tsk-34: Fetch next ready scheduled message (#38)

Closes #34

Reviewed-on: phoenix/textsender-api#38
Co-authored-by: phoenix <kundeng00@pm.me>
Co-committed-by: phoenix <kundeng00@pm.me>

internal/store/mock/scheduled_message_store.go

@@ -47,6 +47,30 @@ func (m *MockScheduledMessageStore) Get(ctx context.Context, id uuid.UUID) (*sch
 	}
 }
 
+func (m *MockScheduledMessageStore) FetchNextScheduledMessage(ctx context.Context) (*scheduling.ScheduledMessage, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.Error != nil {
+		return nil, m.Error
+	}
+
+	var schMsg *scheduling.ScheduledMessage
+	for _, msg := range m.ScheduledMessages {
+		if msg.Status == scheduling.Ready {
+			msg.Status = scheduling.Processing
+			schMsg = msg
+			break
+		}
+	}
+
+	if schMsg == nil {
+		return nil, fmt.Errorf("No scheduled message is ready")
+	} else {
+		return schMsg, nil
+	}
+}
+
 func (m *MockScheduledMessageStore) GetWithUserId(ctx context.Context, userId uuid.UUID) ([]*scheduling.ScheduledMessage, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()

internal/store/scheduled_message_store.go

@@ -13,6 +13,7 @@ import (
 
 type ScheduledMessageStore interface {
 	Get(ctx context.Context, id uuid.UUID) (*scheduling.ScheduledMessage, error)
+	FetchNextScheduledMessage(ctx context.Context) (*scheduling.ScheduledMessage, error)
 	GetWithUserId(ctx context.Context, userId uuid.UUID) ([]*scheduling.ScheduledMessage, error)
 	CreateScheduledMessage(ctx context.Context, schedMsg *scheduling.ScheduledMessage) error
 	UpdateStatus(ctx context.Context, id uuid.UUID, updatedStatus string) (*string, error)
@@ -45,6 +46,33 @@ func (s *PGScheduledMessageStore) Get(ctx context.Context, id uuid.UUID) (*sched
 	return &schMsg, nil
 }
 
+func (s *PGScheduledMessageStore) FetchNextScheduledMessage(ctx context.Context) (*scheduling.ScheduledMessage, error) {
+	query := `
+		UPDATE scheduled_messages
+		SET status = $1
+		WHERE id = (
+			SELECT id FROM scheduled_messages
+			WHERE status = $2
+			ORDER BY id
+			FOR UPDATE SKIP LOCKED
+			LIMIT 1
+		)
+		RETURNING id, scheduled, created, status, user_id
+	`
+	var schMsg scheduling.ScheduledMessage
+	err := s.db.QueryRow(ctx, query, scheduling.Processing, scheduling.Ready).Scan(
+		&schMsg.Id, &schMsg.Scheduled, &schMsg.Created, &schMsg.Status, &schMsg.UserId,
+	)
+
+	if err == pgx.ErrNoRows {
+		return nil, nil
+	} else if err != nil {
+		return nil, fmt.Errorf("Getting scheduled message: %w", err)
+	} else {
+		return &schMsg, nil
+	}
+}
+
 func (s *PGScheduledMessageStore) GetWithUserId(ctx context.Context, userId uuid.UUID) ([]*scheduling.ScheduledMessage, error) {
 	query := `
 		SELECT id, scheduled, created, status, user_id FROM scheduled_messages WHERE user_id = $1