Detected version change on call to package

Another belt to wear with the suspenders. When we build a package that
overwrites another package, if the version has changed, we delete the
old compiled code and the old package and start compilation from
scratch. This definitely prevents old beam files from ending up in the
new package.

apps/server/lib/mix/tasks/package.ex

@@ -79,11 +79,13 @@ defmodule Mix.Tasks.Package do
   @execute_permisson 0o755
 
   def run(args) do
-    Mix.Task.run(:compile)
     {opts, _, _} = OptionParser.parse(args, @options)
     default_path = Path.join([Mix.Project.build_path(), "package", "lexical"])
     package_root = Keyword.get(opts, :path, default_path)
 
+    rebuild_on_version_change(package_root)
+
+    Mix.Task.run(:compile)
     Mix.Shell.IO.info("Assembling build in #{package_root}")
     File.mkdir_p!(package_root)
 
@@ -103,6 +105,21 @@ defmodule Mix.Tasks.Package do
     end
   end
 
+  defp rebuild_on_version_change(package_root) do
+    %{elixir: elixir_current, erlang: erlang_current} = Versions.current()
+
+    with {:ok, %{elixir: elixir_compiled, erlang: erlang_compiled}} <-
+           Versions.read(priv_path(package_root)) do
+      if elixir_compiled != elixir_current or erlang_compiled != erlang_current do
+        Code.put_compiler_option(:ignore_module_conflict, true)
+        Mix.Shell.IO.error("The version of elixir or erlang has changed. Forcing recompilation.")
+        File.rm_rf!(package_root)
+        Mix.Task.clear()
+        Mix.Task.run(:clean, ~w(--deps))
+      end
+    end
+  end
+
   defp prepare(package_root) do
     scratch_directory = Path.join(package_root, "scratch")
     File.mkdir(scratch_directory)
@@ -246,7 +263,7 @@ defmodule Mix.Tasks.Package do
   @priv_apps [:remote_control]
 
   defp copy_priv_files(package_root) do
-    priv_dest_dir = Path.join(package_root, "priv")
+    priv_dest_dir = priv_path(package_root)
 
     Enum.each(@priv_apps, fn app_name ->
       case priv_dir(app_name) do
@@ -261,7 +278,7 @@ defmodule Mix.Tasks.Package do
 
   defp write_vm_versions(package_root) do
     package_root
-    |> Path.join("priv")
+    |> priv_path()
     |> Versions.write()
   end
 
@@ -316,4 +333,8 @@ defmodule Mix.Tasks.Package do
         path
     end
   end
+
+  defp priv_path(package_root) do
+    Path.join(package_root, "priv")
+  end
 end