new: allow compose to work without any ``compose.yml`` found.

As services will have by default charms of the same name, this allows to
launch "charms" (spawn a service of this charm) without defining any yaml.

bin/compose

@@ -979,8 +979,7 @@ get_compose_service_def () {
     fi
 
     [ -z "$service" ] && print_syntax_error "Missing service as first argument."
-
-    docker_compose=$([ "$COMPOSE_YML_FILE" -a -e "$COMPOSE_YML_FILE" ] && cat "$COMPOSE_YML_FILE")
+    docker_compose=$(get_compose_yml_content) || return 1
     result=$(_get_compose_service_def_cached "$service" "$docker_compose") || return 1
     echo "$result" | tee "$cache_file" || return 1
 }
@@ -1664,13 +1663,12 @@ _run_service_action_direct() {
         return 1
     fi
 
-    compose_file=$(get_compose_yml_location) || return 1
     export action_errlvl_file="$state_tmpdir/action-$service-$charm-$action-errlvl"
     export state_tmpdir
     {
         (
             set +e  ## Prevents unwanted leaks from parent shell
-            export COMPOSE_CONFIG=$(cat "$compose_file")
+            export COMPOSE_CONFIG=$(get_compose_yml_content)
             export METADATA_CONFIG=$(charm.metadata "$charm")
             export SERVICE_NAME=$service
             export ACTION_NAME=$action
@@ -1705,7 +1703,6 @@ _run_service_action_relation() {
 
     export RELATION_DATA_FILE=$(get_relation_data_file "$service" "$target_service" "$relation_name" "$relation_config")
 
-    compose_file=$(get_compose_yml_location) || return 1
     export action_errlvl_file="$state_tmpdir/action-$service-$charm-$action-errlvl"
     export state_tmpdir
     {
@@ -2100,7 +2097,7 @@ get_default_project_name() {
             return 0
         fi
     fi
-    echo "project"
+    echo "orphan"
     return 0
 }
 export -f get_default_project_name
@@ -2180,20 +2177,57 @@ get_compose_yml_location() {
     fi
     if [ "$DEFAULT_COMPOSE_FILE" ]; then
         if ! [ -e "$DEFAULT_COMPOSE_FILE" ]; then
-            err "No 'compose.yml' was found in current or parent dirs," \
+            warn "No 'compose.yml' was found in current or parent dirs," \
                 "and \$DEFAULT_COMPOSE_FILE points to an unexistent file." \
                 "(${DEFAULT_COMPOSE_FILE})"
-            die "Please provide a 'compose.yml' file."
+            return 0
         fi
         echo "$DEFAULT_COMPOSE_FILE"
         return 0
     fi
-    err "No 'compose.yml' was found in current or parent dirs, and no \$DEFAULT_COMPOSE_FILE was set."
-    die "Please provide a 'compose.yml' file."
-    return 1
+    warn "No 'compose.yml' was found in current or parent dirs, and no \$DEFAULT_COMPOSE_FILE was set."
+    return 0
 }
 export -f get_compose_yml_location
 
+get_compose_yml_content() {
+    local cache_file="$state_tmpdir/$FUNCNAME.cache"
+    if [ -e "$cache_file" ]; then
+        cat "$cache_file" &&
+        touch "$cache_file" || return 1
+        return 0
+    fi
+
+    COMPOSE_YML_FILE=$(get_compose_yml_location) || exit 1
+    export COMPOSE_YML_FILE
+    if [ -e "$COMPOSE_YML_FILE" ]; then
+        debug "Found $WHITE$exname$NORMAL YAML file in '$COMPOSE_YML_FILE'."
+        COMPOSE_YML_CONTENT=$(cat "$COMPOSE_YML_FILE")
+    else
+        debug "No compose file found. Using an empty one."
+        COMPOSE_YML_CONTENT=""
+    fi
+
+    output=$(echo "$COMPOSE_YML_CONTENT"| shyaml get-value 2>&1)
+    if [ "$?" != 0 ]; then
+        outputed_something=
+        while IFS='' read -r line1 && IFS='' read -r line2; do
+            [ "$outputed_something" ] || err "Invalid YAML in '$COMPOSE_YML_FILE':"
+            outputed_something=true
+            echo "$line1 $GRAY($line2)$NORMAL"
+        done < <(echo "$output" | grep ^yaml.scanner -A 100 |
+                     sed -r 's/^  in "<stdin>", //g' | sed -r 's/^yaml.scanner.[a-zA-Z]+: //g') |
+            prefix "  $GRAY|$NORMAL "
+        [ "$outputed_something" ] || {
+            err "Unexpected error while running 'shyaml get-value' on '$COMPOSE_YML_FILE':"
+            echo "$output" | prefix "  $GRAY|$NORMAL "
+        }
+        exit 1
+    fi
+    echo "$COMPOSE_YML_CONTENT" | tee "$cache_file" || return 1
+}
+export -f get_compose_yml_content
+
 
 get_default_target_services() {
     local services=("$@")
@@ -2769,26 +2803,9 @@ done < <(cla.normalize "$@")
 
 export DOCKER_DATASTORE=${DOCKER_DATASTORE:-/srv/docker-datastore}
 
-COMPOSE_YML_FILE=$(get_compose_yml_location) || exit 1
-export COMPOSE_YML_FILE
-debug "Found $WHITE$exname$NORMAL YAML file in '$COMPOSE_YML_FILE'."
 
-output=$(shyaml get-value < "$COMPOSE_YML_FILE" 2>&1)
-if [ "$?" != 0 ]; then
-    outputed_something=
-    while IFS='' read -r line1 && IFS='' read -r line2; do
-        [ "$outputed_something" ] || err "Invalid YAML in '$COMPOSE_YML_FILE':"
-        outputed_something=true
-        echo "$line1 $GRAY($line2)$NORMAL"
-    done < <(echo "$output" | grep ^yaml.scanner -A 100 |
-                    sed -r 's/^  in "<stdin>", //g' | sed -r 's/^yaml.scanner.[a-zA-Z]+: //g') |
-        prefix "  $GRAY|$NORMAL "
-    [ "$outputed_something" ] || {
-        err "Unexpected error while running 'shyaml get-value' on '$COMPOSE_YML_FILE':"
-        echo "$output" | prefix "  $GRAY|$NORMAL "
-        }
-    exit 1
-fi
+export COMPOSE_YML_CONTENT
+
 
 charm.sanity_checks || die "Sanity checks about charm-store failed. Please correct."
 
@@ -2930,7 +2947,7 @@ case "$action" in
             launch_docker_compose "${compose_opts[@]}" "$action" "${action_opts[@]}" "${action_posargs[@]}" "${remainder_args[@]}"
         fi
         ;;
-esac
+esac || exit 1
 
 
 if [ "$post_hook" -a "${#services[@]}" != 0 ]; then