mangaBookmark/backend/latest.go

package main

import (
	"context"
	"log"
	"net/url"
	"time"
)

// fetcher retrieves a series page. It exists as an interface so tests can inject
// a fake: nothing in the test suite may touch the network or the TLS client.
type fetcher interface {
	Get(ctx context.Context, url string) (body string, status int, err error)
}

// latestPoller re-checks each bookmarked series' newest published chapter on a
// schedule, independent of the userscript's own in-browser checks. The two run
// in parallel and report the same observable fact, so whichever writes last wins
// and neither needs to know about the other.
//
// Two clocks, deliberately independent:
//
//   - interval is how often this goroutine wakes up and looks.
//   - cooldown is how long one bookmark rests since its own last check.
//
// Only the cooldown is per bookmark, and it is enforced by the WHERE clause in
// DueForLatestCheck rather than by any timer. Shortening interval therefore
// cannot shorten anyone's cooldown; it only makes the poller wake up and find
// nothing due more often.
type latestPoller struct {
	store    *Store
	fetch    fetcher
	now      func() time.Time // injected so tests can freeze it
	cooldown time.Duration
	interval time.Duration
	stagger  time.Duration
	batch    int
}

// Run polls until ctx is cancelled.
//
// runOnce is called synchronously, so a batch that overruns the tick delays the
// next one instead of stacking a second batch on top of it. That is the intended
// failure mode for a misconfigured batch x stagger: a slower cadence, never
// concurrent fetch storms.
func (p *latestPoller) Run(ctx context.Context) {
	log.Printf("latest-chapter poller: interval=%s cooldown=%s batch=%d stagger=%s",
		p.interval, p.cooldown, p.batch, p.stagger)
	t := time.NewTicker(p.interval)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			log.Println("latest-chapter poller: stopped")
			return
		case <-t.C:
			p.runOnce(ctx)
		}
	}
}

// runOnce processes one batch of due bookmarks.
func (p *latestPoller) runOnce(ctx context.Context) {
	cutoff := p.now().Add(-p.cooldown).UnixMilli()
	due, err := p.store.DueForLatestCheck(cutoff, p.batch)
	if err != nil {
		log.Printf("latest poll: due query: %v", err)
		return
	}

	checked := 0
	for i, b := range due {
		if ctx.Err() != nil {
			break
		}
		// Staggered rather than fired together: a burst of simultaneous requests
		// from one server IP is the traffic shape most likely to move that IP's
		// bot score. This is the server-side analogue of the userscript's "one
		// series per navigation ... indistinguishable from browsing" (L455-456).
		stopped := false
		if i > 0 && p.stagger > 0 {
			select {
			case <-ctx.Done():
				stopped = true
			case <-time.After(p.stagger):
			}
		}
		if stopped {
			break
		}
		p.checkOne(ctx, b)
		checked++
	}
	// due vs checked is how you tell which constraint is binding: ticks that
	// report due=0 mean the cooldown is the limit, ticks that report due==batch
	// every time mean throughput is.
	log.Printf("latest poll: due=%d checked=%d", len(due), checked)
}

// checkOne re-checks one series. Every failure path here is "log and move on":
// the poller is a best-effort enhancement, and no single bad series may stall a
// batch or take down the process.
func (p *latestPoller) checkOne(ctx context.Context, b Bookmark) {
	defer func() {
		if r := recover(); r != nil {
			log.Printf("latest poll %q: recovered from panic: %v", b.Key, r)
		}
	}()

	// Stamped before the fetch, not after, so an error, a timeout, or a shutdown
	// mid-request still consumes the cooldown. Otherwise a renamed or deleted
	// series would be retried on every single tick forever. The userscript
	// stamps in the same order and for the same reason (L471-473).
	if err := p.store.MarkLatestChecked(b.Key, p.now().UnixMilli()); err != nil {
		log.Printf("latest poll %q: mark checked: %v", b.Key, err)
		return
	}

	// series_url is client-supplied (PUT /bookmarks/{key} accepts any string),
	// so this is not just an optimisation against burning a request on an
	// unknown site: without it, the server would issue a GET from its own
	// network position to whatever URL a token-holder writes, including
	// link-local/internal addresses or non-https schemes. The cooldown above
	// is already consumed, so a row that never passes this check is retried at
	// cooldown pace rather than hot-looping.
	if !fetchableSeriesURL(b.Site, b.SeriesURL) {
		log.Printf("latest poll %q: not fetchable: site=%q url=%q", b.Key, b.Site, b.SeriesURL)
		return
	}

	body, status, err := p.fetch.Get(ctx, b.SeriesURL)
	if err != nil {
		log.Printf("latest poll %q: fetch %s: %v", b.Key, b.SeriesURL, err)
		return
	}
	if status != 200 {
		log.Printf("latest poll %q: fetch %s: status %d", b.Key, b.SeriesURL, status)
		return
	}

	latest, ok := latestChapterFrom(b.Site, b.SeriesURL, body)
	if !ok {
		// Most likely a challenge page or a layout change. Either way the row is
		// already stamped, so this waits out a cooldown instead of hot-looping.
		log.Printf("latest poll %q: no chapter links in %d bytes", b.Key, len(body))
		return
	}

	// Re-read: the row may have been updated or deleted while the fetch was in
	// flight, and writing b back wholesale would undo that.
	//
	// ponytail: non-transactional read-modify-write, wrap Get+Upsert in a tx if
	// this ever runs for more than one user. A client PUT that commits between
	// these two statements is lost to the stale re-read — reverting read
	// progress or a status change, and moving updated_at because the stored
	// value now differs. Accepted for a single-user deployment: the window is
	// milliseconds and the loser is one poll cycle.
	cur, found, err := p.store.Get(b.Key)
	if err != nil {
		log.Printf("latest poll %q: reread: %v", b.Key, err)
		return
	}
	if !found {
		return
	}
	// Equality, not >, mirroring the userscript (L427): a site that retracts a
	// chapter should correct the stored number downward.
	if cur.LatestChapterNum != nil && *cur.LatestChapterNum == latest.Num {
		return
	}

	num := latest.Num
	cur.LatestChapter = latest.Label
	cur.LatestChapterNum = &num
	// A candidate only. last_chapter_num is untouched, so the CASE in Upsert
	// keeps the stored updated_at and the bookmark list does not reorder.
	cur.UpdatedAt = p.now().UnixMilli()
	if _, err := p.store.Upsert(cur); err != nil {
		log.Printf("latest poll %q: upsert: %v", b.Key, err)
		return
	}
	log.Printf("latest poll %q: latest is now %s", b.Key, latest.Label)
}

// fetchableSeriesURL reports whether site is a site latestChapterFrom knows how
// to parse and seriesURL is safe to hand to the fetcher: an https URL with a
// non-empty host. series_url comes from client-supplied PUT bodies, so this is
// a defence against the poller being used to probe arbitrary hosts from the
// server's own network position, not just a check against wasted requests.
func fetchableSeriesURL(site, seriesURL string) bool {
	switch site {
	case "asura", "demonic":
	default:
		return false
	}
	u, err := url.Parse(seriesURL)
	if err != nil {
		return false
	}
	return u.Scheme == "https" && u.Host != ""
}