Fix File.cp/3 in case of a file coping to itself

Author: lexmag

lib/elixir/lib/file.ex

@@ -461,7 +461,7 @@ defmodule File do
   error will be returned.
   """
   @spec cp(Path.t, Path.t, (Path.t, Path.t -> boolean)) :: :ok | {:error, posix}
-  def cp(source, destination, callback \\ fn(_, _) -> true end) do
+  def cp(source, destination, callback \\ &path_differs?/2) do
     source = IO.chardata_to_string(source)
     destination = IO.chardata_to_string(destination)
 
@@ -471,12 +471,19 @@ defmodule File do
     end
   end
 
+  defp path_differs?(path, path),
+    do: false
+
+  defp path_differs?(p1, p2) do
+    Path.expand(p1) !== Path.expand(p2)
+  end
+
   @doc """
   The same as `cp/3`, but raises `File.CopyError` if it fails.
   Returns the list of copied files otherwise.
   """
   @spec cp!(Path.t, Path.t, (Path.t, Path.t -> boolean)) :: :ok | no_return
-  def cp!(source, destination, callback \\ fn(_, _) -> true end) do
+  def cp!(source, destination, callback \\ &path_differs?/2) do
     case cp(source, destination, callback) do
       :ok -> :ok
       {:error, reason} ->
@@ -599,8 +606,6 @@ defmodule File do
         [dest|acc]
       {:error, :eexist} ->
         if callback.(src, dest) do
-          # If rm/1 fails, copy/2 will fail
-          _ = rm(dest)
           case copy(src, dest) do
             {:ok, _} ->
               copy_file_mode!(src, dest)
@@ -621,7 +626,7 @@ defmodule File do
         [dest|acc]
       {:error, :eexist} ->
         if callback.(src, dest) do
-          # If rm/1 fails, iF.make_symlink/2 will fail
+          # If rm/1 fails, F.make_symlink/2 will fail
           _ = rm(dest)
           case F.make_symlink(link, dest) do
             :ok -> [dest|acc]

lib/elixir/test/elixir/file_test.exs

@@ -129,6 +129,19 @@ defmodule FileTest do
       end
     end
 
+    test :cp_file_to_itself do
+      src = dest = tmp_path("tmp.file")
+
+      File.write!(src, "here")
+
+      try do
+        assert File.cp(src, dest) == :ok
+        assert File.read!(dest) == "here"
+      after
+        File.rm(dest)
+      end
+    end
+
     test :cp_r_with_src_file_and_dest_file do
       src  = fixture_path("file.txt")
       dest = tmp_path("sample.txt")