func (p *githubRepoPoller) syncComments(ctx context.Context) error {
        for {
                nums := p.issueNumbersWithStaleCommentSync()
                if len(nums) == 0 {
                        return nil
                }
                remain := len(nums)
                for _, num := range nums {
                        p.logf("comment sync: %d issues remaining; syncing issue %v", remain, num)

                        if err := p.syncCommentsOnIssue(ctx, num); err != nil {
                                p.logf("comment sync on issue %d: %v", num, err)
                                return err
                        }
                        remain--
                }
        }
}

        for n, gi := range p.gr.issues {
                if !gi.commentsSynced() {
                        issueNums = append(issueNums, n)
                }
        }

func (gi *GitHubIssue) commentsSynced() bool {
        if gi.NotExist {
                // Issue doesn't exist, so can't sync its non-issues, 
                // so consider it done.
                return true
        }
        return gi.commentsSyncedAsOf.After(gi.Updated)
}

        commentsSyncedAsOf time.Time                   // as of server's Date header

func (c *Corpus) processGithubIssueMutation(m *maintpb.GithubIssueMutation) {
....
        if m.CommentStatus != nil && m.CommentStatus.ServerDate != nil {
                if serverDate, err := ptypes.Timestamp(m.CommentStatus.ServerDate); err == nil {
                        gi.commentsSyncedAsOf = serverDate.UTC()
                }

func (p *githubRepoPoller) syncCommentsOnIssue(ctx context.Context, issueNum int32) error {

    commentsUpdatedTil time.Time       // max comment modtime seen

        since := issue.commentsUpdatedTil
        p.c.mu.RUnlock()

        owner, repo := p.gr.id.Owner, p.gr.id.Repo
        morePages := true // at least try the first. might be empty.                                                                                                                                                    
        for morePages {
                ics, res, err := p.githubDirect.Issues.ListComments(ctx, owner, repo, int(issueNum), &github.IssueListCommentsOptions{
                        Since:       since,
                        Direction:   "asc",
                        Sort:        "updated",
                        ListOptions: github.ListOptions{PerPage: 100},
                })