new: [compose-core] provide ``get_service_incoming_relations`` to list services linked to a service

bin/compose-core

@@ -2209,6 +2209,37 @@ get_service_relation() {
 export -f get_service_relation
 
 
+## From a service and a relation, get all relations targeting given
+## service with given relation.
+##
+## Returns a NUL separated list of couple of:
+##    (base_service, relation_config)
+##
+get_service_incoming_relations() {
+    local service="$1" relation="$2" cache_file="$state_tmpdir/$FUNCNAME.cache.$(H "$@" "$ALL_RELATIONS")" \
+          s rn ts rc td
+    if [ -e "$cache_file" ]; then
+        #debug "$FUNCNAME: SESSION cache hit $1"
+        cat "$cache_file"
+        return 0
+    fi
+    if [ -z "$ALL_RELATIONS" ]; then
+        err "Can't access global \$ALL_RELATIONS"
+        return 1
+    fi
+
+    while read-0 s rn ts rc _td; do
+        [[ "$ts" == "$service" ]] || continue
+        [[ "$rn" == "$relation" ]] || continue
+        relation_data_file=$(get_relation_data_file "$s" "$ts" "$rn" "$rc") || return 1
+        printf "%s\0" "$s" "$(cat "$relation_data_file")"
+    done < <(cat "$ALL_RELATIONS") > "$cache_file"
+
+    cat "$cache_file"
+}
+export -f get_service_incoming_relations
+
+
 export TRAVERSE_SEPARATOR=:
 ## Traverse on first service satisfying relation
 service:traverse() {