Fix Process.run with closed IO

spec/std/process_spec.cr

@@ -181,6 +181,14 @@ pending_interpreted describe: Process do
       $?.exit_code.should eq(0)
     end
 
+    it "forwards closed io" do
+      closed_io = IO::Memory.new
+      closed_io.close
+      Process.run(*stdin_to_stdout_command, input: closed_io)
+      Process.run(*stdin_to_stdout_command, output: closed_io)
+      Process.run(*stdin_to_stdout_command, error: closed_io)
+    end
+
     it "sets working directory with string" do
       parent = File.dirname(Dir.current)
       command = {% if flag?(:win32) %}

src/crystal/system/unix/process.cr

@@ -292,6 +292,11 @@ struct Crystal::System::Process
   end
 
   private def self.reopen_io(src_io : IO::FileDescriptor, dst_io : IO::FileDescriptor)
+    if src_io.closed?
+      dst_io.close
+      return
+    end
+
     src_io = to_real_fd(src_io)
 
     dst_io.reopen(src_io)

src/process.cr

@@ -294,6 +294,14 @@ class Process
     when IO::FileDescriptor
       stdio
     when IO
+      if stdio.closed?
+        if dst_io == STDIN
+          return File.open(File::NULL, "r").tap(&.close)
+        else
+          return File.open(File::NULL, "w").tap(&.close)
+        end
+      end
+
       if dst_io == STDIN
         fork_io, process_io = IO.pipe(read_blocking: true)