Add autoswitching from eth_subscribe to eth_blockNumber in Staking DApp

[varasev]

Motivation

Currently, Staking DApp only receives a new block number from eth_subscribe connected to WebSocket node. This PR allows the App to additionally use eth_blockNumber when eth_subscribe stops sending notifications.

If eth_subscribe stops working, the DApp waits for 60 seconds and then activates eth_blockNumber requests which are sent every 500 ms. Once the new notification from eth_subscribe is received, eth_blockNumber requests stop and the DApp switches back to eth_subscribe.

Also, this PR makes a couple of minor fixes:

• Likelihood of Becoming a Validator on the Next Epoch now should display correct value in the pool info popup: if there are less than 20 candidates, the likelihood is 100%. Otherwise, it is calculated as before.
• Fix in terminology: the column Potential Reward Percent (Current Reward Percent) in delegator list popup renamed to Potential Reward Share (Current Reward Share)

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 820a586ee1c376efe1cd60e45bdcb8b7e4d546e5-PR-3585

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 66.506%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	1	78.57%

Totals
Change from base Build cbff8a452872b61adebc7d14e03c302648f76112:	-0.6%
Covered Lines:	2071
Relevant Lines:	3114

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💛 - Coveralls

[vbaranov]

@varasev 👍
In overall looks good. I left some nitpicks to increase the manageability of the feature and code readability.

[vbaranov]

Can we add manageability of this param through environment variables?

[varasev]

Yes, added it in 00f3943

[vbaranov]

Let's define 500 ms as a module parameter. Ideally, to make it manageable via Environment variables.

[varasev]

Ok, added it in 00f3943

[vbaranov]

Is this parameters order change needed?

[varasev]

During the work on the module, I moved seen_block from state to ETS since in one of my local versions there was a risk of race condition, but later I removed excess code which could lead to that, so overall the seen_block could stay in the state, but I propose to leave it as it is.

[vbaranov]

I would suggest refactoring fetch_state function in order to eliminate mix credo warning. Refactoring can look like this:

defp fetch_state(state, block_number) do
    if block_number <= get(:seen_block) do
      state
    else
      fetch_state_internal(state, block_number)
    end
  end

  defp fetch_state_internal(state, block_number) do
    # read general info from the contracts (including pool list and validator list)
    global_responses =
      ContractReader.perform_requests(ContractReader.global_requests(block_number), state.contracts, state.abi)

    epoch_very_beginning = global_responses.epoch_start_block == block_number + 1

    start_snapshotting = start_snapshotting?(global_responses)

    active_pools_length = Enum.count(global_responses.active_pools)

    # determine if something changed in contracts state since the previous seen block.
    # if something changed or the `fetch_state` function is called for the first time
    # or we are at the beginning of staking epoch or snapshotting recently finished
    # then we should update database
    last_change_block =
      max(global_responses.staking_last_change_block, global_responses.validator_set_last_change_block)

    should_update_db = should_update_db?(state, start_snapshotting, last_change_block)

    # save the general info to ETS (excluding pool list and validator list)
    save_settings(
      global_responses,
      state,
      block_number,
      epoch_very_beginning,
      start_snapshotting,
      active_pools_length,
      last_change_block
    )

    # we should update database as something changed in contracts state
    if should_update_db do
      update_database(
        epoch_very_beginning,
        start_snapshotting,
        global_responses,
        state.contracts,
        state.abi,
        block_number
      )
    end

    # notify the UI about a new block
    data = %{
      active_pools_length: active_pools_length,
      block_number: block_number,
      epoch_end_block: global_responses.epoch_end_block,
      epoch_number: global_responses.epoch_number,
      max_candidates: global_responses.max_candidates,
      staking_allowed: global_responses.staking_allowed,
      staking_token_defined: get(:token, nil) != nil,
      validator_set_apply_block: global_responses.validator_set_apply_block
    }

    Publisher.broadcast([{:staking_update, data}], :realtime)

    if state.snapshotting_finished do
      %{state | snapshotting_finished: false}
    else
      state
    end
  end

  defp save_settings(
         global_responses,
         state,
         block_number,
         epoch_very_beginning,
         start_snapshotting,
         active_pools_length,
         last_change_block
       ) do
    validator_min_reward_percent =
      get_validator_min_reward_percent(global_responses.epoch_number, block_number, state.contracts, state.abi)

    settings =
      global_responses
      |> get_settings(validator_min_reward_percent, block_number)
      |> Enum.concat(active_pools_length: active_pools_length)
      |> Enum.concat(last_change_block: last_change_block)
      |> Enum.concat(seen_block: block_number)

    :ets.insert(@table_name, settings)

    if epoch_very_beginning or start_snapshotting do
      # if the block_number is the latest block of the finished staking epoch
      # or we are starting Blockscout server, the BlockRewardAuRa contract balance
      # could increase before (without Mint/Transfer events),
      # so we need to update its balance in database
      update_block_reward_balance(block_number)
    end
  end

  defp start_snapshotting?(global_responses) do
    global_responses.epoch_number > get(:snapshotted_epoch_number) && global_responses.epoch_number > 0 &&
      not get(:is_snapshotting)
  end

  defp should_update_db?(state, start_snapshotting, last_change_block) do
    first_fetch = get(:epoch_end_block, 0) == 0
    start_snapshotting or state.snapshotting_finished or first_fetch or last_change_block > get(:last_change_block)
  end

[varasev]

Done in 00f3943