Add DB subscription

pkg/db/subscription.go

@@ -0,0 +1,88 @@
+package db
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+type PollableDBQuery[ValueType any] func(lastSeenID int64, numRows int32) (results []ValueType, lastID int64, err error)
+
+// Poll whenever notified, or at an interval if not notified
+type PollingOptions struct {
+	Interval time.Duration
+	Notifier <-chan bool
+	NumRows  int32
+}
+
+type DBSubscription[ValueType any] struct {
+	ctx        context.Context
+	lastSeenID int64
+	options    PollingOptions
+	query      PollableDBQuery[ValueType]
+	updates    chan<- []ValueType
+}
+
+func NewDBSubscription[ValueType any](
+	ctx context.Context,
+	query PollableDBQuery[ValueType],
+	lastSeenID int64,
+	options PollingOptions,
+) *DBSubscription[ValueType] {
+	return &DBSubscription[ValueType]{
+		ctx:        ctx,
+		lastSeenID: lastSeenID,
+		options:    options,
+		query:      query,
+		updates:    nil,
+	}
+}
+
+func (s *DBSubscription[ValueType]) Start() (<-chan []ValueType, error) {
+	var err error
+	if s.updates != nil {
+		return nil, fmt.Errorf("Already started")
+	}
+	updates := make(chan []ValueType)
+	s.updates = updates
+
+	go func() {
+		if err := s.poll(); err != nil {
+			panic("TODO(rich)")
+		}
+
+		for {
+			select {
+			case <-s.ctx.Done():
+				return
+			case <-s.options.Notifier:
+				if err = s.poll(); err != nil {
+					panic("TODO(rich)")
+				}
+			case <-time.After(s.options.Interval):
+				if err = s.poll(); err != nil {
+					panic("TODO(rich)")
+				}
+			}
+		}
+
+	}()
+
+	return updates, nil
+}
+
+func (s *DBSubscription[ValueType]) poll() error {
+	for {
+		results, lastID, err := s.query(s.lastSeenID, s.options.NumRows)
+		if err != nil {
+			return err
+		}
+		s.lastSeenID = lastID
+		s.updates <- results
+		if int32(len(results)) < s.options.NumRows {
+			break
+		}
+	}
+
+	return nil
+}