buildroot/support/download/wrapper

#!/usr/bin/env bash

# This script is a wrapper to the other download helpers.
# 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:
#   $1: name of the helper (eg. cvs, git, cp...)
#   $2: full path to the file in which to save the download
#   $*: additional arguments to the helper in $1
# Environment:
#   BUILD_DIR: the path to Buildroot's build dir

# 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

helper="${1}"
output="${2}"
shift 2

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

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