ZDDC/zddc/internal/cache/cache.go

// Package cache implements zddc-server's client mode: a downstream
// proxy/cache/mirror that runs the same binary against a master.
// Configured via cfg.Upstream (in main.go), the cache layer replaces
// the master-side dispatcher entirely — every incoming request is
// forwarded to the master with the local instance's bearer token, and
// (in cache or mirror mode) the response body is persisted under
// cfg.Root so subsequent requests serve from disk.
//
// The cache directory layout is intentionally a normal ZDDC root: a
// file fetched from `<master>/foo/bar.txt` is stored at `<root>/foo/
// bar.txt`. No sidecar metadata. The local file's mtime is set to the
// upstream's Last-Modified header so revalidation via
// If-Modified-Since reflects the master's notion of the file's age,
// not when the local cache happened to fetch it. Running
// `zddc-server --root <cache-dir>` without --upstream serves the
// cached files as a regular ZDDC — useful for portable offline
// snapshots and sanity-check inspection.
//
// Phase 2 scope: GET/HEAD only. Range requests, stale-while-
// revalidate, and offline-fallback are supported. Directory listings
// are always proxied live (no listing cache yet); writes (PUT / POST /
// DELETE) and the mirror walker land in later phases.
package cache

import (
	"crypto/tls"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"codeberg.org/VARASYS/ZDDC/zddc/internal/config"
)

// MarkerFile records the upstream URL and first-cached-at timestamp
// in the cache root. Prevents accidentally pointing master mode at a
// cache directory and provides provenance for ops/users.
const MarkerFile = ".zddc-upstream"

// HeaderName is the response header that surfaces cache state to the
// client (and the browser-side UI). Values: hit, revalidated, miss,
// proxy, offline.
const HeaderName = "X-ZDDC-Cache"

// listingCachePrefix is the basename prefix used for directory-listing
// sidecar files. Suffix is the format ("html" or "json"). Dot-prefixed
// so the local "serve cache root as plain master" mode hides them via
// the existing dispatch dot-prefix guard.
const listingCachePrefix = ".zddc-listing."

// Cache is the request handler installed in main.go when cfg.Upstream
// is non-empty. It is safe for concurrent ServeHTTP calls.
type Cache struct {
	root     string // local cache directory (== cfg.Root in client mode)
	upstream string // upstream master URL, no trailing slash
	bearer   string // forwarded as Authorization: Bearer to upstream; "" disables
	mode     string // "proxy" | "cache" | "mirror"
	persist  bool   // mode != "proxy" — write responses to disk
	client   *http.Client

	markerOnce sync.Once

	// onAccess is invoked (when non-nil) after a request is dispatched.
	// The walker scheduler installs this hook to kick mirror walks based
	// on incoming traffic. Always called in a goroutine — must not
	// assume it runs before the response completes. Nil in proxy/cache
	// modes; set in mirror mode.
	onAccess func(urlPath string)
}

// New constructs a Cache from the loaded configuration. Validates
// upstream URL, reads the bearer-file (if configured), prepares the
// HTTP client honoring SkipTLSVerify, and ensures the cache root
// exists.
func New(cfg config.Config) (*Cache, error) {
	if cfg.Upstream == "" {
		return nil, fmt.Errorf("cache.New: cfg.Upstream is empty")
	}
	upstream := strings.TrimRight(cfg.Upstream, "/")
	if _, err := url.Parse(upstream); err != nil {
		return nil, fmt.Errorf("cache.New: invalid upstream %q: %w", upstream, err)
	}

	bearer := ""
	if cfg.BearerFile != "" {
		b, err := os.ReadFile(cfg.BearerFile)
		if err != nil {
			return nil, fmt.Errorf("cache.New: read bearer file: %w", err)
		}
		bearer = strings.TrimSpace(string(b))
		if bearer == "" {
			return nil, fmt.Errorf("cache.New: bearer file %q is empty", cfg.BearerFile)
		}
	}

	transport := &http.Transport{
		MaxIdleConns:        10,
		IdleConnTimeout:     30 * time.Second,
		TLSHandshakeTimeout: 10 * time.Second,
		ResponseHeaderTimeout: 30 * time.Second,
	}
	if cfg.SkipTLSVerify {
		// G402 / CWE-295: deliberate. Documented operator opt-in for
		// dev/internal-CA scenarios; never the default.
		transport.TLSClientConfig = &tls.Config{InsecureSkipVerify: true} //nolint:gosec
		slog.Warn("--skip-tls-verify enabled: upstream TLS certificates will NOT be validated")
	}

