1 files changed, 475 insertions, 0 deletions
diff --git a/srv/chat/chat.go b/srv/chat/chat.go
new file mode 100644
index 0000000..44449cd
--- /dev/null
+++ b/srv/chat/chat.go
@@ -0,0 +1,475 @@
+// Package chat implements a simple chatroom system.
+package chat
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strconv"
+	"sync"
+	"time"
+
+	"github.com/mediocregopher/mediocre-go-lib/v2/mctx"
+	"github.com/mediocregopher/mediocre-go-lib/v2/mlog"
+	"github.com/mediocregopher/radix/v4"
+)
+
+// ErrInvalidArg is returned from methods in this package when a call fails due
+// to invalid input.
+type ErrInvalidArg struct {
+	Err error
+}
+
+func (e ErrInvalidArg) Error() string {
+	return fmt.Sprintf("invalid argument: %v", e.Err)
+}
+
+var (
+	errInvalidMessageID = ErrInvalidArg{Err: errors.New("invalid Message ID")}
+)
+
+// UserID uniquely identifies an individual user who has posted a message in a
+// Room.
+type UserID struct {
+
+	// Name will be the user's chosen display name.
+	Name string `json:"name"`
+
+	// Hash will be a hex string generated from a secret only the user knows.
+	Hash string `json:"id"`
+}
+
+// Message describes a message which has been posted to a Room.
+type Message struct {
+	ID     string `json:"id"`
+	UserID UserID `json:"userID"`
+	Body   string `json:"body"`
+}
+
+func msgFromStreamEntry(entry radix.StreamEntry) (Message, error) {
+
+	// NOTE this should probably be a shortcut in radix
+	var bodyStr string
+	for _, field := range entry.Fields {
+		if field[0] == "json" {
+			bodyStr = field[1]
+			break
+		}
+	}
+
+	if bodyStr == "" {
+		return Message{}, errors.New("no 'json' field")
+	}
+
+	var msg Message
+	if err := json.Unmarshal([]byte(bodyStr), &msg); err != nil {
+		return Message{}, fmt.Errorf(
+			"json unmarshaling body %q: %w", bodyStr, err,
+		)
+	}
+
+	msg.ID = entry.ID.String()
+	return msg, nil
+}
+
+// MessageIterator returns a sequence of Messages which may or may not be
+// unbounded.
+type MessageIterator interface {
+
+	// Next blocks until it returns the next Message in the sequence, or the
+	// context error if the context is cancelled, or io.EOF if the sequence has
+	// been exhausted.
+	Next(context.Context) (Message, error)
+
+	// Close should always be called once Next has returned an error or the
+	// MessageIterator will no longer be used.
+	Close() error
+}
+
+// HistoryOpts are passed into Room's History method in order to affect its
+// result. All fields are optional.
+type HistoryOpts struct {
+	Limit  int    // defaults to, and is capped at, 100.
+	Cursor string // If not given then the most recent Messages are returned.
+}
+
+func (o HistoryOpts) sanitize() (HistoryOpts, error) {
+	if o.Limit < 0 || o.Limit > 100 {
+		o.Limit = 100
+	}
+
+	if o.Cursor != "" {
+		id, err := parseStreamEntryID(o.Cursor)
+		if err != nil {
+			return HistoryOpts{}, fmt.Errorf("parsing Cursor: %w", err)
+		}
+		o.Cursor = id.String()
+	}
+
+	return o, nil
+}
+
+// Room implements functionality related to a single, unique chat room.
+type Room interface {
+
+	// Append accepts a new Message and stores it at the end of the room's
+	// history. The original Message is returned with any relevant fields (e.g.
+	// ID) updated.
+	Append(context.Context, Message) (Message, error)
+
+	// Returns a cursor and the list of historical Messages in time descending
+	// order. The cursor can be passed into the next call to History to receive
+	// the next set of Messages.
+	History(context.Context, HistoryOpts) (string, []Message, error)
+
+	// Listen returns a MessageIterator which will return all Messages appended
+	// to the Room since the given ID. Once all existing messages are iterated
+	// through then the MessageIterator will begin blocking until a new Message
+	// is posted.
+	Listen(ctx context.Context, sinceID string) (MessageIterator, error)
+
+	// Delete deletes a Message from the Room.
+	Delete(ctx context.Context, id string) error
+
+	// Close is used to clean up all resources created by the Room.
+	Close() error
+}
+
+// RoomParams are used to instantiate a new Room. All fields are required unless
+// otherwise noted.
