fix(jwk): keep parsed JWKS in local cache only (#134) (#136)

Under yaegi (Traefik's plugin runtime) json.Marshal exposes unexported
struct fields with an X-prefixed name. parsedJWKS{ keys map[string]
crypto.PublicKey } therefore round-tripped through Redis as
{"Xkeys":{"<kid>":{"N":<huge>,"E":65537}}} — *rsa.PublicKey.N is a
*big.Int that marshals to a JSON number hundreds of digits long. On
read, json.Unmarshal into interface{} parses numbers as float64, which
cannot represent that range:

  Failed to deserialize value for key .../discovery/v2.0/keys:parsed:
  json: cannot unmarshal number 2251513...
    into Go value of type float64

Auth still worked (the JWKCache rebuilt the keys in memory on every
miss) but the error log spammed every request.

Two structural problems were behind it:

* parsedJWKS holds crypto.PublicKey interface values that aren't
  meaningfully JSON-serializable. Even on compiled Go (where the
  unexported field marshals to {}), the post-roundtrip type assertion
  v.(*parsedJWKS) silently failed and the cache was useless.
* The same pattern applied to *JWKSet — the struct shape survived JSON
  but the type assertion still failed, defeating the cache for every
  call that went through Redis.

Both keys now use the new UniversalCache.SetLocal/GetLocal pair, which
skips the configured distributed backend entirely. JWK rotation is rare
and a per-replica HTTP fetch on cold cache is cheap, so cross-replica
coherence buys nothing for these entries.

Stale Redis entries written by previous versions are simply ignored —
the new code never reads under those keys, and Redis TTL retires them.

Includes regression coverage for the Azure round-trip, the
poisoned-stale-data scenario, and the SetLocal/GetLocal isolation
contract.

patch-release

jwk.go

@@ -76,9 +76,15 @@ func NewJWKCache() *JWKCache {
 }
 
 // GetJWKS retrieves JWKS from cache or fetches from the remote URL if not cached.
+//
+// The entry is stored locally only via SetLocal/GetLocal. Going through a
+// distributed backend defeats the cache: JSON round-tripping turns *JWKSet
+// into map[string]interface{}, the type assertion below fails, and every
+// request refetches from the upstream. JWK rotation is rare and a per-replica
+// HTTP fetch on cold cache is cheap, so cross-replica coherence buys nothing.
 func (c *JWKCache) GetJWKS(ctx context.Context, jwksURL string, httpClient *http.Client) (*JWKSet, error) {
 	// Check cache first
-	if cachedValue, found := c.cache.Get(jwksURL); found {
+	if cachedValue, found := c.cache.GetLocal(jwksURL); found {
 		if jwks, ok := cachedValue.(*JWKSet); ok {
 			return jwks, nil
 		}
@@ -88,7 +94,7 @@ func (c *JWKCache) GetJWKS(ctx context.Context, jwksURL string, httpClient *http
 	defer c.mutex.Unlock()
 
 	// Double-check after acquiring lock
-	if cachedValue, found := c.cache.Get(jwksURL); found {
+	if cachedValue, found := c.cache.GetLocal(jwksURL); found {
 		if jwks, ok := cachedValue.(*JWKSet); ok {
 			return jwks, nil
 		}
@@ -105,7 +111,7 @@ func (c *JWKCache) GetJWKS(ctx context.Context, jwksURL string, httpClient *http
 	}
 
 	// Cache for 1 hour
-	_ = c.cache.Set(jwksURL, jwks, 1*time.Hour) // Safe to ignore: cache failures are non-critical
+	_ = c.cache.SetLocal(jwksURL, jwks, 1*time.Hour) // Safe to ignore: cache failures are non-critical
 
 	return jwks, nil
 }
@@ -114,9 +120,17 @@ func (c *JWKCache) GetJWKS(ctx context.Context, jwksURL string, httpClient *http
 // caching the JWKS plus its derived parsedJWKS on miss. The parsed entry is
 // stored alongside the raw JWKSet under a sibling cache key with the same
 // 1-hour TTL, so both invalidate together when the upstream JWKS rotates.
+//
+// parsedJWKS is stored locally only (SetLocal/GetLocal). Its values are
+// crypto.PublicKey interfaces wrapping *rsa.PublicKey/*ecdsa.PublicKey,
+// which contain *big.Int that marshals to a hundreds-digit JSON number.
+// On a distributed backend round-trip, json.Unmarshal into interface{} would
+// try to fit that into float64 and fail with UnmarshalTypeError. Under yaegi
+// the unexported parsedJWKS.keys field is exposed via an X-prefixed name on
+// Marshal, leaking the modulus into the cached payload (issue #134).
 func (c *JWKCache) GetPublicKey(ctx context.Context, jwksURL, kid string, httpClient *http.Client) (crypto.PublicKey, error) {
 	parsedKey := jwksURL + parsedKeysSuffix
-	if v, found := c.cache.Get(parsedKey); found {
+	if v, found := c.cache.GetLocal(parsedKey); found {
 		if pj, ok := v.(*parsedJWKS); ok {
 			if k, ok := pj.keys[kid]; ok {
 				return k, nil
@@ -130,7 +144,7 @@ func (c *JWKCache) GetPublicKey(ctx context.Context, jwksURL, kid string, httpCl
 	}
 
 	pj := buildParsedJWKS(jwks)
-	_ = c.cache.Set(parsedKey, pj, 1*time.Hour) // Safe to ignore: cache failures are non-critical
+	_ = c.cache.SetLocal(parsedKey, pj, 1*time.Hour) // Safe to ignore: cache failures are non-critical
 
 	if k, ok := pj.keys[kid]; ok {
 		return k, nil