Handle :EXIT messages in ingestion WriteBuffer

lib/plausible/ingestion/write_buffer.ex

@@ -66,6 +66,13 @@ defmodule Plausible.Ingestion.WriteBuffer do
     {:noreply, %{state | buffer: [], buffer_size: 0, timer: timer}}
   end
 
+  def handle_info({:EXIT, _from, _reason}, state) do
+    # We trap exits so terminate/2 can flush on shutdown. Linked ports and
+    # processes (e.g. the ClickHouse connection socket) deliver :EXIT messages
+    # here on disconnect, which we ignore to keep the buffer alive.
+    {:noreply, state}
+  end
+
   @impl true
   def handle_call(:flush, _from, state) do
     %{timer: timer, flush_interval_ms: flush_interval_ms} = state

test/plausible/ingestion/write_buffer_test.exs

@@ -0,0 +1,25 @@
+defmodule Plausible.Ingestion.WriteBufferTest do
+  use ExUnit.Case, async: true
+
+  alias Plausible.Ingestion.WriteBuffer
+
+  @opts WriteBuffer.compile_time_prepare(Plausible.ClickhouseEventV2)
+
+  test "survives :EXIT messages from linked processes while trapping exits" do
+    opts =
+      @opts
+      |> Map.take([:header, :insert_sql, :insert_opts])
+      |> Map.to_list()
+      |> Keyword.put(:name, :"write_buffer_exit_#{System.unique_integer([:positive])}")
+      # large buffer so the empty buffer never flushes to ClickHouse during the test
+      |> Keyword.put(:flush_interval_ms, 60_000)
+
+    {:ok, pid} = WriteBuffer.start_link(opts)
+
+    send(pid, {:EXIT, self(), :normal})
+
+    # process is still alive and responsive after the :EXIT message
+    assert Process.alive?(pid)
+    assert WriteBuffer.flush(opts[:name]) == :ok
+  end
+end