feat: downloader with CDN redirect support and retry handling

Author: borzovplus

telegram/cdn.go

@@ -1,31 +1,291 @@
 package telegram
 
 import (
+	"context"
 	"crypto/rsa"
 	"encoding/pem"
 
 	"github.com/go-faster/errors"
 
 	"github.com/gotd/td/crypto"
+	"github.com/gotd/td/exchange"
 	"github.com/gotd/td/tg"
 )
 
-func parseCDNKeys(keys ...tg.CDNPublicKey) ([]*rsa.PublicKey, error) {
-	r := make([]*rsa.PublicKey, 0, len(keys))
+// Single key because help.getCdnConfig has no request params.
+const helpGetCDNConfigSingleflightKey = "help.getCdnConfig"
+
+type cdnKeyEntry struct {
+	dcID int
+	key  *rsa.PublicKey
+}
+
+type fetchedCDNKeys struct {
+	all  []exchange.PublicKey
+	byDC map[int][]exchange.PublicKey
+}
+
+func clonePublicKeys(keys []exchange.PublicKey) []exchange.PublicKey {
+	return append([]exchange.PublicKey(nil), keys...)
+}
+
+func mergePublicKeys(primary, fallback []exchange.PublicKey) []exchange.PublicKey {
+	if len(primary) == 0 && len(fallback) == 0 {
+		return nil
+	}
+
+	out := make([]exchange.PublicKey, 0, len(primary)+len(fallback))
+	seen := make(map[int64]struct{}, len(primary)+len(fallback))
+	appendUnique := func(keys []exchange.PublicKey) {
+		for _, key := range keys {
+			fp := key.Fingerprint()
+			if _, ok := seen[fp]; ok {
+				continue
+			}
+			seen[fp] = struct{}{}
+			out = append(out, key)
+		}
+	}
+
+	// Prefer primary keyset order and use fallback only for missing fingerprints.
+	appendUnique(primary)
+	appendUnique(fallback)
+	return out
+}
+
+func parseCDNKeyEntries(keys ...tg.CDNPublicKey) ([]cdnKeyEntry, error) {
+	r := make([]cdnKeyEntry, 0, len(keys))
 
 	for _, key := range keys {
 		block, _ := pem.Decode([]byte(key.PublicKey))
 		if block == nil {
 			continue
 		}
 
-		key, err := crypto.ParseRSA(block.Bytes)
+		parsedKey, err := crypto.ParseRSA(block.Bytes)
 		if err != nil {
 			return nil, errors.Wrap(err, "parse RSA from PEM")
 		}
 
-		r = append(r, key)
+		r = append(r, cdnKeyEntry{
+			dcID: key.DCID,
+			key:  parsedKey,
+		})
 	}
 
