def find_identifiers(src):
    """
    Search for a valid identifier (DOI, ISBN, arXiv, HAL) in a given file.

    .. note::

        This function returns the first matching identifier, that is the most
        likely to be relevant for this file. However, it may fail and return an
        identifier taken from the references or another paper.

    .. note::

        You will need to have ``pdftotext`` and/or ``djvutxt`` installed \
                system-wide before processing files with this function.


    :params src: Path to the file to scan.

    :returns: a tuple (type, identifier) or ``(None, None)`` if not found or \
            an error occurred.
    """
    if src.endswith(".pdf"):
        totext = subprocess.Popen(["pdftotext", src, "-"],
                                  stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE,
                                  bufsize=1)
    elif src.endswith(".djvu"):
        totext = subprocess.Popen(["djvutxt", src],
                                  stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE,
                                  bufsize=1)
    else:
        return (None, None)

    while totext.poll() is None:
        extract_full = ' '.join([i.decode("utf-8").strip()
                                 for i in totext.stdout.readlines()])
        # Loop over all the valid identifier types
        for identifier in __valid_identifiers__:
            # Dynamically call the ``extract_from_text`` method for the
            # associated module.
            module = sys.modules.get("libbmc.%s" % (identifier,), None)
            if module is None:
                continue
            found_id = getattr(module, "extract_from_text")(extract_full)
            if found_id:
                totext.terminate()
                # found_id is a list of found IDs
                return (identifier, found_id[0])
    return (None, None)