Fix 8-bit decoding to preserve line breaks but restore wrapped lines

lib/mail/encoders/eight_bit.ex

@@ -37,14 +37,19 @@ defmodule Mail.Encoders.EightBit do
   @doc """
   Decodes an 8-bit encoded string.
   """
-  def decode(string), do: do_decode(string, "")
-  defp do_decode(<<>>, acc), do: acc
+  def decode(encoded_string), do: do_decode(encoded_string, "", 0)
 
-  defp do_decode(<<head, tail::binary>>, acc) do
-    {decoded, tail} = decode_char(head, tail)
-    do_decode(tail, acc <> decoded)
+  defp do_decode(<<>>, acc, _line_length), do: acc
+
+  defp do_decode(<<"\r\n", tail::binary>>, acc, 998) do
+    do_decode(tail, acc, 0)
+  end
+
+  defp do_decode(<<head, tail::binary>>, acc, line_length) do
+    {decoded, tail, length} = decode_char(head, tail)
+    do_decode(tail, acc <> decoded, line_length + length)
   end
 
-  defp decode_char(?\r, <<?\n, tail::binary>>), do: {"", tail}
-  defp decode_char(char, <<tail::binary>>), do: {<<char>>, tail}
+  defp decode_char(?\0, _tail), do: raise(ArgumentError, message: "illegal NUL character")
+  defp decode_char(char, <<tail::binary>>), do: {<<char>>, tail, 1}
 end

test/mail/encoders/eight_bit_test.exs

@@ -23,8 +23,21 @@ defmodule Mail.Encoders.EightBitTest do
     assert Mail.Encoders.EightBit.decode("") == ""
   end
 
-  test "decode removes <CR><LF> pairs" do
+  test "decode preserves <CR><LF> pairs" do
     message = "This is a \r\ntest\r\n"
-    assert Mail.Encoders.EightBit.decode(message) == "This is a test"
+    assert Mail.Encoders.EightBit.decode(message) == "This is a \r\ntest\r\n"
+  end
+
+  test "decode removes crlf wrapping characters" do
+    message = String.duplicate("-", 1000)
+    encoded = Mail.Encoders.EightBit.encode(message)
+    assert binary_part(encoded, 998, 2) == "\r\n"
+    assert message == Mail.Encoders.EightBit.decode(encoded)
+  end
+
+  test "decode raises if any character is <NUL>" do
+    assert_raise ArgumentError, fn ->
+      Mail.Encoders.EightBit.decode("\0")
+    end
   end
 end