Read metadata of inserted PDF page?

Question

I use only lualatex, so a compiler-specific answer is OK.

Consider this pseudocode-MWE. The included PDF is always one page:

\documentclass{article}
\usepackage{pdfpages}
\def\includepdfifgood#1{
  % This is where I test the pdfproducer string in #1.
  % If wrong pdfproducer, compile halts with error message. Otherwise:
  \includepdf{#1}
}
\begin{document}
\includepdfifgood{something.pdf}
\end{document}

The included page must be the output of a previous lualatex compile. In that earlier compile, I can (for example) set pdfproducer to a custom string. I know how to do that.

So, what I need to do is inspect pdfproducer and accept or reject the file.

Why I need to do this: PDF/X (and PDF/A) have stringent requirements. If a properly pre-processed PDF page is included in such a document, it can still be PDF/X or PDF/A. Otherwise, compliance may fail (and TeX does not know that it fails). I know how to pre-process. Now I need to ensure that only a pre-processed page is allowed into another PDF.

EDIT2: When I asked this question, I had the PDF Producer string in mind. It turns out that the Producer string is not the best one to use, because it may change when the Producer changes version number (for example). It is better to use something that the user can directly set, such as Title, Author, Subject, or Keywords. Fortunately, the accepted code works for any standard info dictionary entry. There is no need to modify it, since within the luacode, producer is just a local variable name, which works even if you are looking for Author, or whatever.

EDIT: In an earlier question, I received a magnificent answer in luacode, which I am already using. My rationale for asking is the same, but the circumstances are different. Alas, I do not know enough Lua to edit the linked answer, or I would have done it.

EDIT2: The accepted answer has been put to good use! In version 1.50 of novel package, the procedure for creating CMYK 240% PDF/X-1a:2001 color cover artwork has been automated by batch/bash script. First, an RGB image (or PDF) is converted to CMYK 240% PDF (FlateDecode) without TeX, using ImageMagick and Ghostscript. This PDF is not PDF/X. However, using the pdfmark capability of Ghostscript, it has a custom tag in the PDF info dictionary. Then, LuaLaTeX (with novel) processes that file into PDF/X-1a:2001. It inspects for the custom tag, and will only perform the process if the tag is detected. That's how it knows the image is in the correct color space, since no profile is embedded. The results have been validated using Adobe Acrobat Pro (which, I believe, is the only kind of program that could do it, until now).

Answer 1

You can use the following code to verify the entries in the Info dictionary:

\documentclass{scrartcl}
\usepackage{luacode,pdfpages}
\begin{luacode*}
  function utf16to8(u16str)
    local result = ""
    local i = 1
    if #u16str % 2 == 1 then
      print("ERROR")
      return
    end
    while i < #u16str do
      local high, low = u16str:byte(i, i + 1)
      i = i + 2
      local current = bit32.replace(low, high, 8, 8)
      if bit32.band(high, 0xFC) == 0xD8 then
        current = bit32.replace(0, current, 10, 10)
        if i > #u16str then
          print("ERROR")
          return
        end
        high, low = u16str:byte(i, i + 1)
        i = i + 2
        current = bit32.replace(current, bit32.replace(low, high, 8, 8), 0, 10) + 0x10000
      elseif bit32.band(high, 0xFC) == 0xDC then
        print("ERROR")
        return
      end
      result = result .. unicode.utf8.char(current)
    end
    return result
  end
  function normalize_string(str)
    if str:sub(1,2) == "\xFE\xFF" then
      return utf16to8(str:sub(3))
    else
      return str
    end
  end
  function check_pdf_info(name, field, expected)
    local doc = epdf.open(name);
    if doc == nil then
      tex.sprint(luatexbase.catcodetables['latex-package'],
          "\\errmessage{Could not open " .. name .. "}{}{}\\@gobbletwo")
    else
      local producer = doc:getDocInfo():dictLookup(field)
      if not producer:isNull() and normalize_string(producer:getString()) == expected then
        tex.sprint(luatexbase.catcodetables['latex-package'], '\\@firstoftwo')
      else
        tex.sprint(luatexbase.catcodetables['latex-package'], '\\@secondoftwo')
      end
    end
  end
\end{luacode*}
\newcommand\PDFVerifyInfoFieldTF[3]{\directlua{check_pdf_info("\luaescapestring{#1}", "\luaescapestring{#2}", "\luaescapestring{#3}")}}
\begin{document}
\PDFVerifyInfoFieldTF{some_file.pdf}{Producer}{pdfTeX-1.40.8}{%
  \includepdf[pages=-]{some_file.pdf}%
}{%
  \errmessage{some_file.tex has the wrong producer}%
}
\end{document}

Possible fields:

• Producer: For example pdfTeX-1.40.18 or LuaTeX-1.0.4.
• Creator: For example TeX or LaTeX with hyperref package.
• Author
• Title
• Subject
• Keywords
• PTEX.Fullbanner: For example This is LuaTeX, Version 1.0.4 (TeX Live 2017/Arch Linux))
• CreationDate or ModDate These two are probably not very useful in this context.