Fix crash when a server sends an x-hex-message header

[ericmj]

Hex.HTTP.handle_hex_message/1 is meant to surface x-hex-message headers from the server (e.g. deprecation notices) as API warning: / API error: output. It runs :binary.list_to_bin/1 on the header value, which requires a charlist — but decode_header/1 converts every response header value to a binary via List.to_string/1. So any real x-hex-message header raises ArgumentError: not an iodata term inside the HTTP task and fails the request (e.g. mix deps.get).

No hex.pm endpoint currently sends x-hex-message, so the bug has been latent — the existing test only exercised the function with charlists. This switches to IO.iodata_to_binary/1, which accepts both binaries (the real path) and charlists (the existing test path), and adds a binary regression case.

[maennchen]

I think this bug was a blessing in disguise. This would also be called for untrusted servers, right?

There's quite a lot of terminal escape sequences that could potentially be problematic. We should add some escaping.

[maennchen]

Something along those lines:

defmodule TerminalSafe do
  @bidi_controls [
    0x061C,
    0x202A,
    0x202B,
    0x202C,
    0x202D,
    0x202E,
    0x2066,
    0x2067,
    0x2068,
    0x2069
  ]

  def escape(binary) when is_binary(binary) do
    binary
    |> String.to_charlist()
    |> Enum.map_join(fn
      cp when cp in @bidi_controls ->
        "\\u{" <> Integer.to_string(cp, 16) <> "}"

      ?\n ->
        "\n"

      ?\t ->
        "\t"

      # Important terminal controls
      ?\e ->
        "\\e"

      ?\r ->
        "\\r"

      ?\b ->
        "\\b"

      ?\a ->
        "\\a"

      # Other C0 controls
      cp when cp < 0x20 ->
        hex =
          cp
          |> Integer.to_string(16)
          |> String.upcase()
          |> String.pad_leading(2, "0")

        "\\x" <> hex

      # DEL
      0x7F ->
        "\\x7F"

      # C1 controls
      cp when cp >= 0x80 and cp <= 0x9F ->
        "\\u{" <> Integer.to_string(cp, 16) <> "}"

      # Normal printable codepoint
      cp ->
        <<cp::utf8>>
    end)
  end
end