fix(diff): clear unmatching deprecated fields

When user uses decK configuration and they use only
new or the old (deprecated) fields then we don't want to display
incorrect diff. We used to try to "fill" those fields but sometimes
it's impossible to do that. The reason why we need to fill in these
fields is that Kong, in order to be backwards compatible sends, both
new and old fields. This commit removes either the deprecated
or the new fields from the response from Kong before doing the diff
so that there is no false negative difference.

KAG-5577

pkg/diff/diff.go

@@ -612,28 +612,35 @@ func (sc *Syncer) Solve(ctx context.Context, parallelism int, dry bool, isJSONOu
 			pluginCopy := &state.Plugin{Plugin: *plugin.DeepCopy()}
 			e.Obj = pluginCopy
 
-			exists, err := utils.WorkspaceExists(ctx, sc.kongClient)
-			if err == nil && exists {
-				var schema map[string]interface{}
-				schema, err = sc.kongClient.Plugins.GetFullSchema(ctx, pluginCopy.Plugin.Name)
-				if err != nil {
-					return nil, err
-				}
+			if oldPlugin, ok := e.OldObj.(*state.Plugin); ok {
+				oldPluginCopy := &state.Plugin{Plugin: *oldPlugin.DeepCopy()}
+				e.OldObj = oldPluginCopy
+
+				if exists, _ := utils.WorkspaceExists(ctx, sc.kongClient); exists {
+					var schema map[string]interface{}
+					schema, err = sc.kongClient.Plugins.GetFullSchema(ctx, pluginCopy.Plugin.Name)
+					if err != nil {
+						return nil, err
+					}
 
-				// fill defaults and auto fields for the configuration that will be used for the diff
-				newPlugin := &pluginCopy.Plugin
-				if err := kong.FillPluginsDefaults(newPlugin, schema); err != nil {
-					return nil, fmt.Errorf("failed processing auto fields: %w", err)
-				}
+					// fill defaults and auto fields for the configuration that will be used for the diff
+					if err := kong.FillPluginsDefaults(&pluginCopy.Plugin, schema); err != nil {
+						return nil, fmt.Errorf("failed processing auto fields: %w", err)
+					}
+
+					// only fill auto fields for the configuration sent to Kong
+					// this is done because we want to avoid Kong to auto generate fields, which
+					// would make decK's configuration no longer fully "declarative"
+					if err := kong.FillPluginsDefaultsWithOpts(&plugin.Plugin, schema, kong.FillRecordOptions{
+						FillDefaults: false,
+						FillAuto:     true,
+					}); err != nil {
+						return nil, fmt.Errorf("failed processing auto fields: %w", err)
+					}
 
-				// only fill auto fields for the configuration sent to Kong
-				// this is done because we want to avoid Kong to auto generate fields, which
-				// would make decK's configuration no longer fully "declarative"
-				if err := kong.FillPluginsDefaultsWithOpts(&plugin.Plugin, schema, kong.FillRecordOptions{
-					FillDefaults: false,
-					FillAuto:     true,
-				}); err != nil {
-					return nil, fmt.Errorf("failed processing auto fields: %w", err)
+					if err := kong.ClearUnmatchingDeprecations(&pluginCopy.Plugin, &oldPluginCopy.Plugin, schema); err != nil {
+						return nil, fmt.Errorf("failed processing auto fields: %w", err)
+					}
 				}
 			}
 		}

pkg/types/plugin.go

@@ -186,6 +186,10 @@ func (d *pluginDiffer) createUpdatePlugin(plugin *state.Plugin) (*crud.Event, er
 		return nil, fmt.Errorf("failed processing auto fields: %w", err)
 	}
 
+	if err := kong.ClearUnmatchingDeprecations(&pluginWithDefaults.Plugin, &currentPlugin.Plugin, schema); err != nil {
+		return nil, fmt.Errorf("failed clearing unmatching deprecations fields: %w", err)
+	}
+
 	if !currentPlugin.EqualWithOpts(pluginWithDefaults, false, true, false) {
 		return &crud.Event{
 			Op:     crud.Update,