fix: [compose-core] make ``stub`` services un-summonable

bin/compose-core

@@ -3265,16 +3265,38 @@ get_all_relations () {
                 providers=()
                 providers_def=()
                 array_read-0 providers providers_def < <(_get_charm_providing "$relation_name" "${service_list[@]}")
+
+                ## select first provider that is not a stub
+                new_providers=()
+                new_providers_def=()
+                while [[ "${#providers[@]}" != 0 ]]; do
+                    provider="${providers[0]}"
+                    provider_def="${providers_def[0]}"
+                    providers=("${providers[@]:1}")
+                    providers_def=("${providers_def[@]:1}")
+                    type="$(get_service_type "$provider")" || true
+                    [ "$type" == "stub" ] && continue
+                    new_providers+=("$provider")
+                    new_providers_def+=("$provider_def")
+                done
+                providers=("${new_providers[@]}")
+                providers_def=("${new_providers_def[@]}")
+
                 if [ "${#providers[@]}" == 0 ]; then
                     err "Summoning a ${DARKBLUE}$relation_name${NORMAL} provider failed: none were found in charm store."
                     return 1
                 fi
 
                 if [ "${#providers[@]}" -gt 1 ]; then
+                    ## if there are multiple providers (for instance
+                    ## sql-database), there are some case where other
+                    ## services will also summon a more specific
+                    ## postgres-database, that will solve our
+                    ## constraint. So we'd rather pass (and requeue)
                     if [ -z "${summon_requeued[$service/$relation_name]}" ]; then
                         debug "Auto-summon ${DARKYELLOW}$service${NORMAL}" \
                              "--${DARKBLUE}$relation_name${NORMAL}--> ($DARKYELLOW""${providers[@]}""$NORMAL)"\
-                             "(> 1 provider). Requeing."
+                             "(> 1 provider). Requeuing."
                         summon+=("$service" "$relation_name" "$relation_def")  ## re-queue it
                         summon_requeued["$service/$relation_name"]=1
                         continue