	if err := os.MkdirAll(cfg.Root, 0o755); err != nil {
		return nil, fmt.Errorf("cache.New: create cache root %q: %w", cfg.Root, err)
	}

	mode := cfg.Mode
	if mode == "" {
		mode = "cache"
	}

	return &Cache{
		root:     cfg.Root,
		upstream: upstream,
		bearer:   bearer,
		mode:     mode,
		persist:  mode != "proxy",
		client: &http.Client{
			Transport: transport,
			Timeout:   60 * time.Second,
			// Don't follow redirects automatically — pass them through to
			// the client so the browser can update its address bar
			// (e.g. master's no-trailing-slash → trailing-slash 301).
			CheckRedirect: func(req *http.Request, via []*http.Request) error {
				return http.ErrUseLastResponse
			},
		},
	}, nil
}

// Mode returns the configured mode label for diagnostics.
func (c *Cache) Mode() string { return c.mode }

// Upstream returns the upstream master URL for diagnostics.
func (c *Cache) Upstream() string { return c.upstream }

// ServeHTTP is the cache layer's HTTP entry point. Replaces the
// master-side dispatcher in client mode.
func (c *Cache) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Phase 2: read-only. Writes are deferred to the outbox phase.
	// Forward HEAD as GET-without-body to keep the response shape
	// consistent with what http.ServeContent would do.
	if r.Method != http.MethodGet && r.Method != http.MethodHead {
		w.Header().Set("Allow", "GET, HEAD")
		http.Error(w, "Method Not Allowed: writes are not yet supported in client mode", http.StatusMethodNotAllowed)
		return
	}

	// Mirror walker hook: notify the scheduler that this URL was
	// touched so it can decide whether to kick a subtree walk. Runs
	// after we've started serving — the user's request never blocks
	// on walk scheduling.
	if c.onAccess != nil {
		go c.onAccess(r.URL.Path)
	}

	// Directory listings: try sidecar listing-cache, fall back to
	// proxy. The Accept header determines which sidecar (HTML vs
	// JSON) to consult — the master serves both formats and the
	// browser asks for HTML for navigation, JSON for the inline-JS
	// listing fetcher.
	if strings.HasSuffix(r.URL.Path, "/") {
		c.serveDirectory(w, r)
		return
	}

	// File request — try cache first when persisting.
	if c.persist {
		if path, ok := c.cachePathFor(r.URL.Path); ok {
			info, err := os.Stat(path)
			if err == nil && !info.IsDir() {
				c.serveFromDisk(w, r, path, info, "hit")
				// Background revalidate; never block the user response.
				go c.revalidate(r.URL.Path, info.ModTime())
				return
			}
		}
	}

	// Miss (or proxy mode) → forward to upstream and (optionally)
	// persist on the way through.
	c.proxy(w, r, c.persist)
}

// serveDirectory handles directory requests. Tries the sidecar
// listing cache first (selected by Accept), revalidates in
// background on a hit, falls back to upstream proxy on miss.
func (c *Cache) serveDirectory(w http.ResponseWriter, r *http.Request) {
	if c.persist {
		if path, ok := c.listingCachePathFor(r.URL.Path, r.Header.Get("Accept")); ok {
			info, err := os.Stat(path)
			if err == nil && !info.IsDir() {
				c.serveListingFromDisk(w, r, path, info, "hit")
				go c.revalidateListing(r.URL.Path, r.Header.Get("Accept"), info.ModTime())
				return
			}
		}
	}
	c.proxyDirectory(w, r, c.persist)
}

