new: [compose-core] add relation and service traversal utilities

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

bin/compose-core

@@ -2117,6 +2117,74 @@ get_service_relation() {
 export -f get_service_relation
 
 
+export TRAVERSE_SEPARATOR=:
+## Traverse on first service satisfying relation
+service:traverse() {
+    local service_path="$1"
+    {
+        SEPARATOR=:
+        read -d "$TRAVERSE_SEPARATOR" service
+        while read -d "$TRAVERSE_SEPARATOR" relation; do
+            ## XXXvlab: Take only first service
+            if ! read-0 ts _ _ < <(get_service_relation "${service}" "${relation}"); then
+                err "Couldn't find relation ${DARKCYAN}${relation}${NORMAL}" \
+                    "from ${DARKYELLOW}$service${NORMAL}."
+                return 1
+            fi
+            service="$ts"
+        done
+        echo "$service"
+    } < <(e "${service_path}${TRAVERSE_SEPARATOR}")
+}
+export -f service:traverse
+
+
+service:relation-file() {
+    local service_path="$1" relation service relation_file
+    if ! [[ "$service_path" == *"$TRAVERSE_SEPARATOR"* ]]; then
+        err "Invalid argument '$service_path'." \
+            "Must provide a service path (no '${TRAVERSE_SEPARATOR}' found)."
+        return 1
+    fi
+    relation="${service_path##*${TRAVERSE_SEPARATOR}}"
+    service=$(service:traverse "${service_path%${TRAVERSE_SEPARATOR}*}") || return 1
+    if ! read-0 ts _ _ < <(get_service_relation "${service}" "${relation}"); then
+        err "Couldn't find relation ${DARKCYAN}${relation}${NORMAL}" \
+            "from ${DARKYELLOW}$service${NORMAL}."
+        return 1
+    fi
+    relation_dir=$(get_relation_data_dir "$service" "$ts" "$relation") || {
+        err "Failed to find relation file"
+        return 1
+    }
+    echo "$relation_dir/data"
+}
+export -f service:relation-file
+
+
+service:relation-options() {
+    local service_path="$1" relation_file
+    relation_file=$(service:relation-file "$service_path") || {
+        err "Failed to find relation file"
+        return 1
+    }
+    cat "$relation_file"
+}
+export -f service:relation-options
+
+
+relation:get() {
+    local service_path="$1" query="$2" relation_file
+    relation_file=$(service:relation-file "$service_path") || {
+        err "Failed to find relation file"
+        return 1
+    }
+    cfg-get-value "$query" < "$relation_file"
+}
+export -f relation:get
+
+
+
 _get_charm_metadata_uses() {
     local metadata="$1" cache_file="$CACHEDIR/$FUNCNAME.cache.$(printf "%s\0" "$@" | md5_compat)"
     if [ -e "$cache_file" ]; then