Check IO buffer when polling for reading.

Author: aardvark179

lib/truffle/io/wait.rb

@@ -13,6 +13,7 @@ def nread
   end
 
   def ready?
+    ensure_open_and_readable
     Truffle::IOOperations.poll(self, Truffle::IOOperations::POLLIN, 0) > 0
   end
 

src/main/ruby/truffleruby/core/truffle/io_operations.rb

@@ -200,6 +200,10 @@ def self.select(readables, readable_ios, writables, writable_ios, errorables, er
     end
 
     def self.poll(io, events, timeout)
+      if (events & POLLIN) != 0
+        return 1 unless io.__send__(:buffer_empty?)
+      end
+
       if timeout
         unless Primitive.object_kind_of? timeout, Numeric
           raise TypeError, 'Timeout must be numeric'