// proxy forwards the request to upstream and serves the response back
// to the client. When writeToCache is true and the response is a
// cacheable 200, the body is also persisted under cfg.Root.
func (c *Cache) proxy(w http.ResponseWriter, r *http.Request, writeToCache bool) {
	upReq, err := c.buildUpstreamRequest(r)
	if err != nil {
		http.Error(w, "Bad Request: "+err.Error(), http.StatusBadRequest)
		return
	}

	resp, err := c.client.Do(upReq)
	if err != nil {
		// Network error. If we have a cached copy, serve it stale.
		if writeToCache && r.Method == http.MethodGet {
			if path, ok := c.cachePathFor(r.URL.Path); ok {
				if info, sErr := os.Stat(path); sErr == nil && !info.IsDir() {
					c.serveFromDisk(w, r, path, info, "offline")
					return
				}
			}
		}
		slog.Warn("upstream fetch failed", "url", upReq.URL.String(), "err", err)
		w.Header().Set(HeaderName, "offline")
		http.Error(w, "Service Unavailable: upstream unreachable", http.StatusServiceUnavailable)
		return
	}
	defer resp.Body.Close()

	// Forward upstream response headers. Skip hop-by-hop headers (RFC
	// 7230 §6.1) — Go's transport already drops most, but Connection
	// and Transfer-Encoding can sneak through and confuse the client.
	for k, vv := range resp.Header {
		if isHopByHop(k) {
			continue
		}
		for _, v := range vv {
			w.Header().Add(k, v)
		}
	}

	cacheable := writeToCache && resp.StatusCode == http.StatusOK && c.responseCacheable(resp)
	if cacheable {
		w.Header().Set(HeaderName, "miss")
	} else if writeToCache {
		w.Header().Set(HeaderName, "proxy")
	} else {
		w.Header().Set(HeaderName, "proxy")
	}

	w.WriteHeader(resp.StatusCode)

	if r.Method == http.MethodHead || resp.StatusCode == http.StatusNotModified {
		return
	}

	if !cacheable {
		_, _ = io.Copy(w, resp.Body)
		return
	}

	// Stream body to client AND to a tmp file in the cache; rename
	// atomically only on success.
	if err := c.streamAndPersist(w, resp, r.URL.Path); err != nil {
		// Mid-stream error: the client got a partial body (HTTP-normal),
		// and we already abandoned the cache write. Just log.
		slog.Debug("stream-and-persist error", "url", r.URL.Path, "err", err)
	} else {
		c.maybeWriteMarker()
	}
}

// buildUpstreamRequest constructs the outbound request preserving the
// path, query, Range, and Accept headers. Adds the bearer if configured.
func (c *Cache) buildUpstreamRequest(r *http.Request) (*http.Request, error) {
	target := c.upstream + r.URL.RequestURI()
	upReq, err := http.NewRequestWithContext(r.Context(), r.Method, target, nil)
	if err != nil {
		return nil, err
	}
	// Preserve the Range header for resumable / partial transfers.
	if v := r.Header.Get("Range"); v != "" {
		upReq.Header.Set("Range", v)
	}
	if v := r.Header.Get("If-Range"); v != "" {
		upReq.Header.Set("If-Range", v)
	}
	if v := r.Header.Get("Accept"); v != "" {
		upReq.Header.Set("Accept", v)
	}
	if v := r.Header.Get("Accept-Encoding"); v != "" {
		upReq.Header.Set("Accept-Encoding", v)
	}
	upReq.Header.Set("User-Agent", "zddc-server-cache/0.1")
	if c.bearer != "" {
		upReq.Header.Set("Authorization", "Bearer "+c.bearer)
	}
	return upReq, nil
}

// responseCacheable reports whether the response body should be
// persisted. Honors Cache-Control: no-store / private and refuses to
// cache responses without a content body (ranges, 204, etc.).
func (c *Cache) responseCacheable(resp *http.Response) bool {
	cc := resp.Header.Get("Cache-Control")
	low := strings.ToLower(cc)
	if strings.Contains(low, "no-store") || strings.Contains(low, "private") {
		return false
	}
	// Don't cache partial-content responses — the server returned 206
	// for a Range request, which means the body covers only part of
	// the file. Caching that partial body would corrupt subsequent
	// non-range fetches.
	if resp.StatusCode != http.StatusOK {
		return false
	}
	return true
}

