Raise informative error on batched resolver timeout

When the Task that runs a batched resolver times out we currently
raise a generic error like this:

** (exit) exited in: Task.await(%Task{owner: #PID<0.15353.21>, pid: #PID<0.15380.21>, ref: #Reference<0.2441152191.2237136897.192020>}, 5000)

This provides no context to the engineer debugging the error about the resolver
that timed out, making debugging much harder.

This change raises a RuntimeError which includes the resolver that timed out
along with the arguments it received:

Batch resolver timed out after 1 ms.
Batch fun: {Absinthe.Middleware.BatchTest.Schema, :slow_field_by_user_id}
Batch data: [3, 2, 1]
Batch opts: [timeout: 1]

This should make the debugging experience a lot better.

Author: jordi-chacon

lib/absinthe/middleware/batch.ex

@@ -147,7 +147,19 @@ defmodule Absinthe.Middleware.Batch do
     end)
     |> Map.new(fn {batch_opts, task, start_time_mono, metadata} ->
       timeout = Keyword.get(batch_opts, :timeout, 5_000)
-      result = Task.await(task, timeout)
+
+      result =
+        try do
+          Task.await(task, timeout)
+        catch
+          :exit, {:timeout, {Task, :await, _}} ->
+            raise """
+            Batch resolver timed out after #{timeout} ms.
+            Batch fun: #{inspect(metadata.batch_fun)}
+            Batch data: #{inspect(metadata.batch_data)}
+            Batch opts: #{inspect(metadata.batch_opts)}
+            """
+        end
 
       end_time_mono = System.monotonic_time()
       duration = end_time_mono - start_time_mono

test/absinthe/middleware/batch_test.exs

@@ -36,6 +36,17 @@ defmodule Absinthe.Middleware.BatchTest do
           end)
         end
       end
+
+      field :slow_field, :integer do
+        resolve fn user, _, _ ->
+          batch(
+            {__MODULE__, :slow_field_by_user_id},
+            user.id,
+            fn batch -> {:ok, Map.get(batch, user.id)} end,
+            timeout: 1
+          )
+        end
+      end
     end
 
     query do
@@ -69,6 +80,11 @@ defmodule Absinthe.Middleware.BatchTest do
     def otel_ctx(_, _) do
       OpenTelemetry.Ctx.get_value("stored_value", nil)
     end
+
+    def slow_field_by_user_id(_, ids) do
+      :timer.sleep(5000)
+      ids |> Enum.with_index() |> Map.new()
+    end
   end
 
   test "can resolve a field using the normal async helper" do
@@ -150,4 +166,25 @@ defmodule Absinthe.Middleware.BatchTest do
 
     assert {:ok, %{data: %{"ctx" => "some_value"}}} == Absinthe.run(doc, Schema)
   end
+
+  test "raises when batched resolver times out" do
+    doc = """
+    {
+      users {
+        slowField
+      }
+    }
+    """
+
+    assert_raise(
+      RuntimeError,
+      """
+      Batch resolver timed out after 1 ms.
+      Batch fun: {Absinthe.Middleware.BatchTest.Schema, :slow_field_by_user_id}
+      Batch data: [3, 2, 1]
+      Batch opts: [timeout: 1]
+      """,
+      fn -> Absinthe.run(doc, Schema) end
+    )
+  end
 end