gofiber/fiber · error

failed to decode session data

Error message

failed to decode session data: %w

What it means

Thrown by decodeSessionData when gob.Decoder.Decode cannot turn the raw bytes from storage back into s.data.Data. The %w carries the gob decoder's complaint (e.g. EOF, unexpected EOF, gob: type mismatch). This indicates the persisted payload is not valid gob, is truncated, or was written with an incompatible/unregistered type set.

Solutions

1. Treat the decode failure as 'session invalid' and destroy/reset the session so the user gets a fresh one.
2. Verify the Storage backend is not returning truncated or empty payloads (check raw byte length).
3. Keep gob.Register calls identical and additive across versions; never remove a registered type while old sessions may still reference it.
4. Clear the affected session ID from storage to stop the recurrence.

Example fix

// before
sess, err := store.Get(c)
if err != nil { return err } // propagates decode error, breaks user

// after
sess, err := store.Get(c)
if err != nil {
    log.Printf("session decode failed, resetting: %v", err)
    sess.Destroy()
    sess, err = store.Get(c) // fresh session
    if err != nil { return err }
}

// Before trusting stored data, probe-decode it in a scratch buffer.
func probeDecode(storage Storage, id string) error {
    raw, err := storage.Get(context.Background(), id)
    if err != nil || raw == nil { return err }
    var d Data
    return gob.NewDecoder(bytes.NewReader(raw)).Decode(&d)
}

sess, err := store.Get(c)
if err != nil {
    // corrupt/incompatible payload — discard and start fresh
    log.Printf("session decode failed, resetting: %v", err)
    sess.Destroy()
    sess, err = store.Get(c)
}
if err != nil { return err }

When it happens

Trigger: The Storage backend returns bytes that are not a valid gob stream, are truncated (e.g. a value-length prefix cut off), or were produced by a build whose registered types differ from the current one; manually tampered session data; switching storage backends or key encodings.

Common situations: Deploying a new version that changed a stored struct without keeping gob.Register consistent; storage corruption (Redis eviction/overwrite, DB truncation); sharing one session store between two apps with different schemas; partial writes during a crash.

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/2aa537ca5af1c6e6. Report an issue: GitHub.

Appendix: source

Thrown at middleware/session/session.go:588

// decodeSessionData decodes session data from raw bytes
//
// Parameters:
//   - rawData: The raw byte data to decode.
//
// Returns:
//   - error: An error if the decoding fails.
//
// Usage:
//
//	err := s.decodeSessionData(rawData)
func (s *Session) decodeSessionData(rawData []byte) error {
	byteBuffer := byteBufferPool.Get().(*bytes.Buffer) //nolint:forcetypeassert,errcheck // We store nothing else in the pool
	defer byteBufferPool.Put(byteBuffer)
	defer byteBuffer.Reset()
	_, _ = byteBuffer.Write(rawData)
	decCache := gob.NewDecoder(byteBuffer)
	if err := decCache.Decode(&s.data.Data); err != nil {
		return fmt.Errorf("failed to decode session data: %w", err)
	}
	return nil
}

// encodeSessionData encodes session data to raw bytes
//
// Parameters:
//   - rawData: The raw byte data to encode.
//
// Returns:
//   - error: An error if the encoding fails.
//
// Usage:
//
//	err := s.encodeSessionData(rawData)
func (s *Session) encodeSessionData() ([]byte, error) {
	byteBuffer := byteBufferPool.Get().(*bytes.Buffer) //nolint:forcetypeassert,errcheck // We store nothing else in the pool
	defer byteBufferPool.Put(byteBuffer)

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