Fix double incrementing of the packet number (#117)

This commit fixes an issue where the packet number field of the
packet header was incorrectly incremented twice.  This resulted
in the packet number fo a single LOGIN7 packet being 2, rather 1.

This violated the TDS protocol definition.  Furthermore, firewall
rules that restrict traffic based on the TDS protocol filtered out
such violating TDS packets.  Also, it resulted in tools such as
Wireshark not being able to properly parse out the TDS header.

Author: deepankar-j

lib/tds/messages.ex

@@ -601,7 +601,7 @@ defmodule Tds.Messages do
   end
 
   def encode_packets(type, data, id) do
-    header = encode_header(type, data, id: id + 1, status: 1)
+    header = encode_header(type, data, id: id, status: 1)
     [header <> data]
   end
 

test/messages_test.exs

@@ -0,0 +1,113 @@
+defmodule MessagesTest do
+  use ExUnit.Case, async: true
+
+  describe "encode_packets" do
+    test "data length < 4088 is encoded into one packet" do
+      assert [<<
+               # type
+               0x10,
+               # status
+               0x1,
+               # length
+               0x0,
+               0x9,
+               # channel
+               0x0,
+               0x0,
+               # packet number
+               0x1,
+               # window
+               0x0,
+               # data
+               0xFF
+             >>] == Tds.Messages.encode_packets(0x10, <<0xFF>>)
+    end
+
+    test "data length == 4087 is encoded into one packet" do
+      data = :binary.copy(<<0xFF>>, 4087)
+      assert [<<
+               # type
+               0x10,
+               # status
+               0x1,
+               # length
+               0x0F,
+               0xFF,
+               # channel
+               0x0,
+               0x0,
+               # packet number
+               0x1,
+               # window
+               0x0,
+               data::binary
+              >>] == Tds.Messages.encode_packets(0x10, data)
+    end
+
+    test "data length == 4088 is encoded into one packet" do
+      data = :binary.copy(<<0xFF>>, 4088)
+      assert [[<<
+               # type
+               0x10,
+               # status
+               0x1,
+               # length
+               0x10,
+               0x00,
+               # channel
+               0x0,
+               0x0,
+               # packet number
+               0x1,
+               # window
+               0x0
+              >>, <<
+               # data
+               data::binary
+             >>]] == Tds.Messages.encode_packets(0x10, data)
+    end
+
+    test "data length == 4089 is encoded into two packets " do
+      part1 = :binary.copy(<<0xFF>>, 4088)
+      part2 = :binary.copy(<<0xFF>>, 1)
+      data = part1 <> part2
+      assert [[<<
+               # type
+               0x10,
+               # status
+               0x0,
+               # length
+               0x10,
+               0x00,
+               # channel
+               0x0,
+               0x0,
+               # packet number
+               0x1,
+               # window
+               0x0 >>,
+              <<
+               # data
+               part1::binary
+             >>],
+               <<
+               # type
+               0x10,
+               # status
+               0x1,
+               # length
+               0x0,
+               0x9,
+               # channel
+               0x0,
+               0x0,
+               # packet number
+               0x2,
+               # window
+               0x0,
+               # data
+               0xFF
+             >>] == Tds.Messages.encode_packets(0x10, data)
+    end
+  end
+end