fix(purchasing): fix crash completing future more than once

[markspanbroek]

Fixes #962

[benbierens]

\o/

[dryajov]

Is the callback supposed to be called more than once, if not this is probably an issue somewhere else?

[emizzle]

Agreed. This smells like a subscription callback or ethers error, or maybe the event is indeed being fired more than once in the contract.

The root issue will be masked by this change. I don't think it's a big deal personally, but it could potentially hide other issues.

[markspanbroek]

Agreed that it should not be called more than once. However it might be called more than once if the JSON-RPC provider decides to invoke the callback more than once for whatever reason (chain reorg?), so we'd better not crash if it does 😄

[emizzle]

Thanks Mark. As with #1248, this is a fix for preventing the node from crashing, but it doesn't quite explain why this subscription callback is being called twice. Therefore, I'd be hesitant to close #962 when merging this.

[markspanbroek]

I took a look into market.nim the nim-ethers subscription handling, and a possible reason why it could be invoked more than once could be because of the resubscribing to events when we encounter an error in the JSON-RPC response when polling. A good fix for that would be to implement codex-storage/nim-ethers#96.