Demonstrate unexpected timeout behaviour when sources are slow

lib/dataloader.ex

@@ -88,6 +88,11 @@ defmodule Dataloader do
     if pending_batches?(dataloader) do
       fun = fn {name, source} -> {name, Source.run(source)} end
 
+      # This function behaves unexpectedly when the source times out; it drops
+      # the source! For example, if we have a single source and it times out,
+      # `pmap` silently accepts the failure and returns `[]`... which then
+      # turns our `sources` into an empty map here!
+      #
       sources =
         dataloader.sources
         |> pmap(

test/dataloader_test.exs

@@ -18,4 +18,16 @@ defmodule DataloaderTest do
 
     assert log =~ "boom"
   end
+
+  test "fails silently and returns an empty map of sources if things timeout" do
+    source = %Dataloader.TestSource{}
+
+    dataloader =
+      Dataloader.new(timeout: 1) # Note the short timeout
+      |> Dataloader.add_source(:test, source)
+
+    new_dataloader = Dataloader.run(dataloader)
+
+    assert dataloader == new_dataloader
+  end
 end

test/support/test_source.ex

@@ -0,0 +1,19 @@
+defmodule Dataloader.TestSource do
+  defstruct [opts: [], batches: [], results: %{}]
+
+  defimpl Dataloader.Source do
+
+    def load(source, _batch_key, _item_key), do: source
+
+    def fetch(_source, _batch_key, _item_key), do: :error
+
+    def pending_batches?(_source), do: true
+
+    def put(source, _batch_key, _item_key, _item), do: source
+
+    def run(source) do
+      Process.sleep(5) # Just needs to be larger than our Dataloader timeout
+      source
+    end
+  end
+end