+type RoomParams struct {
+	Logger      *mlog.Logger
+	Redis       radix.Client
+	ID          string
+	MaxMessages int
+}
+
+func (p RoomParams) streamKey() string {
+	return fmt.Sprintf("chat:{%s}:stream", p.ID)
+}
+
+type room struct {
+	params RoomParams
+
+	closeCtx    context.Context
+	closeCancel context.CancelFunc
+	wg          sync.WaitGroup
+
+	listeningL      sync.Mutex
+	listening       map[chan Message]struct{}
+	listeningLastID radix.StreamEntryID
+}
+
+// NewRoom initializes and returns a new Room instance.
+func NewRoom(ctx context.Context, params RoomParams) (Room, error) {
+
+	params.Logger = params.Logger.WithNamespace("chat-room")
+
+	r := &room{
+		params:    params,
+		listening: map[chan Message]struct{}{},
+	}
+
+	r.closeCtx, r.closeCancel = context.WithCancel(context.Background())
+
+	// figure out the most recent message, if any.
+	lastEntryID, err := r.mostRecentMsgID(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("discovering most recent entry ID in stream: %w", err)
+	}
+	r.listeningLastID = lastEntryID
+
+	r.wg.Add(1)
+	go func() {
+		defer r.wg.Done()
+		r.readStreamLoop(r.closeCtx)
+	}()
+
+	return r, nil
+}
+
+func (r *room) Close() error {
+	r.closeCancel()
+	r.wg.Wait()
+	return nil
+}
+
+func (r *room) mostRecentMsgID(ctx context.Context) (radix.StreamEntryID, error) {
+
+	var entries []radix.StreamEntry
+	err := r.params.Redis.Do(ctx, radix.Cmd(
+		&entries,
+		"XREVRANGE", r.params.streamKey(), "+", "-", "COUNT", "1",
+	))
+
+	if err != nil || len(entries) == 0 {
+		return radix.StreamEntryID{}, err
+	}
+
+	return entries[0].ID, nil
+}
+
+func (r *room) Append(ctx context.Context, msg Message) (Message, error) {
+	msg.ID = "" // just in case
+
+	b, err := json.Marshal(msg)
+	if err != nil {
+		return Message{}, fmt.Errorf("json marshaling Message: %w", err)
+	}
+
+	key := r.params.streamKey()
+	maxLen := strconv.Itoa(r.params.MaxMessages)
+	body := string(b)
+
+	var id string
+
+	err = r.params.Redis.Do(ctx, radix.Cmd(
+		&id, "XADD", key, "MAXLEN", "=", maxLen, "*", "json", body,
+	))
+
+	if err != nil {
+		return Message{}, fmt.Errorf("posting message to redis: %w", err)
+	}
+
+	msg.ID = id
+	return msg, nil
+}
+
+const zeroCursor = "0-0"
+
+func (r *room) History(ctx context.Context, opts HistoryOpts) (string, []Message, error) {
+	opts, err := opts.sanitize()
+	if err != nil {
+		return "", nil, err
+	}
+
+	key := r.params.streamKey()
+	end := opts.Cursor
+	if end == "" {
+		end = "+"
+	}
+	start := "-"
+	count := strconv.Itoa(opts.Limit)
+
+	msgs := make([]Message, 0, opts.Limit)
+	streamEntries := make([]radix.StreamEntry, 0, opts.Limit)
+
+	err = r.params.Redis.Do(ctx, radix.Cmd(
+		&streamEntries,
+		"XREVRANGE", key, end, start, "COUNT", count,
+	))
+
+	if err != nil {
+		return "", nil, fmt.Errorf("calling XREVRANGE: %w", err)
+	}
+
+	var oldestEntryID radix.StreamEntryID
+
+	for _, entry := range streamEntries {
+		oldestEntryID = entry.ID
+
+		msg, err := msgFromStreamEntry(entry)
+		if err != nil {
+			return "", nil, fmt.Errorf(
+				"parsing stream entry %q: %w", entry.ID, err,
+			)
+		}
+		msgs = append(msgs, msg)
+	}
+
+	if len(msgs) < opts.Limit {
+		return zeroCursor, msgs, nil
+	}
+
+	cursor := oldestEntryID.Prev()
+	return cursor.String(), msgs, nil
+}
+
+func (r *room) readStream(ctx context.Context) error {
+
+	r.listeningL.Lock()
+	lastEntryID := r.listeningLastID
+	r.listeningL.Unlock()
+
+	redisAddr := r.params.Redis.Addr()
+	redisConn, err := radix.Dial(ctx, redisAddr.Network(), redisAddr.String())
+	if err != nil {
+		return fmt.Errorf("creating redis connection: %w", err)
+	}
+	defer redisConn.Close()
+
+	streamReader := (radix.StreamReaderConfig{}).New(
+		redisConn,
+		map[string]radix.StreamConfig{
