Event replay and cleanup

This commit introduces the ability to reconfigure modules/event subscribers by
replaying previously stored events at startup. This way modules/subscribers can
always come up with the latest activity and data that was present at close at the
last run of the program. This process also includes the disposal of unneeded events
to minimize disk use over time. The changes include the following:

1. Event interface is extended with the Disposable() function
   unimplemented stubs for existing interface implementations
2. state.Init() is split into Init() and Start()
   Init() now initialized the file used by eventStream, and puts in place a 0 size
   memory cache as a temporary measure on top of the file.
   Start() reads Pop()s documents one by one from the temp nil-cache eventStream,
   it attempts to dispose of each event and if the event is not disposable it is
   pushed onto a second temporary no-memory-cache buffer which is then reverse ordered.
   The underlying file is truncated and reopened.
   Finally, the real eventStream is allocated, with in memory cache. All events in the
   second buffer are published (sent to subscribers as well as added to the new eventStream).
3. Updates to main() to support Init() vs Start()

Signed-off-by: Ava Affine <ava@sunnypup.io>

internal/state/state.go

@@ -40,12 +40,18 @@ const (
 	BadEventObjUnmarshal       = "failed to unmarshal event map"
 	BadEventUnmarshal          = "failed to unmarshal event"
 	BadEventMissingTypeKey     = "event map missing type key"
+	StartBeforeInitError       = "state machine not initialized"
+	ReopenStoreFileError       = "failed to reopen store file"
 )
 
 const (
 	EventTypeMapKey            = "type"
 )
 
+const (
+	TmpDocBufBackingFile       = "bingobot_events_processing"
+)
+
 var eventMutex sync.RWMutex
 var eventSubscriptionCache = [NumEventTypes][]chan Event{}
 var eventStorageFileName string
@@ -60,7 +66,7 @@ func Init(eventMemCacheSize int, eventStoreFileName string) error {
 	}
 
 	if eventStream, err = docbuf.NewDocumentBuffer(
-		eventMemCacheSize,
+		0, // temporary nil-cache buffer to replay events from
 		file,
 	); err != nil {
 		return errors.Join(errors.New(BadEventStreamInit), err)
@@ -71,6 +77,79 @@ func Init(eventMemCacheSize int, eventStoreFileName string) error {
 	return nil
 }
 
+// replay events and begin state machine
+func Start() error {
+	if eventStream == nil {
+		return errors.New(StartBeforeInitError)
+	}
+
+	tmpBackFile, err := os.CreateTemp("", TmpDocBufBackingFile)
+	if err != nil {
+		return err
+	}
+
+	tmpBuf, err := docbuf.NewDocumentBuffer(0, tmpBackFile)
+	if err != nil {
+		tmpBackFile.Close()
+		return err
+	}
+
+	// filter out disposable events into a reverse ordered on disk stack
+	var doc string
+	for err != nil && err.Error() != docbuf.PopFromEmptyBufferError {
+		doc, err = eventStream.Pop()
+		if err != nil && err.Error() != docbuf.PopFromEmptyBufferError {
+			return err
+		}
+
+		ev, err := EventFromString(doc)
+		if err != nil {
+			logging.Warn("Discarding the following event for not unmarshaling: %s", doc)
+			continue
+		}
+
+		if !ev.Disposable() {
+			tmpBuf.Push(doc)
+		}
+	}
+
+	if err := os.Truncate(eventStorageFileName, 0); err != nil {
+		return errors.Join(errors.New(ReopenStoreFileError), err)
+	}
+
+	file, err := os.OpenFile(eventStorageFileName, os.O_CREATE|os.O_RDWR, 0644)
+	if err != nil {
+		return errors.Join(errors.New(BadEventStoreFilename), err)
+	}
+
+	eventStream, err := docbuf.NewDocumentBuffer(maxEventsInMemory, file)
+	if err != nil {
+		// return error here will panic without truncating or writing to the file
+		return err
+	}
+
+	// unravel tmp stack into properly ordered properly allocated buffer
+	for err != nil && err.Error() != docbuf.PopFromEmptyBufferError {
+		doc, err := eventStream.Pop()
+		if err != nil && err.Error() != docbuf.PopFromEmptyBufferError {
+			logging.Warn("Could not handle the following error: %s", err.Error())
+			continue
+		}
+
+		ev, err := EventFromString(doc)
+		if err != nil {
+			logging.Warn("Could not handle the following error: %s", err.Error())
+			continue
+		}
+
+		if err := PublishEvent(ev); err != nil {
+			logging.Warn("Could not handle the following error: %s", err.Error())
+		}
+	}
+
+	return nil
+}
+
 // return no error. we are handling SIGINT
 func Teardown() {
 	// riskily ignore all locking....
@@ -232,6 +311,9 @@ type Event interface {
 
 	// validates state of internal metadata per EventType
 	Validate() error
+
+	// returns true if event can be discarded
+	Disposable() bool
 }
 
 func EventToString(e Event) (string, error) {