rpc: update updatechanpolicy to allow zero fees

[Roasbeef]

Fixes #3138.

[cfromknecht]

comment looks out of date?

[cfromknecht]

kek should have been more clear that it also says "we'll ensure that the passed fee rate is below 1e-6", instead of above or equal to the minimum.

[Roasbeef]

PTAL @cfromknecht

[cfromknecht]

LGTM 🚀, one non-blocking comment

[cfromknecht]

kek should have been more clear that it also says "we'll ensure that the passed fee rate is below 1e-6", instead of above or equal to the minimum.

[ZapUser77]

Doesn't a fee of 0, 0 break the pathfinding algo?

[cfromknecht]

@ZapUser77 nope, fees aren’t the only consideration in determining distance. That will be even more true with #2802

[ZapUser77]

I remember running into this specific problem. The EdgeWeight can return zero if fee = zero, because INT64 is used instead of float:

func edgeWeight(lockedAmt lnwire.MilliSatoshi, fee lnwire.MilliSatoshi,
timeLockDelta uint16) int64 {
// timeLockPenalty is the penalty for the time lock delta of this channel.
// It is controlled by RiskFactorBillionths and scales proportional
// to the amount that will pass through channel. Rationale is that it if
// a twice as large amount gets locked up, it is twice as bad.
timeLockPenalty := int64(lockedAmt) * int64(timeLockDelta) *
RiskFactorBillionths / 1000000000

return int64(fee) + timeLockPenalty

}

If TimelockPenalty results in number less than one, the return would be fees only... which was almost always.
This resulted in an infinite loop between two nodes if the fees were zero/zero, regardless of the Timelock.
I suggested changing several variable types.

Though, I think that was addressed by changing:
if tempDist >= distance[fromVertex].dist {
return
}
From ">" to ">="

[LNBIG-COM]

@alexbosworth wrote to me in LN chat that: "If you set your fees to zero programmatically you need to restart your node otherwise forwarding will break"
@Roasbeef , can you confirm or deny this? I changed fees through gRPC API. Should i restart LND daemons after setting zero fees in channels? And if yes, why?

[Roasbeef]

Hi, That's correct, we became aware of this issue last week. We have a fix, and it should be implemented before our scheduled 0.8 release. When the fix is available in master, we'll comment on this issue.

…

On Mon, Aug 26, 2019, 7:23 AM LNBIG.com Project ***@***.***> wrote: @alexbosworth <https://github.com/alexbosworth> wrote to me in LN chat that: "If you set your fees to zero programmatically you need to restart your node otherwise forwarding will break" @Roasbeef <https://github.com/Roasbeef> , can you confirm or deny this? I changed fees through gRPC API. Should i restart LND daemons after setting zero fees in channels? And if yes, why? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#3139?email_source=notifications&email_token=AAHTWLX44WIE7OJUKL4ZN2DQGPRORA5CNFSM4HQWIRRKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD5EQXEA#issuecomment-524880784>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAHTWLR5VGMD3I5ESVGJIBTQGPRORANCNFSM4HQWIRRA> .

[gcomte]

Hi @Roasbeef

So I'd assume the fix is available in master as of now?

[cfromknecht]

@gcomte given the merge dates this looks like it shipped in 0.7