Make sure that TelemetryMetricsStatsD can be started if host can't be…

… resolved

If the StatsD collector is not up for some reason and the application can't resolve
the name by calling `:inet.gethostbyname(host)` then it prevents the entire application from starting.

This commit makes sure that the TelemetryMetricsStatsD can be started in the errored state
and depending on the `host_resolution_interval` it can be recovered or not.

If `host_resolution_interval` isn't passed, then TelemetryMetricsStatsD will not try to resolve the host again.
However, when it is passed, it can try to recover it in `host_resolution_interval` milliseconds.

lib/telemetry_metrics_statsd.ex

@@ -425,35 +425,48 @@ defmodule TelemetryMetricsStatsd do
         _ -> configure_host_resolution(options)
       end
 
-    udps =
-      for _ <- 1..options.pool_size do
-        {:ok, udp} = UDP.open(udp_config)
-        {:udp, udp}
-      end
+    case udp_config do
+      %{host: _} = udp_config ->
+        udps =
+          for _ <- 1..options.pool_size do
+            {:ok, udp} = UDP.open(udp_config)
+            {:udp, udp}
+          end
 
-    pool_id = :ets.new(__MODULE__, [:bag, :protected, read_concurrency: true])
-    :ets.insert(pool_id, udps)
-
-    handler_ids =
-      EventHandler.attach(
-        metrics,
-        self(),
-        pool_id,
-        options.mtu,
-        options.prefix,
-        options.formatter,
-        options.global_tags
-      )
+        pool_id = :ets.new(__MODULE__, [:bag, :protected, read_concurrency: true])
+        :ets.insert(pool_id, udps)
+
+        handler_ids =
+          EventHandler.attach(
+            metrics,
+            self(),
+            pool_id,
+            options.mtu,
+            options.prefix,
+            options.formatter,
+            options.global_tags
+          )
 
-    {:ok,
-     %{
-       udp_config: udp_config,
-       handler_ids: handler_ids,
-       pool_id: pool_id,
-       host: options.host,
-       port: options.port,
-       host_resolution_interval: options.host_resolution_interval
-     }}
+        {:ok,
+         %{
+           udp_config: udp_config,
+           handler_ids: handler_ids,
+           pool_id: pool_id,
+           host: options.host,
+           port: options.port,
+           host_resolution_interval: options.host_resolution_interval
+         }}
+      _ ->
+        {:ok,
+        %{
+          udp_config: :error,
+          handler_ids: [],
+          pool_id: nil,
+          host: options.host,
+          port: options.port,
+          host_resolution_interval: options.host_resolution_interval
+        }}
+    end
   end
 
   @impl true
@@ -492,28 +505,42 @@ defmodule TelemetryMetricsStatsd do
 
   @impl true
   def handle_info(:resolve_host, state) do
-    %{host: host, udp_config: %{host: current_address}, host_resolution_interval: interval} =
+    %{host: host, udp_config: udp_config, host_resolution_interval: interval} =
       state
 
     new_state =
       case :inet.gethostbyname(host) do
         {:ok, hostent(h_addr_list: ips)} ->
-          if Enum.member?(ips, current_address) do
-            state
-          else
-            [new_address | _] = ips
-            update_host(state, new_address)
+          cond do
+            udp_config == :error ->
+              state
+
+            Enum.member?(ips, udp_config.host) ->
+              state
+
+            true ->
+              [new_address | _] = ips
+              update_host(state, new_address)
           end
 
         {:error, reason} ->
-          Logger.log(
-            @log_level_warning,
-            "Failed to resolve the hostname #{host}: #{inspect(reason)}. " <>
-              "Using the previously resolved address of #{:inet.ntoa(current_address)}."
-          )
-      end
+          case udp_config do
+            :error ->
+              Logger.log(
+                @log_level_warning,
+                "Failed to resolve the hostname #{host}: #{inspect(reason)}. " <>
+                  "Previously resolved hostname was unsuccessful. The library will not send any metrics."
+              )
+            _ -> Logger.log(
+                @log_level_warning,
+                "Failed to resolve the hostname #{host}: #{inspect(reason)}. " <>
+                  "Using the previously resolved address of #{:inet.ntoa(udp_config.host)}."
+              )
+          end
 
-    Process.send_after(self(), :resolve_host, interval)
+          Process.send_after(self(), :resolve_host, interval)
+          state
+      end
 
     {:noreply, new_state}
   end
@@ -548,18 +575,39 @@ defmodule TelemetryMetricsStatsd do
          host_resolution_interval: interval
        })
        when is_integer(interval) do
-    {:ok, hostent(h_addr_list: [ip | _ips])} = :inet.gethostbyname(host, inet_address_family)
-    Process.send_after(self(), :resolve_host, interval)
-    %{host: ip, port: port, inet_address_family: inet_address_family}
+
+    case :inet.gethostbyname(host, inet_address_family) do
+      {:ok, hostent(h_addr_list: [ip | _ips])} ->
+        %{host: ip, port: port, inet_address_family: inet_address_family}
+
+      {:error, reason} ->
+        Logger.log(
+          @log_level_warning,
+          "Failed to resolve the hostname #{host}: #{inspect(reason)}. " <>
+            "Retrying to resolve it again in #{interval} milliseconds."
+        )
+        Process.send_after(self(), :resolve_host, interval)
+        :error
+    end
+
   end
 
   defp configure_host_resolution(%{
          host: host,
          port: port,
          inet_address_family: inet_address_family
        }) do
-    {:ok, hostent(h_addr_list: [ip | _ips])} = :inet.gethostbyname(host, inet_address_family)
-    %{host: ip, port: port, inet_address_family: inet_address_family}
+    case :inet.gethostbyname(host, inet_address_family) do
+      {:ok, hostent(h_addr_list: [ip | _ips])} ->
+        %{host: ip, port: port, inet_address_family: inet_address_family}
+
+      {:error, reason} ->
+        Logger.log(
+          @log_level_warning,
+          "Failed to resolve the hostname #{host}: #{inspect(reason)}. Metrics will not be sent at all."
+        )
+        :error
+    end
   end
 
   defp update_pool(pool_id, new_host, new_port) do