Introduce basic support for close_read and close_write.

lib/openssl/ssl.rb

@@ -459,6 +459,32 @@ def session
         nil
       end
 
+      # Close the stream for reading.
+      # This method is ignored by OpenSSL as there is no reasonable way to
+      # implement it, but exists for compatibility with IO.
+      def close_read
+        # Unsupported and ignored.
+        # Just don't read any more.
+      end
+
+      # Closes the stream for writing. The behavior of this method depends on
+      # the version of OpenSSL and the TLS protocol in use.
+      # 
+      # In TLS 1.3 and later:
+      # - Sends a 'close_notify' alert to the peer.
+      # - Does not wait for the peer's 'close_notify' alert in response.
+      # 
+      # In TLS 1.2 and earlier:
+      # - Sends a 'close_notify' alert to the peer.
+      # - Waits for the peer's 'close_notify' alert in response.
+      # 
+      # Therefore, on TLS 1.2, this method will cause the connection to be
+      # completely shut down. On TLS 1.3, the connection will remain open for
+      # reading only.
+      def close_write
+        stop
+      end
+
       private
 
       def using_anon_cipher?

test/openssl/test_ssl.rb

@@ -117,6 +117,30 @@ def test_socket_open_with_local_address_port_context
     }
   end
 
+  def test_socket_close_write
+    server_proc = proc do |ctx, ssl|
+      message = ssl.read
+      ssl.write(message)
+      ssl.close_write
+    ensure
+      ssl.close
+    end
+
+    start_server(server_proc: server_proc) do |port|
+      ctx = OpenSSL::SSL::SSLContext.new
+      ssl = OpenSSL::SSL::SSLSocket.open("127.0.0.1", port, context: ctx)
+      ssl.sync_close = true
+      ssl.connect
+
+      message = "abc"*1024
+      ssl.write message
+      ssl.close_write
+      assert_equal message, ssl.read
+    ensure
+      ssl&.close
+    end
+  end
+
   def test_add_certificate
     ctx_proc = -> ctx {
       # Unset values set by start_server