Add an option to make bucket notifications synchronous (#17406)

With the current asynchronous behaviour in sending notification events
to the targets, we can't provide guaranteed delivery as the systems
might go for restarts.

For such event-driven use-cases, we can provide an option to enable
synchronous events where the APIs wait until the event is successfully
sent or persisted.

This commit adds 'MINIO_API_SYNC_EVENTS' env which when set to 'on'
will enable sending/persisting events to targets synchronously.

internal/config/api/api.go

@@ -45,6 +45,7 @@ const (
 	apiDisableODirect              = "disable_odirect"
 	apiGzipObjects                 = "gzip_objects"
 	apiRootAccess                  = "root_access"
+	apiSyncEvents                  = "sync_events"
 
 	EnvAPIRequestsMax             = "MINIO_API_REQUESTS_MAX"
 	EnvAPIRequestsDeadline        = "MINIO_API_REQUESTS_DEADLINE"
@@ -62,6 +63,7 @@ const (
 	EnvAPIDisableODirect              = "MINIO_API_DISABLE_ODIRECT"
 	EnvAPIGzipObjects                 = "MINIO_API_GZIP_OBJECTS"
 	EnvAPIRootAccess                  = "MINIO_API_ROOT_ACCESS" // default "on"
+	EnvAPISyncEvents                  = "MINIO_API_SYNC_EVENTS" // default "off"
 )
 
 // Deprecated key and ENVs
@@ -135,6 +137,10 @@ var (
 			Key:   apiRootAccess,
 			Value: "on",
 		},
+		config.KV{
+			Key:   apiSyncEvents,
+			Value: "off",
+		},
 	}
 )
 
@@ -154,6 +160,7 @@ type Config struct {
 	DisableODirect              bool          `json:"disable_odirect"`
 	GzipObjects                 bool          `json:"gzip_objects"`
 	RootAccess                  bool          `json:"root_access"`
+	SyncEvents                  bool          `json:"sync_events"`
 }
 
 // UnmarshalJSON - Validate SS and RRS parity when unmarshalling JSON.
@@ -269,5 +276,7 @@ func LookupConfig(kvs config.KVS) (cfg Config, err error) {
 	}
 	cfg.StaleUploadsExpiry = staleUploadsExpiry
 
+	cfg.SyncEvents = env.Get(EnvAPISyncEvents, kvs.Get(apiSyncEvents)) == config.EnableOn
+
 	return cfg, nil
 }

internal/config/api/help.go

@@ -104,5 +104,11 @@ var (
 			Optional:    true,
 			Type:        "boolean",
 		},
+		config.HelpKV{
+			Key:         apiSyncEvents,
+			Description: "set to enable synchronous bucket notifications" + defaultHelpPostfix(apiSyncEvents),
+			Optional:    true,
+			Type:        "boolean",
+		},
 	}
 )