implement source DAP request

apps/debug_adapter/lib/debug_adapter/protocol.ex

@@ -38,6 +38,12 @@ defmodule ElixirLS.DebugAdapter.Protocol do
     end
   end
 
+  defmacro source_req(seq, args) do
+    quote do
+      request(unquote(seq), "source", unquote(args))
+    end
+  end
+
   defmacro set_breakpoints_req(seq, source, breakpoints) do
     quote do
       request(unquote(seq), "setBreakpoints", %{

apps/debug_adapter/lib/debug_adapter/server.ex

@@ -885,6 +885,24 @@ defmodule ElixirLS.DebugAdapter.Server do
     {%{}, %{state | config: config}}
   end
 
+  defp handle_request(
+         source_req(_, args),
+         state = %__MODULE__{}
+       ) do
+    path = args["source"]["path"]
+
+    content =
+      if path == "replinput" do
+        # this is a special path that VSCode uses for debugger console
+        # return an empty string as we do not need anything there
+        ""
+      else
+        File.read!(path)
+      end
+
+    {%{"content" => content}, state}
+  end
+
   defp handle_request(
          set_breakpoints_req(_, %{"path" => _path}, _breakpoints),
          %__MODULE__{config: %{"noDebug" => true}}

apps/debug_adapter/test/debugger_test.exs

@@ -3865,4 +3865,43 @@ defmodule ElixirLS.DebugAdapter.ServerTest do
       assert Process.alive?(server)
     end)
   end
+
+  test "source", %{server: server} do
+    in_fixture(__DIR__, "mix_project", fn ->
+      Server.receive_packet(server, initialize_req(1, %{}))
+      assert_receive(response(_, 1, "initialize", _))
+
+      Server.receive_packet(
+        server,
+        %{
+          "arguments" => %{
+            "sourceReference" => 0,
+            "source" => %{"path" => "lib/crash.ex"}
+          },
+          "command" => "source",
+          "seq" => 1,
+          "type" => "request"
+        }
+      )
+
+      assert_receive(%{"body" => %{"content" => "defmodule MixProject.Crash do" <> _}}, 10000)
+
+      Server.receive_packet(
+        server,
+        %{
+          "arguments" => %{
+            "sourceReference" => 0,
+            "source" => %{"path" => "replinput"}
+          },
+          "command" => "source",
+          "seq" => 1,
+          "type" => "request"
+        }
+      )
+
+      assert_receive(%{"body" => %{"content" => ""}}, 10000)
+
+      assert Process.alive?(server)
+    end)
+  end
 end