// streamAndPersist writes resp.Body simultaneously to the client and
// to a temp file in the cache. Renames the temp atomically on success.
// Sets the local file's mtime to upstream's Last-Modified (if
// present) so subsequent revalidations send If-Modified-Since with a
// timestamp upstream can compare against its own state.
func (c *Cache) streamAndPersist(w http.ResponseWriter, resp *http.Response, urlPath string) error {
	finalPath, ok := c.cachePathFor(urlPath)
	if !ok {
		_, err := io.Copy(w, resp.Body)
		return err
	}
	if err := os.MkdirAll(filepath.Dir(finalPath), 0o755); err != nil {
		_, copyErr := io.Copy(w, resp.Body)
		if copyErr != nil {
			return copyErr
		}
		return err
	}
	tmp, err := os.CreateTemp(filepath.Dir(finalPath), ".zddc-cache-tmp-*")
	if err != nil {
		_, copyErr := io.Copy(w, resp.Body)
		if copyErr != nil {
			return copyErr
		}
		return err
	}
	tmpName := tmp.Name()
	mw := io.MultiWriter(tmp, w)
	if _, err := io.Copy(mw, resp.Body); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return err
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return err
	}
	if lm := resp.Header.Get("Last-Modified"); lm != "" {
		if t, err := http.ParseTime(lm); err == nil {
			_ = os.Chtimes(tmpName, t, t)
		}
	}
	return os.Rename(tmpName, finalPath)
}

// serveFromDisk serves a cached file via http.ServeContent (which
// handles Range requests, If-Modified-Since, and conditional GETs
// natively). cacheState is the X-ZDDC-Cache value to surface.
func (c *Cache) serveFromDisk(w http.ResponseWriter, r *http.Request, path string, info os.FileInfo, cacheState string) {
	f, err := os.Open(path)
	if err != nil {
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	defer f.Close()
	w.Header().Set(HeaderName, cacheState)
	http.ServeContent(w, r, filepath.Base(path), info.ModTime(), f)
}

// revalidate fires a conditional GET against upstream after a cache
// hit. 304 = no-op (cache is fresh). 200 = update cache. 403/404 =
// purge (ACL revoked or upstream deleted). Network errors are
// swallowed — staleness via offline is the documented behavior.
func (c *Cache) revalidate(urlPath string, mtime time.Time) {
	target := c.upstream + urlPath
	req, err := http.NewRequest(http.MethodGet, target, nil)
	if err != nil {
		return
	}
	if !mtime.IsZero() {
		req.Header.Set("If-Modified-Since", mtime.UTC().Format(http.TimeFormat))
	}
	if c.bearer != "" {
		req.Header.Set("Authorization", "Bearer "+c.bearer)
	}
	resp, err := c.client.Do(req)
	if err != nil {
		return
	}
	defer resp.Body.Close()
	switch resp.StatusCode {
	case http.StatusNotModified:
		return
	case http.StatusOK:
		if !c.responseCacheable(resp) {
			return
		}
		if err := c.persistOnly(resp, urlPath); err != nil {
			slog.Debug("revalidate persist error", "url", urlPath, "err", err)
		}
	case http.StatusForbidden, http.StatusNotFound:
		if path, ok := c.cachePathFor(urlPath); ok {
			_ = os.Remove(path)
			slog.Info("purged cached entry after upstream 4xx", "url", urlPath, "status", resp.StatusCode)
		}
	}
}

// persistOnly writes resp.Body to the cache without forwarding it
// anywhere. Used by revalidate (the user's request was already served
// from disk; we just refresh the cache in the background).
func (c *Cache) persistOnly(resp *http.Response, urlPath string) error {
	finalPath, ok := c.cachePathFor(urlPath)
	if !ok {
		_, _ = io.Copy(io.Discard, resp.Body)
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(finalPath), 0o755); err != nil {
		_, _ = io.Copy(io.Discard, resp.Body)
		return err
	}
	tmp, err := os.CreateTemp(filepath.Dir(finalPath), ".zddc-cache-tmp-*")
	if err != nil {
		_, _ = io.Copy(io.Discard, resp.Body)
		return err
	}
	tmpName := tmp.Name()
	if _, err := io.Copy(tmp, resp.Body); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return err
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return err
	}
	if lm := resp.Header.Get("Last-Modified"); lm != "" {
		if t, err := http.ParseTime(lm); err == nil {
			_ = os.Chtimes(tmpName, t, t)
		}
	}
	return os.Rename(tmpName, finalPath)
}

