Add a weekly dump of the Garmin database

garmin-dump.timer runs garmin-dump weekly, keeping the two newest ISO weeks in
~/.local/share/negsplit/dumps, which simple_backup then copies; the login comes
from ~/.config/negsplit/dump.cnf (not tracked).

.local/bin/garmin-dump

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Dump the Garmin database to a per-ISO-week file, keeping the two newest weeks.
+set -euo pipefail
+
+cnf=${XDG_CONFIG_HOME:-$HOME/.config}/negsplit/dump.cnf
+dir=${XDG_DATA_HOME:-$HOME/.local/share}/negsplit/dumps
+out=$dir/Garmin-$(date +%G-W%V).sql.gz
+keep=2
+
+umask 077
+mkdir -p "$dir"
+
+# Write to a temporary file so a failed dump never replaces a good one
+tmp=$(mktemp "$dir/.dump.XXXXXX")
+trap 'rm -f "$tmp"' EXIT
+
+mariadb-dump --defaults-extra-file="$cnf" --single-transaction --databases Garmin | gzip > "$tmp"
+mv "$tmp" "$out"
+
+# Names sort by week, so all but the last $keep are the old ones
+dumps=("$dir"/Garmin-*.sql.gz)
+if (( ${#dumps[@]} > keep )); then
+    rm -f -- "${dumps[@]:0:${#dumps[@]}-keep}"
+fi