ZDDC/zddc/internal/cache/walker.go

package cache

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"strings"
	"sync"
	"time"
)

// MirrorStateFile is the persisted scheduler state — last walk time
// per subtree. Lives at the cache root so state survives restarts.
const MirrorStateFile = ".zddc-mirror-state.json"

// DefaultMirrorMinInterval is the gate between walks for the same
// subtree. Active mirrors stay current via repeated visits; idle
// mirrors don't generate any upstream traffic until someone hits them.
const DefaultMirrorMinInterval = time.Hour

// DefaultMirrorParallelism caps how many concurrent file fetches a
// single walk fires. Bound is per-walk, not global — multiple
// subtrees walked at once each get their own budget. Conservative
// enough not to starve interactive requests sharing the same
// connection pool.
const DefaultMirrorParallelism = 4

// MirrorScheduler owns the per-subtree access-triggered walk policy.
// Built once at startup from cfg.MirrorSubtree[]; the cache layer
// installs Trigger as its onAccess hook.
type MirrorScheduler struct {
	cache       *Cache
	subtrees    []string // normalized: leading "/", no trailing "/"
	minInterval time.Duration
	parallelism int

	mu       sync.Mutex
	state    map[string]subtreeState
	statePath string
	inFlight map[string]bool // tracked separately from on-disk state — restarts forget
}

type subtreeState struct {
	LastWalkAt time.Time `json:"last_walk_at"`
}

type persistedState struct {
	Subtrees map[string]subtreeState `json:"subtrees"`
}

// NewMirrorScheduler constructs a scheduler attached to the given
// cache. Returns nil scheduler (and nil error) when no subtrees are
// configured — caller checks for that.
func NewMirrorScheduler(c *Cache, subtrees []string, minInterval time.Duration, parallelism int) (*MirrorScheduler, error) {
	if len(subtrees) == 0 {
		return nil, nil
	}
	if minInterval <= 0 {
		minInterval = DefaultMirrorMinInterval
	}
	if parallelism <= 0 {
		parallelism = DefaultMirrorParallelism
	}
	normalized := make([]string, 0, len(subtrees))
	for _, s := range subtrees {
		s = strings.TrimSpace(s)
		if s == "" || s == "/" {
			s = "/"
		} else {
			if !strings.HasPrefix(s, "/") {
				s = "/" + s
			}
			s = strings.TrimRight(s, "/")
		}
		normalized = append(normalized, s)
	}
	statePath := filepath.Join(c.root, MirrorStateFile)
	state := loadMirrorState(statePath)
	s := &MirrorScheduler{
		cache:       c,
		subtrees:    normalized,
		minInterval: minInterval,
		parallelism: parallelism,
		state:       state,
		statePath:   statePath,
		inFlight:    make(map[string]bool),
	}
	c.onAccess = s.Trigger
	return s, nil
}

// Subtrees returns the configured subtree list (read-only).
func (s *MirrorScheduler) Subtrees() []string {
	out := make([]string, len(s.subtrees))
	copy(out, s.subtrees)
	return out
}

// MinInterval returns the configured min walk interval.
func (s *MirrorScheduler) MinInterval() time.Duration { return s.minInterval }

// Trigger inspects an incoming URL and, if it falls under a
// configured subtree AND a walk is due, kicks one in the background.
// Always non-blocking. Safe to call concurrently.
func (s *MirrorScheduler) Trigger(urlPath string) {
	subtree := s.matchSubtree(urlPath)
	if subtree == "" {
		return
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.inFlight[subtree] {
		return
	}
	st := s.state[subtree]
	if !st.LastWalkAt.IsZero() && time.Since(st.LastWalkAt) < s.minInterval {
		return
	}
	s.inFlight[subtree] = true
	go s.runWalk(subtree)
}

// runWalk does one walk-and-record cycle for subtree. Always called
// in a goroutine from Trigger.
func (s *MirrorScheduler) runWalk(subtree string) {
	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Minute)
	defer cancel()
	slog.Info("mirror walk start", "subtree", subtree)
	start := time.Now()
	stats, err := s.walkSubtree(ctx, subtree)
	dur := time.Since(start)
	if err != nil {
		slog.Warn("mirror walk failed", "subtree", subtree, "duration", dur, "err", err)
	} else {
		slog.Info("mirror walk done", "subtree", subtree, "duration", dur,
			"dirs", stats.dirs, "files", stats.files, "fetched", stats.fetched, "purged", stats.purged)
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	s.inFlight[subtree] = false
	s.state[subtree] = subtreeState{LastWalkAt: time.Now()}
	persistMirrorState(s.statePath, s.state)
}

