base-files: sysupgrade: fix streaming backup archives to stdout

Due to previous refactoring in sysupgrade, writing backup archives to
stdout became impossible since the hardcoded gzip output redirection
did not account for the `-` special case filename.

Fix this issue by substituting `-` with `/proc/self/fd/1` in the tar
archive output path variable.

Also remove a redundant `rm -f` of the target file path that occurs
before the file could've possibly been written.

Fixes: #14773
Fixes: https://github.com/openwrt/luci/issues/6961
Fixes: e36cc53092 ("base-files: sysupgrade: use tar helper to include installed_packages.txt")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

diff --git a/package/base-files/files/sbin/sysupgrade b/package/base-files/files/sbin/sysupgrade
index 78ec455067910aea384338c4d0242a419d717515..720f3da3fa45476107e461151e925a2576fe0867 100755 (executable)
--- a/package/base-files/files/sbin/sysupgrade
+++ b/package/base-files/files/sbin/sysupgrade
@@ -241,12 +241,13 @@ create_backup_archive() {
        [ "$(rootfs_type)" = "tmpfs" ] && {
                echo "Cannot save config while running from ramdisk." >&2
                ask_bool 0 "Abort" && exit
-               rm -f "$conf_tar"
                return 0
        }
        run_hooks "$CONFFILES" $sysupgrade_init_conffiles
        ask_bool 0 "Edit config file list" && vi "$CONFFILES"
 
+       [ "$conf_tar" != "-" ] || conf_tar=""
+
        v "Saving config files..."
        [ "$VERBOSE" -gt 1 ] && TAR_V="v" || TAR_V=""
        sed -i -e 's,^/,,' "$CONFFILES"
@@ -272,12 +273,12 @@ create_backup_archive() {
 
                # Rest of archive with config files and ending padding
                tar c${TAR_V} -C / -T "$CONFFILES"
-       } | gzip > "$conf_tar"
+       } | gzip > "${conf_tar:-/proc/self/fd/1}"
 
        local err=$?
        if [ "$err" -ne 0 ]; then
                echo "Failed to create the configuration backup."
-               rm -f "$conf_tar"
+               [ -f "$conf_tar" ] && rm -f "$conf_tar"
        fi
 
        rm -f "$CONFFILES"