x: add Redis backend for cache

Adds a RedisCache config block and redisCache implementation for the
generic cache.Cache interface, enabling distributed HTTP response
caching across gost instances.

go-redis/redis/v8 was already a dependency (used by recorder and
internal loader) — no new imports needed.

cache/redis.go

@@ -0,0 +1,119 @@
+package cache
+
+import (
+	"context"
+	"time"
+
+	"github.com/go-gost/core/cache"
+	"github.com/go-gost/core/logger"
+	"github.com/go-redis/redis/v8"
+)
+
+// defaultKeyPrefix is prepended to cache keys when RedisOptions.KeyPrefix is empty.
+const defaultKeyPrefix = "gost:cache:"
+
+// RedisOptions configures a Redis cache.
+type RedisOptions struct {
+	// Addr is the Redis server address (host:port).
+	Addr string
+	// DB is the Redis database number.
+	DB int
+	// Username for Redis authentication. Optional.
+	Username string
+	// Password for Redis authentication. Optional.
+	Password string
+	// DefaultTTL is applied to entries stored without an explicit TTL.
+	// A non-positive value means entries never expire by default.
+	DefaultTTL time.Duration
+	// KeyPrefix is prepended to all cache keys. Defaults to "gost:cache:".
+	KeyPrefix string
+	// Logger is used for debug logging. Optional.
+	Logger logger.Logger
+}
+
+// redisCache is a cache.Cache backed by a Redis server.
+//
+// Redis expires keys natively, so an expired entry is a true miss here — the
+// interface's serve-stale contract (returning expired entries from Get) does
+// not apply to this backend.
+type redisCache struct {
+	client     *redis.Client
+	defaultTTL time.Duration
+	keyPrefix  string
+	logger     logger.Logger
+}
+
+// NewRedisCache creates a Redis-backed cache. Close releases the underlying
+// client connections.
+func NewRedisCache(opts RedisOptions) cache.Cache {
+	keyPrefix := opts.KeyPrefix
+	if keyPrefix == "" {
+		keyPrefix = defaultKeyPrefix
+	}
+
+	return &redisCache{
+		client: redis.NewClient(&redis.Options{
+			Addr:     opts.Addr,
+			Username: opts.Username,
+			Password: opts.Password,
+			DB:       opts.DB,
+		}),
+		defaultTTL: opts.DefaultTTL,
+		keyPrefix:  keyPrefix,
+		logger:     opts.Logger,
+	}
+}
+
+// Get returns the entry for key, or cache.ErrNotFound if the key is absent
+// (including keys Redis has already expired).
+func (c *redisCache) Get(ctx context.Context, key string) (*cache.Entry, error) {
+	k := c.keyPrefix + key
+
+	pipe := c.client.Pipeline()
+	getCmd := pipe.Get(ctx, k)
+	ttlCmd := pipe.PTTL(ctx, k)
+	if _, err := pipe.Exec(ctx); err != nil {
+		if err == redis.Nil {
+			return nil, cache.ErrNotFound
+		}
+		return nil, err
+	}
+
+	entry := &cache.Entry{
+		Data: []byte(getCmd.Val()),
+	}
+	if ttl := ttlCmd.Val(); ttl > 0 {
+		entry.Expiration = time.Now().Add(ttl)
+	}
+	return entry, nil
+}
+
+// Set stores data under key. A non-positive TTL (after applying the default)
+// stores the entry without expiration.
+func (c *redisCache) Set(ctx context.Context, key string, data []byte, opts ...cache.SetOption) error {
+	var options cache.SetOptions
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	ttl := options.TTL
+	if ttl <= 0 {
+		ttl = c.defaultTTL
+	}
+	if ttl < 0 {
+		ttl = 0 // redis: 0 = no expiration
+	}
+
+	return c.client.Set(ctx, c.keyPrefix+key, data, ttl).Err()
+}
+
+// Delete removes key from the cache. Deleting an absent key is a no-op.
+func (c *redisCache) Delete(ctx context.Context, key string) error {
+	return c.client.Del(ctx, c.keyPrefix+key).Err()
+}
+
+// Close releases the Redis client. Implements io.Closer so the registry
+// closes the client on hot reload.
+func (c *redisCache) Close() error {
+	return c.client.Close()
+}