Realtime fetcher retries for unknown block number

Why:

* We want to add a retry mechanism, instead of waiting half a second for
every block the realtime fetcher attempts to fetch and index. We had
this half a second wait in place in an attempt to avoid "Unknown block
number" errors. The half a second wait was effective at reducing these
errors but it also meant we were awaiting unnecessarily for most
fetches. Retrying after half a second, when Parity says it
doesn't yet know of a given block number, makes more sense.
* Issue link: n/a

This change addresses the need by:

* Editing `Indexer.Block.Realtime.Fetcher.fetch_and_import_block/3` to
retry failed fetch/import attempts because of "Unknown block number"
errors. It waits half a second before retrying and retries a maximum of
three times.

apps/indexer/lib/indexer/block/realtime/fetcher.ex

@@ -124,10 +124,7 @@ defmodule Indexer.Block.Realtime.Fetcher do
     end
   end
 
-  def fetch_and_import_block(block_number_to_fetch, block_fetcher) do
-    # Wait half a second to give Parity/Geth time to sync.
-    :timer.sleep(500)
-
+  def fetch_and_import_block(block_number_to_fetch, block_fetcher, retry \\ 3) do
     case fetch_and_import_range(block_fetcher, block_number_to_fetch..block_number_to_fetch) do
       {:ok, {_inserted, _next}} ->
         Logger.debug(fn ->
@@ -148,6 +145,14 @@ defmodule Indexer.Block.Realtime.Fetcher do
         end)
 
       {:error, changesets} when is_list(changesets) ->
+        params = %{
+          changesets: changesets,
+          block_number_to_fetch: block_number_to_fetch,
+          block_fetcher: block_fetcher,
+          retry: retry
+        }
+
+        if retry_fetch_and_import_block(params) == :ignore do
           Logger.error(fn ->
             [
               "realtime indexer failed to validate for block ",
@@ -157,6 +162,7 @@ defmodule Indexer.Block.Realtime.Fetcher do
               ".  Block will be retried by catchup indexer."
             ]
           end)
+        end
 
       {:error, {step, failed_value, _changes_so_far}} ->
         Logger.error(fn ->
@@ -173,6 +179,27 @@ defmodule Indexer.Block.Realtime.Fetcher do
     end
   end
 
+  defp retry_fetch_and_import_block(%{retry: retry}) when retry < 1, do: :ignore
+
+  defp retry_fetch_and_import_block(%{changesets: changesets} = params) do
+    if unknown_block_number_error?(changesets) do
+      # Wait half a second to give Parity time to sync.
+      :timer.sleep(500)
+
+      number = params.block_number_to_fetch
+      fetcher = params.block_fetcher
+      updated_retry = params.retry - 1
+
+      fetch_and_import_block(number, fetcher, updated_retry)
+    else
+      :ignore
+    end
+  end
+
+  defp unknown_block_number_error?(changesets) do
+    Enum.any?(changesets, &(Map.get(&1, :message) == "Unknown block number"))
+  end
+
   defp async_import_remaining_block_data(%{block_second_degree_relations: block_second_degree_relations, tokens: tokens}) do
     tokens
     |> Enum.map(& &1.contract_address_hash)