new: [compose-core] add new ``stub`` service type

``stub`` services do not have any container running nor images.
They are intended to stand for externally managed services.

bin/compose-core

@@ -678,13 +678,13 @@ get_service_type() {
         cat "$cache_file"
         return 0
     fi
-
-    charm=$(get_service_charm "$service") || return 1
+    master_target_service="$(get_top_master_service_for_service "$service")" || return 1
+    charm=$(get_service_charm "$master_target_service") || return 1
     metadata=$(charm.metadata "$charm") || return 1
     printf "%s" "$metadata" | shyaml get-value type service 2>/dev/null |
         tee "$cache_file"
 }
-
+export -f get_service_type
 
 are_files_locked_in_dir() {
     local dir="$1" device hdev ldev
@@ -1059,6 +1059,7 @@ get_docker_compose_links() {
         master_target_service="$(get_top_master_service_for_service "$target_service")" || return 1
         [ "$master_service" == "$master_target_service" ] && continue
         type="$(get_service_type "$target_service")" || return 1
+        [ "$type" == "stub" ] && continue
         [ "$type" == "run-once" ] && continue
         if [ "$tech_dep" == "reversed" ]; then
             deps+=("$(echo -en "$master_target_service:\n  links:\n    - $master_service")")
@@ -1124,6 +1125,9 @@ _get_docker_compose_service_mixin() {
         cat "$cache_file"
         return 0
     fi
+    type=$(get_service_type "$service") || return 1
+    [ "$type" == "stub" ] && return 0
+
     master_service=$(get_top_master_service_for_service "$service") || {
         err "Failed to get top master service for service $DARKYELLOW$service$NORMAL"
         return 1
@@ -1562,11 +1566,14 @@ run_service_hook () {
 
             charm=$(get_service_charm "$subservice") || return 1
             charm.has_hook "$charm" "$action" >/dev/null || continue
+            type=$(get_service_type "$subservice") || return 1
 
             PROJECT_NAME=$(get_default_project_name) || return 1
             MASTER_BASE_SERVICE_NAME=$(get_top_master_service_for_service "$subservice") || return 1
             MASTER_BASE_CHARM_NAME=$(get_service_charm "$MASTER_BASE_SERVICE_NAME") || return 1
-            DOCKER_BASE_IMAGE=$(service_base_docker_image "$MASTER_BASE_SERVICE_NAME") || return 1
+            if [ "$type" != "stub" ]; then
+                DOCKER_BASE_IMAGE=$(service_base_docker_image "$MASTER_BASE_SERVICE_NAME") || return 1
+            fi
 
             Wrap "${wrap_opts[@]}" -d "running $YELLOW$action$NORMAL hook of $DARKYELLOW$subservice$NORMAL in charm $DARKPINK$charm$NORMAL" <<EOF || return 1
 
@@ -1919,7 +1926,10 @@ _run_service_relation () {
         verb "Running ${DARKBLUE}$relation_name${NORMAL} relation-joined script" \
              "for target $DARKYELLOW$target_service$NORMAL (charm $DARKPINK$target_charm$NORMAL)"
         RELATION_CONFIG="$relation_dir/config_provider"
-        DOCKER_BASE_IMAGE=$(service_base_docker_image "$target_service") || return 1
+        type=$(get_service_type "$target_service") || return 1
+        if [ "$type" != "stub" ]; then
+            DOCKER_BASE_IMAGE=$(service_base_docker_image "$target_service") || return 1
+        fi
         export DOCKER_BASE_IMAGE RELATION_CONFIG RELATION_DATA
         {
             (
@@ -4445,6 +4455,7 @@ if [ "$is_docker_compose_action" -a "${#services_args[@]}" -gt 0 ]; then
             type="$(get_service_type "$mservice")" || exit 1
             ## remove run-once
             [ "$type" == "run-once" ] && continue
+            [ "$type" == "stub" ] && continue
             seen[$mservice]=1
             action_posargs+=("$mservice")
         done