new: [compose-core] make ``service:relation-file`` always return an existing file

It must create it with ``compose`` / ``default-options`` values by
default if not already created.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

bin/compose-core

@@ -2148,7 +2148,7 @@ service:relation-file() {
     fi
     relation="${service_path##*${TRAVERSE_SEPARATOR}}"
     service=$(service:traverse "${service_path%${TRAVERSE_SEPARATOR}*}") || return 1
-    if ! read-0 ts _ _ < <(get_service_relation "${service}" "${relation}"); then
+    if ! read-0 ts rc _ < <(get_service_relation "${service}" "${relation}"); then
         err "Couldn't find relation ${DARKCYAN}${relation}${NORMAL}" \
             "from ${DARKYELLOW}$service${NORMAL}."
         return 1
@@ -2157,7 +2157,12 @@ service:relation-file() {
         err "Failed to find relation file"
         return 1
     }
-    echo "$relation_dir/data"
+    relation_file="$relation_dir/data"
+    if ! [ -e "$relation_file" ]; then
+        e "$rc" > "$relation_file"
+        chmod go-rwx "$relation_file"  ## protecting this file
+    fi
+    echo "$relation_file"
 }
 export -f service:relation-file