les: implement ultralight client

[b00ris]

Implements ultra light client feature.
https://github.com/zsfelfoldi/ethereum-docs/blob/master/les/tasks/ulc.txt
More information about ULC https://hackmd.io/s/HJy7jjZpm

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[b00ris]

@zsfelfoldi Hi! Could you give me a review update? I can't reach you.

[zsfelfoldi]

A few cosmetic fixes suggested in the comments. If these are fixed, please squash commits and rebase, then I will approve.

[zsfelfoldi]

please squash into a single commit

[JekaMas]

@karalabe please, give us your comments and we'll fix them all quickly. We're really looking to finish the PR this year :)

[zsfelfoldi]

I think the most imporant suggestion was to simplify the handling of trusted peers in the server pool. I have that thing in my head now and I will quickly do it in a separate branch so that we can finalize this PR after the holidays. Our next release will be a major one (1.9) which is a great opportunity to include new features like this one.

[JekaMas]

Great news! If you need any help please ping @b00ris or me.

[b00ris]

Wow! Great news!

[zsfelfoldi]

@b00ris @JekaMas here is my modification: https://github.com/zsfelfoldi/go-ethereum/tree/ulc-serverpool-fix
Since p2p.Server automatically tries to keep static nodes connected it is cleaner if we just add them once at the beginning. We don't need to create a separate pool or even pool entries for them. Just remember the IDs so that we can ignore them and not add them to the known/new selection pools either. Btw your version worked fine too but during the review we concluded that it is best to not complicate this logic if not necessary. I tried it and it seems to work in both normal and ulc mode but please try it in your environment too, I don't want to screw it up.
@karalabe @fjl also PTAL

[zsfelfoldi]

@b00ris please keep your version too until we all have a consensus about it

[zsfelfoldi]

Also there was a suggestion to remove "ulc.config" option for a separate ULC config file because we already have a config file. Right now I am in favor of keeping it unless there is a strong argument against so I did not remove it. ULC config can be an application vendor-specific thing while other config parameters are more user environment-specific so maybe having an optional separate config file is not a bad idea.
If I remember correctly the rest of the PR was generally considered OK and mergeable.

[b00ris]

@zsfelfoldi I've reviewed your suggestion and I like it. It's cleaner than mine. I've also tried it in my environment and it works well.
I thought to delete ulc.config, but it was so useful for testing and I decided to keep it. I think it would be better to change it to toml to keep consistency.

[shazow]

What's the next step here? Should @b00ris merge @zsfelfoldi's changes into this PR?

Are we still blocked on a review from @karalabe?

[b00ris]

@zsfelfoldi, @karalabe Could you give a feedback?

[zsfelfoldi]

@b00ris sure, I pushed my changes on this branch, @fjl will take a last look and then we'll merge. It will definitely go into the next release.

[zsfelfoldi]

@fjl @karalabe I am going to merge this PR on Friday if there is no objection

[zsfelfoldi]

Probably not this week because we don't want to merge any big changes until the delayed fork issue is settled. It is still going into 1.9 of course.

[ssl-prime]

please correct variable name in 'light/lightchain.go' from 'mu' to 'chainmu'
line no 537, 538, 544, 545

[JekaMas]

@b00ris