gofiber/fiber · error

failed to decode session data

Error message

failed to decode session data: %w

What it means

Thrown during session acquisition (store Get/Acquire) when the session ID already has raw data in Storage but decoding that data via decodeSessionData fails. It is the acquisition-path equivalent of 201: the persisted payload for an existing session is corrupt or type-incompatible.

Solutions

1. Handle the error by destroying the session and issuing a fresh one (do not keep retrying the same corrupt payload).
2. Verify the Storage backend integrity and that it returns full payloads.
3. Keep gob.Register consistent/additive across deployments.

Example fix

// before
sess, err := store.Get(c)
if err != nil { return err } // user locked out by corrupt session

// after
sess, err := store.Get(c)
if err != nil {
    c.Cookie(&fiber.Cookie{Name: store.SessionName, Expires: time.Unix(0,0)})
    sess, err = store.Get(c)
    if err != nil { return err }
}

// Validate a stored session before handing it to users.
func sessionPayloadValid(raw []byte) bool {
    if raw == nil { return true }
    var d Data
    return gob.NewDecoder(bytes.NewReader(raw)).Decode(&d) == nil
}

sess, err := store.Get(c)
if err != nil {
    log.Printf("acquire decode failed, resetting: %v", err)
    store.Storage.Delete(context.Background(), id)
    sess, err = store.Get(c)
}

When it happens

Trigger: Storage.Get returns non-nil bytes for the session ID, but those bytes fail gob-decoding — truncation, corruption, or a type set mismatch between the writer and reader builds.

Common situations: Deploying a version that changed a stored struct without keeping gob.Register additive; storage corruption; one shared session store used by two apps with different schemas; migrating session backends.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to decode session data
• failed to encode data
• failed to encode session data
• cache: failed to marshal key
• cache: failed to unmarshal key

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/d3e0efc3a1f05f3c. Report an issue: GitHub.

Appendix: source

Thrown at middleware/session/store.go:179

	sess := acquireSession()

	sess.mu.Lock()

	sess.ctx = c
	sess.config = s
	sess.id = id
	sess.isFresh = isFresh
	sess.extractor = selectedExtractor

	// Decode session data if found
	if rawData != nil {
		sess.data.Lock()
		err := sess.decodeSessionData(rawData)
		sess.data.Unlock()
		if err != nil {
			sess.mu.Unlock()
			sess.Release()
			return nil, fmt.Errorf("failed to decode session data: %w", err)
		}
	}

	sess.mu.Unlock()

	if isFresh && s.AbsoluteTimeout > 0 {
		sess.setAbsExpiration(time.Now().Add(s.AbsoluteTimeout))
	} else if sess.isAbsExpired() {
		if err := sess.Reset(); err != nil {
			return nil, fmt.Errorf("failed to reset session: %w", err)
		}
		sess.setAbsExpiration(time.Now().Add(s.AbsoluteTimeout))
	}

	return sess, nil
}

// getSessionID returns the session ID using the configured extractor.

View on GitHub (pinned to a105acad6c)