New static configuration loading system.

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

pkg/config/env/env.go

@@ -0,0 +1,50 @@
+// Package env implements encoding and decoding between environment variable and a typed Configuration.
+package env
+
+import (
+	"strings"
+
+	"github.com/containous/traefik/pkg/config/parser"
+)
+
+// Decode decodes the given environment variables into the given element.
+// The operation goes through four stages roughly summarized as:
+// env vars -> map
+// map -> tree of untyped nodes
+// untyped nodes -> nodes augmented with metadata such as kind (inferred from element)
+// "typed" nodes -> typed element
+func Decode(environ []string, element interface{}) error {
+	vars := make(map[string]string)
+	for _, evr := range environ {
+		n := strings.SplitN(evr, "=", 2)
+		if strings.HasPrefix(strings.ToUpper(n[0]), "TRAEFIK_") {
+			key := strings.ReplaceAll(strings.ToLower(n[0]), "_", ".")
+			vars[key] = n[1]
+		}
+	}
+
+	return parser.Decode(vars, element)
+}
+
+// Encode encodes the configuration in element into the environment variables represented in the returned Flats.
+// The operation goes through three stages roughly summarized as:
+// typed configuration in element -> tree of untyped nodes
+// untyped nodes -> nodes augmented with metadata such as kind (inferred from element)
+// "typed" nodes -> environment variables with default values (determined by type/kind)
+func Encode(element interface{}) ([]parser.Flat, error) {
+	if element == nil {
+		return nil, nil
+	}
+
+	node, err := parser.EncodeToNode(element, false)
+	if err != nil {
+		return nil, err
+	}
+
+	err = parser.AddMetadata(element, node)
+	if err != nil {
+		return nil, err
+	}
+
+	return parser.EncodeToFlat(element, node, parser.FlatOpts{Case: "upper", Separator: "_"})
+}