Handle :EXIT messages in ingestion WriteBuffer

lib/plausible/ingestion/write_buffer.ex

@@ -66,6 +66,11 @@ defmodule Plausible.Ingestion.WriteBuffer do
     {:noreply, %{state | buffer: [], buffer_size: 0, timer: timer}}
   end
 
+  def handle_info({:EXIT, _from, reason}, state) do
+    Logger.warning("#{state.name} received EXIT, keeping buffer alive: #{inspect(reason)}")
+    {: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 +1,46 @@
+defmodule Plausible.Ingestion.WriteBufferTest do
+  use ExUnit.Case, async: true
+
+  import ExUnit.CaptureLog
+
+  alias Plausible.Ingestion.WriteBuffer
+
+  @compile_time_opts WriteBuffer.compile_time_prepare(Plausible.ClickhouseEventV2)
+
+  test "keeps running and logs the reason when a linked process exits", %{test: test} do
+    opts = [
+      header: @compile_time_opts.header,
+      insert_sql: @compile_time_opts.insert_sql,
+      insert_opts: @compile_time_opts.insert_opts,
+      # unique atom is at our disposal already
+      name: test,
+      # no need to ever flush automatically on slow CI
+      flush_interval_ms: 1_000_000_000
+    ]
+
+    {:ok, pid} = WriteBuffer.start_link(opts)
+
+    # The buffer traps exits, so a linked process exiting delivers a real
+    # {:EXIT, from, reason} message to handle_info/2. Linking a throwaway
+    # process to the buffer and killing it reproduces the crash in #6382
+    # without sending a synthetic message.
+    child =
+      spawn(fn ->
+        Process.link(pid)
+        Process.sleep(:infinity)
+      end)
+
+    ref = Process.monitor(child)
+
+    log =
+      capture_log(fn ->
+        Process.exit(child, :shutdown)
+        assert_receive {:DOWN, ^ref, :process, ^child, _}, 1_000
+        assert Process.alive?(pid)
+        assert WriteBuffer.flush(test) == :ok
+      end)
+
+    assert log =~ "received EXIT, keeping buffer alive"
+    assert log =~ ":shutdown"
+  end
+end