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#!/usr/bin/env bash

# This script is a wrapper to the other download backends.
# Its role is to ensure atomicity when saving downloaded files
# back to BR2_DL_DIR, and not clutter BR2_DL_DIR with partial,
# failed downloads.
#
# Call it with -h to see some help.

# To avoid cluttering BR2_DL_DIR, we download to a trashable
# location, namely in $(BUILD_DIR).
# Then, we move the downloaded file to a temporary file in the
# same directory as the final output file.
# This allows us to finally atomically rename it to its final
# name.
# If anything goes wrong, we just remove all the temporaries
# created so far.

# We want to catch any unexpected failure, and exit immediately.
set -e

main() {
    local OPT OPTARG
    local backend output

    # Parse our options; anything after '--' is for the backend
    while getopts :hb:o: OPT; do
        case "${OPT}" in
        h)  help; exit 0;;
        b)  backend="${OPTARG}";;
        o)  output="${OPTARG}";;
        :)  error "option '%s' expects a mandatory argument\n" "${OPTARG}";;
        \?) error "unknown option '%s'\n" "${OPTARG}";;
        esac
    done
    # Forget our options, and keep only those for the backend
    shift $((OPTIND-1))

    if [ -z "${backend}" ]; then
        error "no backend specified, use -b\n"
    fi
    if [ -z "${output}" ]; then
        error "no output specified, use -o\n"
    fi

    # tmpd is a temporary directory in which backends may store intermediate
    # by-products of the download.
    # tmpf is the file in which the backends should put the downloaded content.
    # tmpd is located in $(BUILD_DIR), so as not to clutter the (precious)
    # $(BR2_DL_DIR)
    # We let the backends create tmpf, so they are able to set whatever
    # permission bits they want (although we're only really interested in
    # the executable bit.)
    tmpd="$(mktemp -d "${BUILD_DIR}/.${output##*/}.XXXXXX")"
    tmpf="${tmpd}/output"

    # Helpers expect to run in a directory that is *really* trashable, so
    # they are free to create whatever files and/or sub-dirs they might need.
    # Doing the 'cd' here rather than in all backends is easier.
    cd "${tmpd}"

    # If the backend fails, we can just remove the temporary directory to
    # remove all the cruft it may have left behind. Then we just exit in
    # error too.
    if ! "${OLDPWD}/support/download/${backend}" "${tmpf}" "${@}"; then
        rm -rf "${tmpd}"
        exit 1
    fi

    # cd back to free the temp-dir, so we can remove it later
    cd "${OLDPWD}"

    # tmp_output is in the same directory as the final output, so we can
    # later move it atomically.
    tmp_output="$(mktemp "${output}.XXXXXX")"

    # 'mktemp' creates files with 'go=-rwx', so the files are not accessible
    # to users other than the one doing the download (and root, of course).
    # This can be problematic when a shared BR2_DL_DIR is used by different
    # users (e.g. on a build server), where all users may write to the shared
    # location, since other users would not be allowed to read the files
    # another user downloaded.
    # So, we restore the 'go' access rights to a more sensible value, while
    # still abiding by the current user's umask. We must do that before the
    # final 'mv', so just do it now.
    # Some backends (cp and scp) may create executable files, so we need to
    # carry the executable bit if needed.
    [ -x "${tmpf}" ] && new_mode=755 || new_mode=644
    new_mode=$(printf "%04o" $((0${new_mode} & ~0$(umask))))
    chmod ${new_mode} "${tmp_output}"

    # We must *not* unlink tmp_output, otherwise there is a small window
    # during which another download process may create the same tmp_output
    # name (very, very unlikely; but not impossible.)
    # Using 'cp' is not reliable, since 'cp' may unlink the destination file
    # if it is unable to open it with O_WRONLY|O_TRUNC; see:
    #   http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cp.html
    # Since the destination filesystem can be anything, it might not support
    # O_TRUNC, so 'cp' would unlink it first.
    # Use 'cat' and append-redirection '>>' to save to the final location,
    # since that is the only way we can be 100% sure of the behaviour.
    if ! cat "${tmpf}" >>"${tmp_output}"; then
        rm -rf "${tmpd}" "${tmp_output}"
        exit 1
    fi
    rm -rf "${tmpd}"

    # tmp_output and output are on the same filesystem, so POSIX guarantees
    # that 'mv' is atomic, because it then uses rename() that POSIX mandates
    # to be atomic, see:
    #   http://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html
    if ! mv -f "${tmp_output}" "${output}"; then
        rm -f "${tmp_output}"
        exit 1
    fi
}

help() {
    cat <<_EOF_
NAME
    ${my_name} - download wrapper for Buildroot

SYNOPSIS
    ${my_name} [OPTION]... -- [BACKEND OPTION]...

DESCRIPTION
    Wrapper script around different download mechanisms. Ensures that
    concurrent downloads do not conflict, that partial downloads are
    properly evicted without leaving temporary files, and that access
    rights are maintained.

    -h  This help text.

    -b BACKEND
        Wrap the specified BACKEND. Known backends are:
            bzr     Bazaar
            cp      Local files
            cvs     Concurrent Versions System
            git     Git
            hg      Mercurial
            scp     Secure copy
            svn     Subversion
            wget    HTTP download

    -o FILE
        Store the downloaded archive in FILE.

  Exit status:
    0   if OK
    !0  in case of error

ENVIRONMENT

    BUILD_DIR
        The path to Buildroot's build dir
_EOF_
}

trace()  { local msg="${1}"; shift; printf "%s: ${msg}" "${my_name}" "${@}"; }
warn()   { trace "${@}" >&2; }
errorN() { local ret="${1}"; shift; warn "${@}"; exit ${ret}; }
error()  { errorN 1 "${@}"; }

my_name="${0##*/}"
main "${@}"