Fix transfer plugin unpack configuration

Remove default unpack configuration to prevent duplication of
configuration from toml decoder appending to the default. When no unpack
configuration is provided, use the default.

Signed-off-by: Derek McGowan <[email protected]>

Author: dmcgowan

plugins/transfer/plugin.go

@@ -77,6 +77,17 @@ func init() {
 			var lc local.TransferConfig
 			lc.MaxConcurrentDownloads = config.MaxConcurrentDownloads
 			lc.MaxConcurrentUploadedLayers = config.MaxConcurrentUploadedLayers
+
+			// If UnpackConfiguration is not defined, set the default.
+			// If UnpackConfiguration is defined and empty, ignore.
+			if config.UnpackConfiguration == nil {
+				config.UnpackConfiguration = []unpackConfiguration{
+					{
+						Platform:    platforms.Format(platforms.DefaultSpec()),
+						Snapshotter: defaults.DefaultSnapshotter,
+					},
+				}
+			}
 			for _, uc := range config.UnpackConfiguration {
 				p, err := platforms.Parse(uc.Platform)
 				if err != nil {
@@ -148,7 +159,7 @@ type transferConfig struct {
 	MaxConcurrentUploadedLayers int `toml:"max_concurrent_uploaded_layers"`
 
 	// UnpackConfiguration is used to read config from toml
-	UnpackConfiguration []unpackConfiguration `toml:"unpack_config"`
+	UnpackConfiguration []unpackConfiguration `toml:"unpack_config,omitempty"`
 
 	// RegistryConfigPath is a path to the root directory containing registry-specific configurations
 	RegistryConfigPath string `toml:"config_path"`
@@ -169,11 +180,5 @@ func defaultConfig() *transferConfig {
 	return &transferConfig{
 		MaxConcurrentDownloads:      3,
 		MaxConcurrentUploadedLayers: 3,
-		UnpackConfiguration: []unpackConfiguration{
-			{
-				Platform:    platforms.Format(platforms.DefaultSpec()),
-				Snapshotter: defaults.DefaultSnapshotter,
-			},
-		},
 	}
 }