Fix IO.copy_stream with a Tempfile destination

* Fixes #3280

CHANGELOG.md

@@ -6,6 +6,7 @@ New features:
 Bug fixes:
 
 * Fix `rb_enc_left_char_head()` so it is not always `ArgumentError` (#3267, @eregon).
+* Fix `IO.copy_stream` with a `Tempfile` destination (#3280, @eregon).
 
 Compatibility:
 

spec/ruby/core/io/copy_stream_spec.rb

@@ -69,9 +69,12 @@
   end
 
   it "raises an IOError if the destination IO is not open for writing" do
-    @to_io.close
-    @to_io = new_io @to_name, "r"
-    -> { IO.copy_stream @object.from, @to_io }.should raise_error(IOError)
+    to_io = new_io __FILE__, "r"
+    begin
+      -> { IO.copy_stream @object.from, to_io }.should raise_error(IOError)
+    ensure
+      to_io.close
+    end
   end
 
   it "does not close the destination IO" do
@@ -109,7 +112,8 @@
   end
 
   after :each do
-    rm_r @to_name, @from_bigfile
+    rm_r @to_name if @to_name
+    rm_r @from_bigfile
   end
 
   describe "from an IO" do
@@ -164,6 +168,25 @@
       it_behaves_like :io_copy_stream_to_io, nil, IOSpecs::CopyStream
       it_behaves_like :io_copy_stream_to_io_with_offset, nil, IOSpecs::CopyStream
     end
+
+    describe "to a Tempfile" do
+      before :all do
+        require 'tempfile'
+      end
+
+      before :each do
+        @to_io = Tempfile.new("rubyspec_copy_stream", encoding: Encoding::BINARY, mode: File::RDONLY)
+        @to_name = @to_io.path
+      end
+
+      after :each do
+        @to_io.close!
+        @to_name = nil # do not rm_r it, already done by Tempfile#close!
+      end
+
+      it_behaves_like :io_copy_stream_to_io, nil, IOSpecs::CopyStream
+      it_behaves_like :io_copy_stream_to_io_with_offset, nil, IOSpecs::CopyStream
+    end
   end
 
   describe "from a file name" do
@@ -277,10 +300,8 @@
       @io.should_not_receive(:pos)
       IO.copy_stream(@io, @to_name)
     end
-
   end
 
-
   describe "with a destination that does partial reads" do
     before do
       @from_out, @from_in = IO.pipe

src/main/ruby/truffleruby/core/io.rb

@@ -356,24 +356,19 @@ def initialize(from, to, length, offset)
       @method = read_method @from
     end
 
+    # From copy_stream_body in io.c in CRuby
+    # The first element is true if obj can be used as an IO directly
     def to_io(obj, mode)
-      if Primitive.is_a?(obj, IO)
-        flag = true
-        io = obj
+      io = IO.try_convert(obj)
+      if io
+        [true, io]
+      elsif Primitive.is_a?(obj, String) or obj.respond_to? :to_path
+        path = Truffle::Type.coerce_to obj, String, :to_path
+        io = File.open path, mode
+        [false, io]
       else
-        flag = false
-
-        if Primitive.is_a?(obj, String)
-          io = File.open obj, mode
-        elsif obj.respond_to? :to_path
-          path = Truffle::Type.coerce_to obj, String, :to_path
-          io = File.open path, mode
-        else
-          io = obj
-        end
+        [false, obj]
       end
-
-      [flag, io]
     end
 
     def read_method(obj)