 	return r, nil
 }
+
+func buildCDNKeysCache(entries []cdnKeyEntry) fetchedCDNKeys {
+	result := fetchedCDNKeys{
+		all:  make([]exchange.PublicKey, 0, len(entries)),
+		byDC: make(map[int][]exchange.PublicKey),
+	}
+
+	seenAll := make(map[int64]struct{}, len(entries))
+	seenByDC := make(map[int]map[int64]struct{})
+
+	for _, entry := range entries {
+		key := exchange.PublicKey{RSA: entry.key}
+		fingerprint := key.Fingerprint()
+
+		if _, ok := seenAll[fingerprint]; !ok {
+			seenAll[fingerprint] = struct{}{}
+			result.all = append(result.all, key)
+		}
+
+		seen, ok := seenByDC[entry.dcID]
+		if !ok {
+			seen = map[int64]struct{}{}
+			seenByDC[entry.dcID] = seen
+		}
+		if _, ok := seen[fingerprint]; ok {
+			continue
+		}
+		seen[fingerprint] = struct{}{}
+		result.byDC[entry.dcID] = append(result.byDC[entry.dcID], key)
+	}
+
+	return result
+}
+
+func copyCDNKeysByDC(byDC map[int][]exchange.PublicKey) map[int][]exchange.PublicKey {
+	if len(byDC) == 0 {
+		return nil
+	}
+
+	r := make(map[int][]exchange.PublicKey, len(byDC))
+	for dcID, keys := range byDC {
+		r[dcID] = append([]exchange.PublicKey(nil), keys...)
+	}
+	return r
+}
+
+func cloneFetchedCDNKeys(keys fetchedCDNKeys) fetchedCDNKeys {
+	return fetchedCDNKeys{
+		all:  clonePublicKeys(keys.all),
+		byDC: copyCDNKeysByDC(keys.byDC),
+	}
+}
+
+func (c *Client) cachedCDNKeys() ([]exchange.PublicKey, bool, uint64) {
+	c.cdnKeysMux.Lock()
+	defer c.cdnKeysMux.Unlock()
+
+	return clonePublicKeys(c.cdnKeys), c.cdnKeysSet, c.cdnKeysGen
+}
+
+func (c *Client) cachedCDNKeysForDC(dcID int) ([]exchange.PublicKey, bool) {
+	c.cdnKeysMux.Lock()
+	defer c.cdnKeysMux.Unlock()
+
+	return clonePublicKeys(c.cdnKeysByDC[dcID]), c.cdnKeysSet
+}
+
+func (c *Client) cdnConfigFetchContext(caller context.Context) context.Context {
+	if c.ctx != nil {
+		// Bind network request lifetime to client lifecycle, not to the first
+		// singleflight caller.
+		return c.ctx
+	}
+
+	// Caller cancellation is handled outside singleflight wait loop; request
+	// itself should not inherit first caller deadline/cancellation.
+	return context.WithoutCancel(caller)
+}
+
+func (c *Client) loadCDNKeys(ctx context.Context) (fetchedCDNKeys, error) {
+	resultCh := c.cdnKeysLoad.DoChan(helpGetCDNConfigSingleflightKey, func() (interface{}, error) {
+		// singleflight ensures only one goroutine issues help.getCdnConfig;
+		// others wait and reuse same result.
+		cfg, err := c.tg.HelpGetCDNConfig(c.cdnConfigFetchContext(ctx))
+		if err != nil {
+			return nil, errors.Wrap(err, "help.getCdnConfig")
+		}
+
+		entries, err := parseCDNKeyEntries(cfg.PublicKeys...)
+		if err != nil {
+			return nil, errors.Wrap(err, "parse CDN public keys")
+		}
+		return buildCDNKeysCache(entries), nil
+	})
+
+	select {
+	case <-ctx.Done():
+		return fetchedCDNKeys{}, ctx.Err()
+	case result := <-resultCh:
+		if result.Err != nil {
+			return fetchedCDNKeys{}, result.Err
+		}
+
+		keys, ok := result.Val.(fetchedCDNKeys)
+		if !ok {
+			return fetchedCDNKeys{}, errors.Errorf("unexpected CDN keys type %T", result.Val)
+		}
+		return cloneFetchedCDNKeys(keys), nil
+	}
+}
+
+func (c *Client) fetchCDNKeys(ctx context.Context) ([]exchange.PublicKey, error) {
+	const maxVersionRetries = 3
+	for attempt := 0; attempt < maxVersionRetries; attempt++ {
+		// Fast path: fully cached, no network requests.
+		cached, set, startGen := c.cachedCDNKeys()
+		if set {
+			return cached, nil
+		}
+		// Snapshot generation to detect invalidation races after in-flight load.
+
+		keys, err := c.loadCDNKeys(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		c.cdnKeysMux.Lock()
+		switch {
+		case c.cdnKeysSet:
+			// Another goroutine already populated cache while we were waiting.
+			cached := clonePublicKeys(c.cdnKeys)
+			c.cdnKeysMux.Unlock()
+			return cached, nil
+		case c.cdnKeysGen != startGen:
+			// Cache was invalidated (fingerprint miss) during in-flight request.
+			// Discard stale result and retry from fresh generation.
+			c.cdnKeysMux.Unlock()
+			continue
+		default:
+			// Safe to commit fetched keys into cache.
+			c.cdnKeys = clonePublicKeys(keys.all)
+			c.cdnKeysByDC = copyCDNKeysByDC(keys.byDC)
+			c.cdnKeysSet = true
+			cached := clonePublicKeys(c.cdnKeys)
+			c.cdnKeysMux.Unlock()
+			return cached, nil
+		}
+	}
+
+	return nil, errors.New("cdn keys cache changed concurrently")
+}
+
+func (c *Client) refreshCDNKeys(ctx context.Context) ([]exchange.PublicKey, error) {
+	const maxVersionRetries = 3
+	for attempt := 0; attempt < maxVersionRetries; attempt++ {
+		c.cdnKeysMux.Lock()
+		startGen := c.cdnKeysGen
+		c.cdnKeysMux.Unlock()
+
+		keys, err := c.loadCDNKeys(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		c.cdnKeysMux.Lock()
+		if c.cdnKeysGen != startGen {
+			// Fingerprint invalidation happened while refresh was in-flight.
+			// Discard stale result and refetch for fresh generation.
+			c.cdnKeysMux.Unlock()
+			continue
+		}
+		c.cdnKeys = clonePublicKeys(keys.all)
+		c.cdnKeysByDC = copyCDNKeysByDC(keys.byDC)
+		c.cdnKeysSet = true
+		cached := clonePublicKeys(c.cdnKeys)
+		c.cdnKeysMux.Unlock()
+
+		return cached, nil
+	}
+
+	return nil, errors.New("cdn keys cache changed concurrently")
+}
+
+func (c *Client) fetchCDNKeysForDC(ctx context.Context, dcID int) ([]exchange.PublicKey, error) {
+	keys, set := c.cachedCDNKeysForDC(dcID)
+	if !set {
+		if _, err := c.fetchCDNKeys(ctx); err != nil {
+			return nil, err
+		}
+	}
+
+	const maxRefreshAttempts = 3
+	for attempt := 0; attempt < maxRefreshAttempts; attempt++ {
+		if err := ctx.Err(); err != nil {
+			return nil, err
+		}
+
+		keys, _ = c.cachedCDNKeysForDC(dcID)
+		if len(keys) > 0 {
+			return keys, nil
+		}
+		if attempt == maxRefreshAttempts-1 {
+			break
+		}
+
+		// Requested CDN DC is missing in current snapshot; retry bounded
+		// help.getCdnConfig refreshes to handle eventual config propagation.
+		if _, err := c.refreshCDNKeys(ctx); err != nil {
+			return nil, err
+		}
+	}
+
+	return nil, errors.Errorf("no CDN public keys for CDN DC %d after %d refresh attempts", dcID, maxRefreshAttempts)
+}

