feat: store and push last valid config

Signed-off-by: Mattia Lavacca <lavacca.mattia@gmail.com>

internal/dataplane/kong_client.go

@@ -117,6 +117,9 @@ type KongClient struct {
 	// SHAs is a slice is configuration hashes send in last batch send.
 	SHAs []string
 
+	// TODO: comment
+	LastValidKongState *kongstate.KongState
+
 	// clientsProvider allows retrieving the most recent set of clients.
 	clientsProvider clients.AdminAPIClientsProvider
 
@@ -386,6 +389,13 @@ func (c *KongClient) Update(ctx context.Context) error {
 
 	// In case of a failure in syncing configuration with Gateways, propagate the error.
 	if gatewaysSyncErr != nil {
+		if c.LastValidKongState != nil {
+			_, fallbackSyncErr := c.sendOutToGatewayClients(ctx, c.LastValidKongState, c.kongConfig)
+			if fallbackSyncErr != nil {
+				return errors.Join(gatewaysSyncErr, fallbackSyncErr)
+			}
+			c.logger.Debug("due to errors in the current config, the last valid config has been pushed to Gateways")
+		}
 		return gatewaysSyncErr
 	}
 
@@ -503,6 +513,10 @@ func (c *KongClient) sendToClient(
 	// update the lastConfigSHA with the new updated checksum
 	client.SetLastConfigSHA(newConfigSHA)
 
+	if len(entityErrors) == 0 {
+		c.LastValidKongState = s
+	}
+
 	return string(newConfigSHA), nil
 }
 

test/integration/config_error_fallback_test.go

@@ -0,0 +1,14 @@
+//go:build integration_tests
+
+package integration
+
+import (
+	"testing"
+)
+
+func TestConfigErrorWithFallback(t *testing.T) {
+	// This test is NOT parallel.
+	// The broken configuration prevents all updates and will break unrelated tests
+
+	// TODO: add test
+}