Process an extended or global header as described in
           POSIX.1-2008.

def _proc_pax(self, tarfile):
        """Process an extended or global header as described in
           POSIX.1-2008.
        """
        # Read the header information.
        buf = tarfile.fileobj.read(self._block(self.size))

        # A pax header stores supplemental information for either
        # the following file (extended) or all following files
        # (global).
        if self.type == XGLTYPE:
            pax_headers = tarfile.pax_headers
        else:
            pax_headers = tarfile.pax_headers.copy()

        # Check if the pax header contains a hdrcharset field. This tells us
        # the encoding of the path, linkpath, uname and gname fields. Normally,
        # these fields are UTF-8 encoded but since POSIX.1-2008 tar
        # implementations are allowed to store them as raw binary strings if
        # the translation to UTF-8 fails.
        match = re.search(br"\d+ hdrcharset=([^\n]+)\n", buf)
        if match is not None:
            pax_headers["hdrcharset"] = match.group(1).decode("utf8")

        # For the time being, we don't care about anything other than "BINARY".
        # The only other value that is currently allowed by the standard is
        # "ISO-IR 10646 2000 UTF-8" in other words UTF-8.
        hdrcharset = pax_headers.get("hdrcharset")
        if hdrcharset == "BINARY":
            encoding = tarfile.encoding
        else:
            encoding = "utf8"

        # Parse pax header information. A record looks like that:
        # "%d %s=%s\n" % (length, keyword, value). length is the size
        # of the complete record including the length field itself and
        # the newline. keyword and value are both UTF-8 encoded strings.
        regex = re.compile(br"(\d+) ([^=]+)=")
        pos = 0
        while True:
            match = regex.match(buf, pos)
            if not match:
                break

            length, keyword = match.groups()
            length = int(length)
            value = buf[match.end(2) + 1:match.start(1) + length - 1]

            # Normally, we could just use "utf8" as the encoding and "strict"
            # as the error handler, but we better not take the risk. For
            # example, GNU tar <= 1.23 is known to store filenames it cannot
            # translate to UTF-8 as raw strings (unfortunately without a
            # hdrcharset=BINARY header).
            # We first try the strict standard encoding, and if that fails we
            # fall back on the user's encoding and error handler.
            keyword = self._decode_pax_field(keyword, "utf8", "utf8",
                    tarfile.errors)
            if keyword in PAX_NAME_FIELDS:
                value = self._decode_pax_field(value, encoding, tarfile.encoding,
                        tarfile.errors)
            else:
                value = self._decode_pax_field(value, "utf8", "utf8",
                        tarfile.errors)

            pax_headers[keyword] = value
            pos += length

        # Fetch the next header.
        try:
            next = self.fromtarfile(tarfile)
        except HeaderError:
            raise SubsequentHeaderError("missing or bad subsequent header")

        # Process GNU sparse information.
        if "GNU.sparse.map" in pax_headers:
            # GNU extended sparse format version 0.1.
            self._proc_gnusparse_01(next, pax_headers)

        elif "GNU.sparse.size" in pax_headers:
            # GNU extended sparse format version 0.0.
            self._proc_gnusparse_00(next, pax_headers, buf)

        elif pax_headers.get("GNU.sparse.major") == "1" and pax_headers.get("GNU.sparse.minor") == "0":
            # GNU extended sparse format version 1.0.
            self._proc_gnusparse_10(next, pax_headers, tarfile)

        if self.type in (XHDTYPE, SOLARIS_XHDTYPE):
            # Patch the TarInfo object with the extended header info.
            next._apply_pax_info(pax_headers, tarfile.encoding, tarfile.errors)
            next.offset = self.offset

            if "size" in pax_headers:
                # If the extended header replaces the size field,
                # we need to recalculate the offset where the next
                # header starts.
                offset = next.offset_data
                if next.isreg() or next.type not in SUPPORTED_TYPES:
                    offset += next._block(next.size)
                tarfile.offset = offset

        return next