decode: recursively unslice opaque config

Also handle []interface{} in HookWeakDecodeFromSlice

Without this change only the top level []map[string]interface{} will be
unpacked as a single item. With this change any nested config will be
unpacked.

lib/decode/decode.go

@@ -7,6 +7,8 @@ package decode
 import (
 	"reflect"
 	"strings"
+
+	"github.com/mitchellh/reflectwalk"
 )
 
 // HookTranslateKeys is a mapstructure decode hook which translates keys in a
@@ -120,15 +122,39 @@ func HookWeakDecodeFromSlice(from, to reflect.Type, data interface{}) (interface
 
 	switch d := data.(type) {
 	case []map[string]interface{}:
-		switch {
-		case len(d) == 0:
-			return nil, nil
-		case len(d) == 1:
-			return d[0], nil
-		default:
+		if len(d) == 1 {
+			return unSlice(d[0])
+		}
+	// the JSON decoder can apparently decode slices as []interface{}
+	case []interface{}:
+		if len(d) == 1 {
+			return unSlice(d[0])
+		}
+	}
 	return data, nil
 }
-	default:
-		return data, nil
+
+func unSlice(data interface{}) (interface{}, error) {
+	err := reflectwalk.Walk(data, &unSliceWalker{})
+	return data, err
 }
+
+type unSliceWalker struct{}
+
+func (u *unSliceWalker) Map(_ reflect.Value) error {
+	return nil
+}
+
+func (u *unSliceWalker) MapElem(m, k, v reflect.Value) error {
+	if !v.IsValid() || v.Kind() != reflect.Interface {
+		return nil
+	}
+
+	v = v.Elem() // unpack the value from the interface{}
+	if v.Kind() != reflect.Slice || v.Len() != 1 {
+		return nil
+	}
+
+	m.SetMapIndex(k, v.Index(0))
+	return nil
 }

lib/decode/decode_test.go

@@ -272,3 +272,39 @@ item {
 	}
 	require.Equal(t, target, expected)
 }
+
+func TestHookWeakDecodeFromSlice_NestedOpaqueConfig(t *testing.T) {
+	source := `
+service {
+  proxy {
+    config {
+      envoy_gateway_bind_addresses {
+        all-interfaces {
+          address = "0.0.0.0"
+          port = 8443
+        }
+      }
+    }
+  }
+}`
+
+	target := map[string]interface{}{}
+	err := decodeHCLToMapStructure(source, &target)
+	require.NoError(t, err)
+
+	expected := map[string]interface{}{
+		"service": map[string]interface{}{
+			"proxy": map[string]interface{}{
+				"config": map[string]interface{}{
+					"envoy_gateway_bind_addresses": map[string]interface{}{
+						"all-interfaces": map[string]interface{}{
+							"address": "0.0.0.0",
+							"port":    8443,
+						},
+					},
+				},
+			},
+		},
+	}
+	require.Equal(t, target, expected)
+}