Ensure ERTS is not copied into release (#9752)

The default behaviour when starting a new mix project is to
`include_erts: true`.

If at some point you choose to `include_erts: false`, but have the ERTS
artifact hanging around from previous builds, it gets included in future
builds, therefore rendering `include_erts: false` broken when running on
systems that have built with `include_erts: true` for the same
environment.

`make_tar/1` currently always tries to include the ERTS artifact into
the archive, regardless of our `include_erts` configuration. this fails
in 1.9.4 if no previous artifact is present, and seems to be worked
around in the current implementation by simply ignoring source
directories that don't exist.

This adds a regression test for the behaviour by "simulating" an ERTS
leftover from a previous build.

This also adds logic to not try to include ERTS into archives in the
first place when our release is configured with `include_erts: false`.

lib/mix/lib/mix/tasks/release.ex

@@ -1075,14 +1075,20 @@ defmodule Mix.Tasks.Release do
         [Path.join("lib", "#{name}-#{vsn}") | acc]
       end)
 
+    erts_dir =
+      case release.erts_source do
+        nil -> []
+        _ -> ["erts-#{release.erts_version}"]
+      end
+
     release_files =
       for basename <- File.ls!(Path.join(release.path, "releases")),
           not File.dir?(Path.join([release.path, "releases", basename])),
           do: Path.join("releases", basename)
 
     dirs =
-      ["bin", Path.join("releases", release.version), "erts-#{release.erts_version}"] ++
-        lib_dirs ++ release_files
+      ["bin", Path.join("releases", release.version)] ++
+        erts_dir ++ lib_dirs ++ release_files
 
     files =
       dirs

lib/mix/test/mix/tasks/release_test.exs

@@ -208,6 +208,34 @@ defmodule Mix.Tasks.ReleaseTest do
         end)
       end)
     end
+
+    test "without ERTS when a previous build included ERTS" do
+      in_fixture("release_test", fn ->
+        config = [releases: [demo: [include_erts: false, steps: [:assemble, :tar]]]]
+
+        Mix.Project.in_project(:release_test, ".", config, fn _ ->
+          root = Path.absname("_build/#{Mix.env()}/rel/demo")
+
+          erts_dir_from_previous_build =
+            Path.absname("_build/#{Mix.env()}/rel/demo/erts-#{@erts_version}")
+
+          File.mkdir_p!(erts_dir_from_previous_build)
+
+          Mix.Task.run("release")
+          tar_path = Path.expand(Path.join([root, "..", "..", "demo-0.1.0.tar.gz"]))
+          message = "* building #{tar_path}"
+          assert_received {:mix_shell, :info, [^message]}
+          assert File.exists?(tar_path)
+
+          {:ok, files} = String.to_charlist(tar_path) |> :erl_tar.table([:compressed])
+          files = Enum.map(files, &to_string/1)
+
+          assert "bin/demo" in files
+          refute Enum.any?(files, &(&1 =~ "erts"))
+          refute Enum.any?(files, &(&1 =~ "stdlib"))
+        end)
+      end)
+    end
   end
 
   test "assembles a bootable release with ERTS" do