Fix race condition in chain exchange

[lemmih]

Issue summary

The handle_chain_exchange_request function downloads blocks (aka tipsets) from peers. It does this by sending the same request to multiple peers and accepting the first valid response. However, a concurrency bug is triggered if the number of peers is low.

As seen in the code snippet below, the function waits for a valid result OR all the tasks to finish. If all the tasks finish, it assumes no valid responses and returns an error. This logic needs to be corrected: We should only fail when we've received invalid responses from all peers.

Other information and links

forest/blockchain/chain_sync/src/network_context.rs

Lines 275 to 282 in 2f6025d

	tokio::select! {
	result = result_rx.recv_async() => {
	tasks.abort_all();
	log::debug!("Succeed: handle_chain_exchange_request");
	result.map_err(|e| e.to_string())?
	},
	_ = wait_all(&mut tasks) => return Err(make_failure_message()),
	}

[lemmih]

@elmattic Do you want to have a look at this one?

[elmattic]

Sure.

[hanabi1224]

As a side note, the error message in #2755 looks related