Fix Path.absname/1 to correctly handle UNC paths on Windows (#9689)

Now excludes files that does not exist on the file system. This could
happen e.g. if the release is built with `include_erts: false`.

Otherwise building the tar would fail with `:enoent`.

lib/elixir/lib/path.ex

@@ -95,6 +95,8 @@ defmodule Path do
     absname(absname_join(name), cwd)
   end
 
+  @slash [?/, ?\\]
+
   # Joins a list
   defp absname_join([name1, name2 | rest]), do: absname_join([absname_join(name1, name2) | rest])
 
@@ -110,6 +112,11 @@ defmodule Path do
     do_absname_join(rest, relativename, [?:, uc_letter + ?a - ?A], :win32)
   end
 
+  defp do_absname_join(<<c1, c2, rest::binary>>, relativename, [], :win32)
+       when c1 in @slash and c2 in @slash do
+    do_absname_join(rest, relativename, '//', :win32)
+  end
+
   defp do_absname_join(<<?\\, rest::binary>>, relativename, result, :win32),
     do: do_absname_join(<<?/, rest::binary>>, relativename, result, :win32)
 
@@ -254,8 +261,6 @@ defmodule Path do
   defp unix_pathtype([list | rest]) when is_list(list), do: unix_pathtype(list ++ rest)
   defp unix_pathtype(relative), do: {:relative, relative}
 
-  @slash [?/, ?\\]
-
   defp win32_pathtype([list | rest]) when is_list(list), do: win32_pathtype(list ++ rest)
 
   defp win32_pathtype([char, list | rest]) when is_list(list),

lib/elixir/test/elixir/path_test.exs

@@ -39,6 +39,13 @@ defmodule PathTest do
   describe "Windows" do
     @describetag :windows
 
+    test "absname/1" do
+      assert Path.absname("//host/path") == "//host/path"
+      assert Path.absname("\\\\host\\path") == "//host/path"
+      assert Path.absname("\\/host\\path") == "//host/path"
+      assert Path.absname("/\\host\\path") == "//host/path"
+    end
+
     test "relative/1" do
       assert Path.relative("C:/usr/local/bin") == "usr/local/bin"
       assert Path.relative("C:\\usr\\local\\bin") == "usr\\local\\bin"
@@ -67,6 +74,11 @@ defmodule PathTest do
       assert Path.type("/usr/local/bin") == :volumerelative
       assert Path.type('usr/local/bin') == :relative
       assert Path.type("../usr/local/bin") == :relative
+
+      assert Path.type("//host/path") == :absolute
+      assert Path.type("\\\\host\\path") == :absolute
+      assert Path.type("/\\host\\path") == :absolute
+      assert Path.type("\\/host\\path") == :absolute
     end
 
     test "split/1" do

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

@@ -1033,7 +1033,9 @@ defmodule Mix.Tasks.Release do
         lib_dirs ++ release_files
 
     files =
-      Enum.map(dirs, &{String.to_charlist(&1), String.to_charlist(Path.join(release.path, &1))})
+      dirs
+      |> Enum.filter(&File.exists?(Path.join(release.path, &1)))
+      |> Enum.map(&{String.to_charlist(&1), String.to_charlist(Path.join(release.path, &1))})
 
     File.rm(out_path)
     :ok = :erl_tar.create(String.to_charlist(out_path), files, [:dereference, :compressed])

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

@@ -67,6 +67,8 @@ defmodule Mix.Tasks.ReleaseTest do
           assert "releases/0.1.0/vm.args" in files
           assert "releases/COOKIE" in files
           assert "releases/start_erl.data" in files
+          assert Enum.any?(files, &(&1 =~ "erts"))
+          assert Enum.any?(files, &(&1 =~ "stdlib"))
 
           for dir <- files_with_versions -- ["ignored_app-0.1.0"] do
             [name | _] = String.split(dir, "-")
@@ -79,6 +81,29 @@ defmodule Mix.Tasks.ReleaseTest do
       end)
     end
 
+    test "tar without 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")
+
+          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
+
     test "steps" do
       in_fixture("release_test", fn ->
         last_step = fn release ->