IO#sysread Check for readable

src/io.c

@@ -637,6 +637,9 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
   }
 
   fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);
+  if (!fptr->readable) {
+    mrb_raise(mrb, E_IO_ERROR, "not opened for reading");
+  }
   ret = read(fptr->fd, RSTRING_PTR(buf), maxlen);
   switch (ret) {
     case 0: /* EOF */

test/io.rb

@@ -238,6 +238,11 @@ assert('IO.sysopen, IO#sysread') do
   assert_raise(IOError) { io.sysread(1) }
   assert_raise(ArgumentError) { io.sysread(-1) }
   io.closed?
+
+  fd = IO.sysopen $mrbtest_io_wfname, "w"
+  io = IO.new fd, "w"
+  assert_raise(IOError) { io.sysread(1) }
+  io.close
 end
 
 assert('IO.sysopen, IO#syswrite') do