multiple realms fix (#102)

* Allow to use multiple realms

This change is a ressurection of PR #88 which can't be merged due to significant refactor of the codebase.

* Fix the autocleanup routine to handle multiple realms correctly, update tests.

* Metadata rediscovery when provider is unavailable for any reason during the start.

This one prevents the permanent 503 from the plugin when OIDC provider was for some reason unavailable during the start.

middleware.go

@@ -50,6 +50,20 @@ func (t *TraefikOidc) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		t.metadataMu.RUnlock()
 
 		if issuerURL == "" {
+			// Provider metadata initialization failed - try to recover
+			// Retry every 30 seconds to allow automatic recovery when provider comes back online
+			t.metadataRetryMutex.Lock()
+			shouldRetry := time.Since(t.lastMetadataRetryTime) >= 30*time.Second
+			if shouldRetry {
+				t.lastMetadataRetryTime = time.Now()
+			}
+			t.metadataRetryMutex.Unlock()
+
+			if shouldRetry && t.providerURL != "" {
+				t.logger.Info("Attempting to recover OIDC provider metadata...")
+				go t.attemptMetadataRecovery()
+			}
+
 			t.logger.Error("OIDC provider metadata initialization failed or incomplete")
 			t.sendErrorResponse(rw, req, "OIDC provider metadata initialization failed - please check provider availability and configuration", http.StatusServiceUnavailable)
 			return