type walkStats struct {
	dirs    int
	files   int
	fetched int
	purged  int
}

// walkSubtree recursively walks the upstream listings under subtree,
// fetching missing/stale files and purging local files no longer in
// the upstream's filtered listing. Bounded parallelism via a
// semaphore. Errors fetching individual files are logged but don't
// abort the walk.
func (s *MirrorScheduler) walkSubtree(ctx context.Context, subtree string) (walkStats, error) {
	sem := make(chan struct{}, s.parallelism)
	var stats walkStats
	var statsMu sync.Mutex
	addFetched := func() { statsMu.Lock(); stats.fetched++; statsMu.Unlock() }
	addPurged := func() { statsMu.Lock(); stats.purged++; statsMu.Unlock() }
	addDirs := func() { statsMu.Lock(); stats.dirs++; statsMu.Unlock() }
	addFiles := func() { statsMu.Lock(); stats.files++; statsMu.Unlock() }

	var fetchWG sync.WaitGroup

	var walkDir func(dirURL string) error
	walkDir = func(dirURL string) error {
		if !strings.HasSuffix(dirURL, "/") {
			dirURL += "/"
		}
		addDirs()
		entries, err := s.fetchListing(ctx, dirURL)
		if err != nil {
			return err
		}

		// Build a set of names the upstream listing exposes so we can
		// purge anything local-but-removed-upstream after walking.
		upstreamNames := make(map[string]bool, len(entries))
		for _, e := range entries {
			name := strings.TrimSuffix(e.Name, "/")
			upstreamNames[name] = true
		}

		for _, e := range entries {
			if ctx.Err() != nil {
				return ctx.Err()
			}
			childURL := dirURL + url.PathEscape(strings.TrimSuffix(e.Name, "/"))
			if e.IsDir {
				childURL += "/"
				if err := walkDir(childURL); err != nil {
					slog.Debug("walk subdir failed", "url", childURL, "err", err)
				}
				continue
			}
			addFiles()
			fetchWG.Add(1)
			sem <- struct{}{}
			go func(fileURL string) {
				defer fetchWG.Done()
				defer func() { <-sem }()
				if s.fetchFileIfNeeded(ctx, fileURL) {
					addFetched()
				}
			}(childURL)
		}

		// Purge local files no longer present upstream. Only acts on
		// regular files inside this directory — listing sidecars,
		// dot-prefix internals, and subdirectories are left alone.
		s.purgeOrphans(dirURL, upstreamNames, addPurged)
		return nil
	}

	err := walkDir(subtree + "/")
	fetchWG.Wait()
	return stats, err
}

// purgeOrphans removes local cached files in dirURL that are absent
// from the upstream listing. Only operates on plain files; ignores
// dot-prefix entries (cache state) and subdirectories (they get their
// own pass during walkSubtree's recursion).
func (s *MirrorScheduler) purgeOrphans(dirURL string, upstreamNames map[string]bool, addPurged func()) {
	rel := filepath.FromSlash(strings.Trim(dirURL, "/"))
	localDir := filepath.Join(s.cache.root, rel)
	entries, err := os.ReadDir(localDir)
	if err != nil {
		return
	}
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		name := e.Name()
		if strings.HasPrefix(name, ".") {
			continue
		}
		if upstreamNames[name] {
			continue
		}
		_ = os.Remove(filepath.Join(localDir, name))
		addPurged()
		slog.Info("purged orphan", "path", path.Join(dirURL, name))
	}
}

// fetchListing GETs the upstream JSON listing for dirURL and parses
// the entries. Caches the JSON sidecar as a side-effect (so offline
// directory browsing works for walked subtrees).
func (s *MirrorScheduler) fetchListing(ctx context.Context, dirURL string) ([]listingEntry, error) {
	target := s.cache.upstream + dirURL
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("Accept", "application/json")
	if s.cache.bearer != "" {
		req.Header.Set("Authorization", "Bearer "+s.cache.bearer)
	}
	resp, err := s.cache.client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("fetch listing %s: %w", target, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("listing %s: status %d", target, resp.StatusCode)
	}
	bodyBytes, err := io.ReadAll(io.LimitReader(resp.Body, 16*1024*1024))
	if err != nil {
		return nil, fmt.Errorf("read listing body: %w", err)
	}
	// Persist the JSON sidecar so the directory is browsable offline.
	if path, ok := s.cache.listingCachePathFor(dirURL, "application/json"); ok {
		if err := os.MkdirAll(filepath.Dir(path), 0o755); err == nil {
			tmp, err := os.CreateTemp(filepath.Dir(path), ".zddc-cache-tmp-*")
			if err == nil {
				tmpName := tmp.Name()
				if _, err := tmp.Write(bodyBytes); err == nil {
					_ = tmp.Close()
					if lm := resp.Header.Get("Last-Modified"); lm != "" {
						if t, err := http.ParseTime(lm); err == nil {
							_ = os.Chtimes(tmpName, t, t)
						}
					}
					_ = os.Rename(tmpName, path)
					s.cache.maybeWriteMarker()
				} else {
					_ = tmp.Close()
					_ = os.Remove(tmpName)
				}
			}
		}
	}
	var parsed []listingEntry
	if err := json.Unmarshal(bodyBytes, &parsed); err != nil {
		return nil, fmt.Errorf("parse listing %s: %w", target, err)
	}
	return parsed, nil
}

