arch-system: borgbackup: better connectivity check

Instead of trying to execute borg, which would give us a false positive
if borg itself isn't working on the remote, just connect to the server
without executing a command.

We check for 255 because `man 1 ssh` states the following:
    ssh exits with the exit status of the remote command or with 255 if
    an error occured.

1 files changed, 6 insertions, 5 deletions

diff --git a/arch-system/borgbackup b/arch-system/borgbackup
index 66d1a36..fdd410d 100755
--- a/arch-system/borgbackup
+++ b/arch-system/borgbackup
@@ -5,11 +5,11 @@ set -o errexit -o errtrace
 BORG_HOST="borg@borg1.th73.ovh"
 export BORG_REPO="$BORG_HOST:."
 export BORG_PASSPHRASE="$(< ~/.borg_secret)"
-export BORG_RSH="ssh -o IdentitiesOnly=yes -o ConnectionAttempts=20 -i ~/.ssh/borg_key"
+export BORG_RSH="ssh -o IdentitiesOnly=yes -o ConnectionAttempts=20 \
+                     -o ConnectTimeout=30 -i ~/.ssh/borg_key"
 export LC_COLLATE=C
 
 SNAPSHOT_TARGET="/.snapshots/home-$(date "+%Y-%m-%dT%H:%M:%S")"
-NETWORK_TIMEOUT=30
 
 cleanup() {
     cd /
@@ -66,12 +66,13 @@ if [ ! -d "$SNAPSHOT_TARGET" ]; then
     sudo -n backup-sudo snapshot "$SNAPSHOT_TARGET"
 fi
 
-if ! ssh -To BatchMode=yes -o ConnectTimeout=$NETWORK_TIMEOUT \
-         -o IdentitiesOnly=yes -o ConnectionAttempts=20 -i ~/.ssh/borg_key \
-         "$BORG_HOST" "borg --version" > /dev/null; then
+$BORG_RSH "$BORG_HOST" -f -N || rc=$?
+if [ ${rc-0} -eq 255 ]; then
     notify_user "Backup: upload skipped"
     echo "SSH server not reachable, skipping upload" >&2
     exit 0
+elif [ ${rc-0} -ne 0 ]; then
+    handle_failure
 fi
 
 # break locks in case the previous run was interrupted