Connection pool: implement logging and telemetry events

lib/pleroma/telemetry/logger.ex

@@ -0,0 +1,62 @@
+defmodule Pleroma.Telemetry.Logger do
+  @moduledoc "Transforms Pleroma telemetry events to logs"
+
+  require Logger
+
+  @events [
+    [:pleroma, :connection_pool, :reclaim, :start],
+    [:pleroma, :connection_pool, :reclaim, :stop],
+    [:pleroma, :connection_pool, :provision_failure]
+  ]
+  def attach do
+    :telemetry.attach_many("pleroma-logger", @events, &handle_event/4, [])
+  end
+
+  # Passing anonymous functions instead of strings to logger is intentional,
+  # that way strings won't be concatenated if the message is going to be thrown
+  # out anyway due to higher log level configured
+
+  def handle_event(
+        [:pleroma, :connection_pool, :reclaim, :start],
+        _,
+        %{max_connections: max_connections, reclaim_max: reclaim_max},
+        _
+      ) do
+    Logger.debug(fn ->
+      "Connection pool is exhausted (reached #{max_connections} connections). Starting idle connection cleanup to reclaim as much as #{
+        reclaim_max
+      } connections"
+    end)
+  end
+
+  def handle_event(
+        [:pleroma, :connection_pool, :reclaim, :stop],
+        %{reclaimed_count: 0},
+        _,
+        _
+      ) do
+    Logger.error(fn ->
+      "Connection pool failed to reclaim any connections due to all of them being in use. It will have to drop requests for opening connections to new hosts"
+    end)
+  end
+
+  def handle_event(
+        [:pleroma, :connection_pool, :reclaim, :stop],
+        %{reclaimed_count: reclaimed_count},
+        _,
+        _
+      ) do
+    Logger.debug(fn -> "Connection pool cleaned up #{reclaimed_count} idle connections" end)
+  end
+
+  def handle_event(
+        [:pleroma, :connection_pool, :provision_failure],
+        %{opts: [key | _]},
+        _,
+        _
+      ) do
+    Logger.error(fn ->
+      "Connection pool had to refuse opening a connection to #{key} due to connection limit exhaustion"
+    end)
+  end
+end