telegram/cdn_conn_dead.go

@@ -0,0 +1,60 @@
+package telegram
+
+import (
+	"github.com/go-faster/errors"
+	"go.uber.org/zap"
+
+	"github.com/gotd/td/exchange"
+	"github.com/gotd/td/mtproto"
+	"github.com/gotd/td/pool"
+)
+
+func (c *Client) handleCDNConnDead(dcID int, err error) {
+	if errors.Is(err, exchange.ErrKeyFingerprintNotFound) {
+		c.log.Warn("Resetting cached CDN keys after fingerprint miss",
+			zap.Int("dc_id", dcID),
+		)
+		c.cdnKeysMux.Lock()
+		c.cdnKeys = nil
+		c.cdnKeysByDC = nil
+		c.cdnKeysSet = false
+		// Bump generation so in-flight help.getCdnConfig results are discarded if
+		// they were started before invalidation.
+		c.cdnKeysGen++
+		c.cdnKeysMux.Unlock()
+		// Drop current singleflight entry so next attempt refetches keys.
+		c.cdnKeysLoad.Forget(helpGetCDNConfigSingleflightKey)
+
+		// Close asynchronously: callback may be invoked from pool worker
+		// goroutine, and synchronous self-close can deadlock on Wait().
+		// Queue closes through bounded workers to avoid unbounded goroutine fan-out.
+		c.cdnPools.invalidateDC(dcID)
+		// Fingerprint miss is recoverable and handled internally by invalidation
+		// + reconnect with fresh keys, no external onDead signal is needed.
+		return
+	}
+
+	if !errors.Is(err, mtproto.ErrPFSDropKeysRequired) {
+		// Keep legacy callback semantics for all non-PFS errors.
+		if c.onDead != nil {
+			c.onDead(err)
+		}
+		return
+	}
+
+	c.log.Warn("Dropping stored CDN session key after PFS key reset request",
+		zap.Int("dc_id", dcID),
+	)
+	c.sessionsMux.Lock()
+	s, ok := c.cdnSessions[dcID]
+	if !ok {
+		s = pool.NewSyncSession(pool.Session{DC: dcID})
+		c.cdnSessions[dcID] = s
+	}
+	s.Store(pool.Session{DC: dcID})
+	c.sessionsMux.Unlock()
+
+	if c.onDead != nil {
+		c.onDead(err)
+	}
+}