Do not crash on fake down messages on stream monitor, closes #9803

Author: josevalim

lib/elixir/lib/task/supervised.ex

@@ -539,7 +539,7 @@ defmodule Task.Supervised do
 
       # One of the spawned processes went down. We inform the parent process of
       # this and keep going.
-      {:DOWN, ref, _, _, reason} ->
+      {:DOWN, ref, _, _, reason} when is_map_key(running_tasks, ref) ->
         {task, running_tasks} = Map.pop(running_tasks, ref)
         %{position: position, timer_ref: timer_ref, timed_out?: timed_out?} = task
 

lib/elixir/test/elixir/task_test.exs

@@ -584,6 +584,21 @@ defmodule TaskTest do
       [ok: :ok] = Task.async_stream([1], fn _ -> :ok end, timeout: :infinity) |> Enum.to_list()
     end
 
+    test "streams with fake down messages on the inbox" do
+      parent = self()
+
+      assert Task.async_stream([:ok], fn :ok ->
+               {:links, links} = Process.info(self(), :links)
+
+               for link <- links do
+                 send(link, {:DOWN, make_ref(), :process, parent, :oops})
+               end
+
+               :ok
+             end)
+             |> Enum.to_list() == [ok: :ok]
+    end
+
     test "with $callers" do
       grandparent = self()