// listingEntry mirrors the subset of zddc/internal/listing/types.go's
// FileInfo we need. Extra fields ignored.
type listingEntry struct {
	Name  string `json:"name"`
	IsDir bool   `json:"is_dir"`
}

// fetchFileIfNeeded conditional-GETs a file URL. Returns true when
// the cache was updated (200 + write); false when no work was needed
// (304 or skipped).
func (s *MirrorScheduler) fetchFileIfNeeded(ctx context.Context, fileURL string) bool {
	cachePath, ok := s.cache.cachePathFor(fileURL)
	if !ok {
		return false
	}
	target := s.cache.upstream + fileURL
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
	if err != nil {
		return false
	}
	if info, err := os.Stat(cachePath); err == nil {
		req.Header.Set("If-Modified-Since", info.ModTime().UTC().Format(http.TimeFormat))
	}
	if s.cache.bearer != "" {
		req.Header.Set("Authorization", "Bearer "+s.cache.bearer)
	}
	resp, err := s.cache.client.Do(req)
	if err != nil {
		return false
	}
	defer resp.Body.Close()
	if resp.StatusCode == http.StatusNotModified {
		return false
	}
	if resp.StatusCode != http.StatusOK {
		// 4xx → ACL revoked or upstream gone; remove local cache.
		if resp.StatusCode == http.StatusForbidden || resp.StatusCode == http.StatusNotFound {
			_ = os.Remove(cachePath)
		}
		return false
	}
	if !s.cache.responseCacheable(resp) {
		return false
	}
	if err := s.cache.persistOnly(resp, fileURL); err != nil {
		slog.Debug("walker persist failed", "url", fileURL, "err", err)
		return false
	}
	s.cache.maybeWriteMarker()
	return true
}

// matchSubtree returns the longest configured subtree that's a prefix
// of urlPath, or "" if none match. "/" matches everything.
func (s *MirrorScheduler) matchSubtree(urlPath string) string {
	if urlPath == "" {
		urlPath = "/"
	}
	var best string
	for _, st := range s.subtrees {
		if st == "/" {
			if len(best) == 0 {
				best = "/"
			}
			continue
		}
		if strings.HasPrefix(urlPath, st+"/") || urlPath == st {
			if len(st) > len(best) {
				best = st
			}
		}
	}
	return best
}

// loadMirrorState reads the persisted state from disk. Returns an
// empty (initialized) map on missing/corrupt input.
func loadMirrorState(path string) map[string]subtreeState {
	state := make(map[string]subtreeState)
	bytes, err := os.ReadFile(path)
	if err != nil {
		return state
	}
	var parsed persistedState
	if err := json.Unmarshal(bytes, &parsed); err != nil {
		return state
	}
	if parsed.Subtrees != nil {
		state = parsed.Subtrees
	}
	return state
}

// persistMirrorState writes the state file atomically (temp + rename).
// Best-effort: errors are logged but non-fatal.
func persistMirrorState(path string, state map[string]subtreeState) {
	bytes, err := json.MarshalIndent(persistedState{Subtrees: state}, "", "  ")
	if err != nil {
		slog.Debug("marshal mirror state", "err", err)
		return
	}
	tmp, err := os.CreateTemp(filepath.Dir(path), ".zddc-mirror-state-tmp-*")
	if err != nil {
		slog.Debug("create mirror state tmp", "err", err)
		return
	}
	tmpName := tmp.Name()
	if _, err := tmp.Write(bytes); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return
	}
	if err := os.Rename(tmpName, path); err != nil {
		_ = os.Remove(tmpName)
	}
}