Optimize cookie parsing (#1310)

Use length tracking and binary_part/3 instead of repeated binary concatenation to avoid allocating new binaries during decoding.

Author: preciz

lib/plug/conn/cookies.ex

@@ -49,40 +49,49 @@ defmodule Plug.Conn.Cookies do
 
   defp decode_kv("", acc), do: acc
   defp decode_kv(<<h, t::binary>>, acc) when h in [?\s, ?\t], do: decode_kv(t, acc)
-  defp decode_kv(kv, acc) when is_binary(kv), do: decode_key(kv, "", acc)
+  defp decode_kv(kv, acc) when is_binary(kv), do: decode_key(kv, kv, 0, acc)
 
-  defp decode_key(<<h, t::binary>>, _key, acc) when h in [?\s, ?\t, ?\r, ?\n, ?\v, ?\f],
-    do: skip_until_cc(t, acc)
+  defp decode_key(<<h, t::binary>>, _key_rest, _len, acc)
+       when h in [?\s, ?\t, ?\r, ?\n, ?\v, ?\f],
+       do: skip_until_cc(t, acc)
 
-  defp decode_key(<<?;, t::binary>>, _key, acc), do: decode_kv(t, acc)
-  defp decode_key(<<?=, t::binary>>, "", acc), do: skip_until_cc(t, acc)
-  defp decode_key(<<?=, t::binary>>, key, acc), do: decode_value(t, "", 0, key, acc)
-  defp decode_key(<<h, t::binary>>, key, acc), do: decode_key(t, <<key::binary, h>>, acc)
-  defp decode_key(<<>>, _key, acc), do: acc
+  defp decode_key(<<?;, t::binary>>, _key_rest, _len, acc), do: decode_kv(t, acc)
+  defp decode_key(<<?=, t::binary>>, _key_rest, 0, acc), do: skip_until_cc(t, acc)
 
-  defp decode_value(<<?;, t::binary>>, value, spaces, key, acc),
-    do: decode_kv(t, [{key, trim_spaces(value, spaces)} | acc])
+  defp decode_key(<<?=, t::binary>>, key_rest, len, acc) do
+    key = binary_part(key_rest, 0, len)
+    decode_value(t, t, 0, 0, key, acc)
+  end
 
-  defp decode_value(<<?\s, t::binary>>, value, spaces, key, acc),
-    do: decode_value(t, <<value::binary, ?\s>>, spaces + 1, key, acc)
+  defp decode_key(<<_, t::binary>>, key_rest, len, acc), do: decode_key(t, key_rest, len + 1, acc)
+  defp decode_key(<<>>, _key_rest, _len, acc), do: acc
 
-  defp decode_value(<<h, t::binary>>, _value, _spaces, _key, acc)
+  defp decode_value(<<?;, t::binary>>, val_rest, len, spaces, key, acc) do
+    value = binary_part(val_rest, 0, len - spaces)
+    decode_kv(t, [{key, value} | acc])
+  end
+
+  defp decode_value(<<?\s, t::binary>>, val_rest, len, spaces, key, acc) do
+    decode_value(t, val_rest, len + 1, spaces + 1, key, acc)
+  end
+
+  defp decode_value(<<h, t::binary>>, _val_rest, _len, _spaces, _key, acc)
        when h in [?\t, ?\r, ?\n, ?\v, ?\f],
        do: skip_until_cc(t, acc)
 
-  defp decode_value(<<h, t::binary>>, value, _spaces, key, acc),
-    do: decode_value(t, <<value::binary, h>>, 0, key, acc)
+  defp decode_value(<<_, t::binary>>, val_rest, len, _spaces, key, acc) do
+    decode_value(t, val_rest, len + 1, 0, key, acc)
+  end
 
-  defp decode_value(<<>>, value, spaces, key, acc),
-    do: [{key, trim_spaces(value, spaces)} | acc]
+  defp decode_value(<<>>, val_rest, len, spaces, key, acc) do
+    value = binary_part(val_rest, 0, len - spaces)
+    [{key, value} | acc]
+  end
 
   defp skip_until_cc(<<?;, t::binary>>, acc), do: decode_kv(t, acc)
   defp skip_until_cc(<<_, t::binary>>, acc), do: skip_until_cc(t, acc)
   defp skip_until_cc(<<>>, acc), do: acc
 
-  defp trim_spaces(value, 0), do: value
-  defp trim_spaces(value, spaces), do: binary_part(value, 0, byte_size(value) - spaces)
-
   @doc """
   Encodes the given cookies as expected in a response header.