   if(input.prevout.txid == myTxo.txid && input.prevout.index == myTxo.index)
   {
      // tx spent the txo you were looking for
   }
}