tx_memory_pool: make double spends a no-drop offense

[jeffro256]

Nodes who see different txs in a double spend attack will drop each other, splitting the network. Issue found by @Boog900.

[UkoeHB]

Doesn't this change make it easier to do large-scale double-spend attacks, because now nodes that originate double-spend attacks will stay connected (and hence have higher throughput)?

[SChernykh]

Nodes that originate double spends don't get disconnected even with the old code because their "neighbours" haven't seen the other transaction as well.

The only nodes that suffer from disconnects are the ones where both "tx wavefronts" meet, and those are not the attacker's nodes in 99% of cases.

[jeffro256]

Let's say that we have 10 nodes with connections:

A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J

The attacker can start propagating tx T1 at node A and double-spend tx T2 at node J. The flow of txs over time could look like this:

T1                                                    T2
A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J
---------------------------------------------------------------
T1    T1                                        T2    T2
A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J
---------------------------------------------------------------
T1    T1    T1                            T2    T2    T2
A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J
---------------------------------------------------------------
T1    T1    T1    T1                T2    T2    T2    T2
A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J
---------------------------------------------------------------
T1    T1    T1    T1    T1    T2    T2    T2    T2    T2
A <-> B <-> C <-> D <-> E <-> F <-> G <-> H <-> I <-> J
---------------------------------------------------------------

At this point E and F will drop each other, despite not being close to the origin of the double spend, and we have 2 split networks. In the real world, relationships are much more complicated, so it won't be this easy, but the point remains.

[jeffro256]

We could implement a long term mitigation against spamming double spends by keeping a map of key images -> txid per host and checking if if 1 host sent 2 different txs that share 1 key image. Then we could block them