Handle result from realtime tasks

Previously, BlockFetcher only had handle_info for the DOWN message from
realtime tasks, but when the reason is :normal, the task would have
succeeded and therefore, the BlockFetcher would also have received a
`{ref, :ok}` that was not being matched.

apps/indexer/lib/indexer/block_fetcher.ex

@@ -64,7 +64,7 @@ defmodule Indexer.BlockFetcher do
             catchup_task: nil,
             catchup_block_number: nil,
             catchup_bound_interval: nil,
-            realtime_tasks: [],
+            realtime_task_by_ref: %{},
             realtime_interval: nil,
             blocks_batch_size: @blocks_batch_size,
             blocks_concurrency: @blocks_concurrency,
@@ -103,12 +103,6 @@ defmodule Indexer.BlockFetcher do
     {:noreply, %{state | catchup_task: catchup_task}}
   end
 
-  def handle_info(:realtime_index, %__MODULE__{realtime_tasks: realtime_tasks} = state) when is_list(realtime_tasks) do
-    realtime_task = Task.Supervisor.async_nolink(Indexer.TaskSupervisor, fn -> realtime_task(state) end)
-
-    {:noreply, %{state | realtime_tasks: [realtime_task | realtime_tasks]}}
-  end
-
   def handle_info(
         {ref, missing_block_count},
         %__MODULE__{
@@ -152,26 +146,53 @@ defmodule Indexer.BlockFetcher do
     {:noreply, %__MODULE__{state | catchup_task: nil}}
   end
 
-  def handle_info({:DOWN, ref, :process, pid, reason}, %__MODULE__{realtime_tasks: realtime_tasks} = state)
-      when is_list(realtime_tasks) do
-    {down_realtime_tasks, running_realtime_tasks} =
-      Enum.split_with(realtime_tasks, fn
-        %Task{pid: ^pid, ref: ^ref} -> true
-        _ -> false
+  def handle_info(:realtime_index, %__MODULE__{} = state) do
+    %Task{ref: ref} =
+      realtime_task = Task.Supervisor.async_nolink(Indexer.TaskSupervisor, fn -> realtime_task(state) end)
+
+    new_state = put_in(state.realtime_task_by_ref[ref], realtime_task)
+
+    {:noreply, new_state}
+  end
+
+  def handle_info({ref, :ok = result}, %__MODULE__{realtime_task_by_ref: realtime_task_by_ref} = state) do
+    {realtime_task, running_realtime_task_by_ref} = Map.pop(realtime_task_by_ref, ref)
+
+    case realtime_task do
+      nil ->
+        Logger.error(fn ->
+          "Unknown ref (#{inspect(ref)}) that is neither the catchup index" <>
+            " nor a realtime index Task ref returned result (#{inspect(result)})"
         end)
 
-    case {Enum.empty?(down_realtime_tasks), reason} do
-      {true, :normal} ->
+      _ ->
         :ok
+    end
+
+    Process.demonitor(ref, [:flush])
+
+    {:noreply, %__MODULE__{state | realtime_task_by_ref: running_realtime_task_by_ref}}
+  end
+
+  def handle_info({:DOWN, ref, :process, pid, reason}, %__MODULE__{realtime_task_by_ref: realtime_task_by_ref} = state) do
+    {realtime_task, running_realtime_task_by_ref} = Map.pop(realtime_task_by_ref, ref)
 
-      {true, reason} ->
-        Logger.error(fn -> "Realtime index stream exited with reason (#{inspect(reason)}). Restarting" end)
+    case realtime_task do
+      nil ->
+        Logger.error(fn ->
+          "Unknown ref (#{inspect(ref)}) that is neither the catchup index" <>
+            " nor a realtime index Task ref reports unknown pid (#{pid}) DOWN due to reason (#{reason}})"
+        end)
 
-      {_, reason} ->
-        Logger.error(fn -> "Unexpected pid (#{inspect(pid)}) exited with reason (#{inspect(reason)})." end)
+      _ ->
+        Logger.error(fn ->
+          "Realtime index stream exited with reason (#{inspect(reason)}).  " <>
+            "The next realtime index task will fill the missing block " <>
+            "if the lastest block number has not advanced by then or the catch up index will fill the missing block."
+        end)
     end
 
-    {:noreply, %__MODULE__{state | realtime_tasks: running_realtime_tasks}}
+    {:noreply, %__MODULE__{state | realtime_task_by_ref: running_realtime_task_by_ref}}
   end
 
   defp cap_seq(seq, next, range) do