// cachePathFor maps a URL path to a local filesystem path under the
// cache root. Returns ok=false on inputs that would escape the root,
// reserve a marker filename, or otherwise be unsafe to write.
func (c *Cache) cachePathFor(urlPath string) (string, bool) {
	if urlPath == "" || urlPath == "/" {
		return "", false
	}
	if strings.Contains(urlPath, "..") {
		return "", false
	}
	clean := filepath.FromSlash(strings.TrimPrefix(urlPath, "/"))
	abs := filepath.Join(c.root, clean)
	if !strings.HasPrefix(abs, c.root+string(filepath.Separator)) && abs != c.root {
		return "", false
	}
	// Don't let URLs collide with internal markers.
	if filepath.Base(abs) == MarkerFile {
		return "", false
	}
	return abs, true
}

// maybeWriteMarker writes the .zddc-upstream provenance file once,
// the first time the cache stores anything. Best-effort: an error
// here doesn't fail the request.
func (c *Cache) maybeWriteMarker() {
	c.markerOnce.Do(func() {
		marker := filepath.Join(c.root, MarkerFile)
		if _, err := os.Stat(marker); err == nil {
			return
		}
		body := fmt.Sprintf("upstream: %s\nfirst_cached: %s\nmode: %s\n",
			c.upstream, time.Now().UTC().Format(time.RFC3339), c.mode)
		_ = os.WriteFile(marker, []byte(body), 0o644)
	})
}

// listingFormat collapses an Accept header to "json" or "html". Mirrors
// the master's content-negotiation: anything Accept-ing application/
// json wins JSON, otherwise HTML.
func listingFormat(accept string) string {
	if strings.Contains(strings.ToLower(accept), "application/json") {
		return "json"
	}
	return "html"
}

// listingCachePathFor returns the on-disk sidecar path for a cached
// directory listing. urlPath is the directory URL (always ending in
// "/"); accept is the request's Accept header.
func (c *Cache) listingCachePathFor(urlPath, accept string) (string, bool) {
	if !strings.HasSuffix(urlPath, "/") {
		return "", false
	}
	if strings.Contains(urlPath, "..") {
		return "", false
	}
	rel := filepath.FromSlash(strings.Trim(urlPath, "/"))
	dir := filepath.Join(c.root, rel)
	if !strings.HasPrefix(dir, c.root) {
		return "", false
	}
	return filepath.Join(dir, listingCachePrefix+listingFormat(accept)), true
}

// serveListingFromDisk serves a cached directory listing. Like
// serveFromDisk but always sets Content-Type from the format suffix
// (the master's headers may have been gzip-stripped or otherwise
// massaged by the time we cached them).
func (c *Cache) serveListingFromDisk(w http.ResponseWriter, r *http.Request, path string, info os.FileInfo, cacheState string) {
	f, err := os.Open(path)
	if err != nil {
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	defer f.Close()
	if strings.HasSuffix(path, ".json") {
		w.Header().Set("Content-Type", "application/json; charset=utf-8")
	} else {
		w.Header().Set("Content-Type", "text/html; charset=utf-8")
	}
	w.Header().Set(HeaderName, cacheState)
	http.ServeContent(w, r, filepath.Base(path), info.ModTime(), f)
}

// proxyDirectory is proxy() with directory-specific persistence: the
// listing sidecar (selected by Accept) is the cache target instead of
// the literal URL path. Falls back to serving stale on network error.
func (c *Cache) proxyDirectory(w http.ResponseWriter, r *http.Request, writeToCache bool) {
	upReq, err := c.buildUpstreamRequest(r)
	if err != nil {
		http.Error(w, "Bad Request: "+err.Error(), http.StatusBadRequest)
		return
	}
	resp, err := c.client.Do(upReq)
	if err != nil {
		if writeToCache && r.Method == http.MethodGet {
			if path, ok := c.listingCachePathFor(r.URL.Path, r.Header.Get("Accept")); ok {
				if info, sErr := os.Stat(path); sErr == nil && !info.IsDir() {
					c.serveListingFromDisk(w, r, path, info, "offline")
					return
				}
			}
		}
		slog.Warn("upstream directory fetch failed", "url", upReq.URL.String(), "err", err)
		w.Header().Set(HeaderName, "offline")
		http.Error(w, "Service Unavailable: upstream unreachable", http.StatusServiceUnavailable)
		return
	}
	defer resp.Body.Close()

