Bugfix: don't make collision from "tried" a feeler #19906

The purpose of feelers is to move addrs from "new" to "tried." We should not select a node evicted from collisions, as they are already "tried".

Can you explain a bit more what this PR is trying to accomplish? Is this just about naming conventions, or something else?

@sdaftuar does the first commit message not sound clear?

If we're going to connect to a feeler (as decided by a timer),
we should not "waste" this iteration on connecting to a collision.
The collisions belong to the "tried" table, so connecting to them
don't accomplish the goal of feelers.

We are supposed to attempt a feeler "transition" every 2 minutes. But instead, when a timer hits, we might connect to a tried address (from tried collisions), instead of a feeler. And the next feeler will be attempted after 2 more minutes.

I don't think it's exploitable: an attacker can't force us to do this all the time. It's just currently logically incorrect.

Got it, thanks for clarifying -- I didn't realize that the concern was around the frequency that we pull things out of the new table to potentially move them to the tried table.

I think the underlying question here is how often we want to be making outbound connections to the network (this is a question I was thinking about in the context of #19858 as well). Both uses of FEELER connections today (moving things from new to tried, as well as the test-before-evict logic of resolving tried-table collisions) have the same property that their connections last a very short time. So I think that we could ask ourselves, given a certain rate at which we're comfortable making new outbound connections, how many of those should go to the test-before-evict logic, and how many to the logic that is pulling entries from the new-table?

In practice, it seems to me that tried-table collisions are relatively infrequent, probably less than a few times per day on nodes I've looked at, if I remember right. So the effect of this on the new-table FEELER logic is low. On the other hand, if it were to have a big effect somehow, then changing the logic so that we still make feeler connections to new-table peers at the once-every-2-minute rate will mean we're connecting to the network much more (assuming that we always make the test-before-evict connections when we need to).

So my inclination is that we don't need this change? Or if we do want it, we should just put a rate limit in place so that we can reason about what bounds are achievable in the event of tried-table-collisions.

BTW, the logic presented in this PR is incorrect, I think, because it only tries to resolve tried-table-collisions when we'd make a non-FEELER connection. However, once we have all our outbound peers in place, we never try to make new non-FEELER connections (well, at least until #19858), so I think this basically breaks the test-before-evict logic as drafted now.

We currently have the following documentation:

    /**
     * Feeler connections are short lived connections used to increase the
     * number of connectable addresses in our AddrMan. Approximately every
     * FEELER_INTERVAL, we attempt to connect to a random address from the new
     * table. If successful, we add it to the tried table.
     */
    FEELER,

It's not accurate since test-before-evict is not about new->tried, and it's not mentioned here. I'm pretty sure it may be inaccurate in other places as well, I didn't even know that this is done via feelers before you told me so here.

Considering what's above, this PR attempted (not so well yet) to separate these two uses, and make feelers do new->tried exclusively.

Now that I know that this other case was actually intended (at least by the author), there are 2 paths we may take:

• update the documentation to mention that case as well (+ maybe don't reset feeler timer when resolve collisions, related to #19869)
• OR have a new conn type @sdaftuar what do you think? Perhaps I'll sketch what would the latter option look like in the code later.

Now that I know that this other case was actually intended (at least by the author), there are 2 paths we may take:

• update the documentation to mention that case as well (+ maybe don't reset feeler timer when resolve collisions, related to #19869)
• OR have a new conn type

I agree that it's confusing that the term "feeler" has encompassed both behaviors (perhaps @EthanHeilman has a view as well?) In Eclipse Attacks on Bitcoin’s Peer-to-Peer Network, the "feeler" and "test-before-evict" countermeasures do not seem to use overlapping terminology, so my suggestion is that our code be clearer about mapping to those countermeasure names as well.

My suggestion would be that we rename ConnectionType::FEELER to something like ConnectionType::ADDRMAN_HELPER or ConnectionType::TEST_CONNECT or something to indicate either that the connection is short-lived, or just for the addrman, or both. And then document in our code that we use this brief-addrman-helper conn_type for both the feeler and test-before-evict peer selection logic.

I think we could also rename the feeler timer to be the "addrman-helper-timer", and differentiate the two behaviors in code comments better, if that would make the logic clearer.

I don't think it makes sense to add a second conn_type that has exactly the same properties as the first, just with a different name. Perhaps if we thought those behaviors might diverge in the future, it might make sense; but I don't anticipate that myself.

closing in favour of #19958