+			r.params.streamKey(): {After: lastEntryID},
+		},
+	)
+
+	for {
+		dlCtx, dlCtxCancel := context.WithTimeout(ctx, 10*time.Second)
+		_, streamEntry, err := streamReader.Next(dlCtx)
+		dlCtxCancel()
+
+		if errors.Is(err, radix.ErrNoStreamEntries) {
+			continue
+		} else if err != nil {
+			return fmt.Errorf("fetching next entry from stream: %w", err)
+		}
+
+		msg, err := msgFromStreamEntry(streamEntry)
+		if err != nil {
+			return fmt.Errorf("parsing stream entry %q: %w", streamEntry, err)
+		}
+
+		r.listeningL.Lock()
+
+		var dropped int
+		for ch := range r.listening {
+			select {
+			case ch <- msg:
+			default:
+				dropped++
+			}
+		}
+
+		if dropped > 0 {
+			ctx := mctx.Annotate(ctx, "msgID", msg.ID, "dropped", dropped)
+			r.params.Logger.WarnString(ctx, "some listening channels full, messages dropped")
+		}
+
+		r.listeningLastID = streamEntry.ID
+
+		r.listeningL.Unlock()
+	}
+}
+
+func (r *room) readStreamLoop(ctx context.Context) {
+	for {
+		err := r.readStream(ctx)
+		if errors.Is(err, context.Canceled) {
+			return
+		} else if err != nil {
+			r.params.Logger.Error(ctx, "reading from redis stream", err)
+		}
+	}
+}
+
+type listenMsgIterator struct {
+	ch           <-chan Message
+	missedMsgs   []Message
+	sinceEntryID radix.StreamEntryID
+	cleanup      func()
+}
+
+func (i *listenMsgIterator) Next(ctx context.Context) (Message, error) {
+
+	if len(i.missedMsgs) > 0 {
+		msg := i.missedMsgs[0]
+		i.missedMsgs = i.missedMsgs[1:]
+		return msg, nil
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return Message{}, ctx.Err()
+		case msg := <-i.ch:
+
+			entryID, err := parseStreamEntryID(msg.ID)
+			if err != nil {
+				return Message{}, fmt.Errorf("parsing Message ID %q: %w", msg.ID, err)
+
+			} else if !i.sinceEntryID.Before(entryID) {
+				// this can happen if someone Appends a Message at the same time
+				// as another calls Listen. The Listener might have already seen
+				// the Message by calling History prior to the stream reader
+				// having processed it and updating listeningLastID.
+				continue
+			}
+
+			return msg, nil
+		}
+	}
+}
+
+func (i *listenMsgIterator) Close() error {
+	i.cleanup()
+	return nil
+}
+
+func (r *room) Listen(
+	ctx context.Context, sinceID string,
+) (
+	MessageIterator, error,
+) {
+
+	var sinceEntryID radix.StreamEntryID
+
+	if sinceID != "" {
+		var err error
+		if sinceEntryID, err = parseStreamEntryID(sinceID); err != nil {
+			return nil, fmt.Errorf("parsing sinceID: %w", err)
+		}
+	}
+
+	ch := make(chan Message, 32)
+
+	r.listeningL.Lock()
+	lastEntryID := r.listeningLastID
+	r.listening[ch] = struct{}{}
+	r.listeningL.Unlock()
+
+	cleanup := func() {
+		r.listeningL.Lock()
+		defer r.listeningL.Unlock()
+		delete(r.listening, ch)
+	}
+
+	key := r.params.streamKey()
+	start := sinceEntryID.Next().String()
+	end := "+"
+	if lastEntryID != (radix.StreamEntryID{}) {
+		end = lastEntryID.String()
+	}
+
+	var streamEntries []radix.StreamEntry
+
+	err := r.params.Redis.Do(ctx, radix.Cmd(
+		&streamEntries,
+		"XRANGE", key, start, end,
+	))
+
+	if err != nil {
+		cleanup()
+		return nil, fmt.Errorf("retrieving missed stream entries: %w", err)
+	}
+
+	missedMsgs := make([]Message, len(streamEntries))
+
+	for i := range streamEntries {
+
+		msg, err := msgFromStreamEntry(streamEntries[i])
+		if err != nil {
+			cleanup()
+			return nil, fmt.Errorf(
+				"parsing stream entry %q: %w", streamEntries[i].ID, err,
+			)
+		}
+
+		missedMsgs[i] = msg
+	}
+
+	return &listenMsgIterator{
+		ch:           ch,
+		missedMsgs:   missedMsgs,
+		sinceEntryID: sinceEntryID,
+		cleanup:      cleanup,
+	}, nil
+}
+
+func (r *room) Delete(ctx context.Context, id string) error {
+	return r.params.Redis.Do(ctx, radix.Cmd(
+		nil, "XDEL", r.params.streamKey(), id,
+	))
+}