	for k, vv := range resp.Header {
		if isHopByHop(k) {
			continue
		}
		for _, v := range vv {
			w.Header().Add(k, v)
		}
	}
	cacheable := writeToCache && resp.StatusCode == http.StatusOK && c.responseCacheable(resp)
	if cacheable {
		w.Header().Set(HeaderName, "miss")
	} else {
		w.Header().Set(HeaderName, "proxy")
	}
	w.WriteHeader(resp.StatusCode)
	if r.Method == http.MethodHead || resp.StatusCode == http.StatusNotModified {
		return
	}
	if !cacheable {
		_, _ = io.Copy(w, resp.Body)
		return
	}
	if err := c.streamAndPersistListing(w, resp, r.URL.Path, r.Header.Get("Accept")); err != nil {
		slog.Debug("listing stream-and-persist error", "url", r.URL.Path, "err", err)
	} else {
		c.maybeWriteMarker()
	}
}

// streamAndPersistListing is streamAndPersist for directory listings,
// resolving the cache target via listingCachePathFor (Accept-aware).
func (c *Cache) streamAndPersistListing(w http.ResponseWriter, resp *http.Response, urlPath, accept string) error {
	finalPath, ok := c.listingCachePathFor(urlPath, accept)
	if !ok {
		_, err := io.Copy(w, resp.Body)
		return err
	}
	if err := os.MkdirAll(filepath.Dir(finalPath), 0o755); err != nil {
		_, copyErr := io.Copy(w, resp.Body)
		if copyErr != nil {
			return copyErr
		}
		return err
	}
	tmp, err := os.CreateTemp(filepath.Dir(finalPath), ".zddc-cache-tmp-*")
	if err != nil {
		_, copyErr := io.Copy(w, resp.Body)
		if copyErr != nil {
			return copyErr
		}
		return err
	}
	tmpName := tmp.Name()
	mw := io.MultiWriter(tmp, w)
	if _, err := io.Copy(mw, resp.Body); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return err
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return err
	}
	if lm := resp.Header.Get("Last-Modified"); lm != "" {
		if t, err := http.ParseTime(lm); err == nil {
			_ = os.Chtimes(tmpName, t, t)
		}
	}
	return os.Rename(tmpName, finalPath)
}

// revalidateListing is the listing-cache analogue of revalidate.
func (c *Cache) revalidateListing(urlPath, accept string, mtime time.Time) {
	target := c.upstream + urlPath
	req, err := http.NewRequest(http.MethodGet, target, nil)
	if err != nil {
		return
	}
	if !mtime.IsZero() {
		req.Header.Set("If-Modified-Since", mtime.UTC().Format(http.TimeFormat))
	}
	if accept != "" {
		req.Header.Set("Accept", accept)
	}
	if c.bearer != "" {
		req.Header.Set("Authorization", "Bearer "+c.bearer)
	}
	resp, err := c.client.Do(req)
	if err != nil {
		return
	}
	defer resp.Body.Close()
	switch resp.StatusCode {
	case http.StatusNotModified:
		return
	case http.StatusOK:
		if !c.responseCacheable(resp) {
			return
		}
		path, ok := c.listingCachePathFor(urlPath, accept)
		if !ok {
			return
		}
		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
			return
		}
		tmp, err := os.CreateTemp(filepath.Dir(path), ".zddc-cache-tmp-*")
		if err != nil {
			return
		}
		tmpName := tmp.Name()
		if _, err := io.Copy(tmp, resp.Body); err != nil {
			_ = tmp.Close()
			_ = os.Remove(tmpName)
			return
		}
		_ = 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)
	case http.StatusForbidden, http.StatusNotFound:
		if path, ok := c.listingCachePathFor(urlPath, accept); ok {
			_ = os.Remove(path)
		}
	}
}

// isHopByHop reports whether a header name is hop-by-hop per RFC 7230
// §6.1 — these must not be forwarded by a proxy.
func isHopByHop(name string) bool {
	switch http.CanonicalHeaderKey(name) {
	case "Connection",
		"Keep-Alive",
		"Proxy-Authenticate",
		"Proxy-Authorization",
		"Te",
		"Trailer",
		"Transfer-Encoding",
		"Upgrade":
		return true
	}
	return false
}