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

Author: Dmitry Kakurin

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