gofiber/fiber · critical · ErrTypeAssertionFailed

failed to type-assert to *Middleware

Error message

failed to type-assert to *Middleware

What it means

Declared as ErrTypeAssertionFailed and used only inside acquireMiddleware, which does middlewarePool.Get().(*Middleware) and panics with this message if the assertion fails. The pool's New func always returns *Middleware, so under normal use this cannot happen — it is a defensive panic guarding against external corruption of the package's internal sync.Pool. Hitting it means the pool was tampered with or memory was corrupted.

Source

Thrown at middleware/session/middleware.go:36

type Middleware struct {
	Session     *Session
	ctx         fiber.Ctx
	config      Config
	mu          sync.RWMutex
	isDestroyed bool
}

// Context key for session middleware lookup.
type middlewareKey int

const (
	// middlewareContextKey is the key used to store the *Middleware in the context locals.
	middlewareContextKey middlewareKey = iota
)

var (
	// ErrTypeAssertionFailed occurs when a type assertion fails.
	ErrTypeAssertionFailed = errors.New("failed to type-assert to *Middleware")

	// Pool for reusing middleware instances.
	middlewarePool = &sync.Pool{
		New: func() any {
			return &Middleware{}
		},
	}
)

// New initializes session middleware with optional configuration.
//
// Parameters:
//   - config: Variadic parameter to override default config.
//
// Returns:
//   - fiber.Handler: The Fiber handler for the session middleware.
//
// Usage:

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Stop mutating or sharing the session package's internal pool; only use session.New/NewWithStore.
2. Reproduce under `go test -race` to find any data race corrupting pool state.
3. Ensure exactly one gofiber/fiber/v3 module version is in the build (`go mod graph`) — mixed versions can alias types unexpectedly.
4. If using a fork, diff middleware/session/middleware.go against upstream to confirm the pool New func is intact.

// There is no public pre-check; the assertion guards an internal sync.Pool.
// Validate your environment instead: ensure a single Fiber module version and
// no third-party mutation of session package internals.
func assertSingleFiberVersion() error {
    out, err := exec.Command("go", "list", "-m", "github.com/gofiber/fiber/v3").Output()
    if err != nil { return err }
    fmt.Println("fiber version:", strings.TrimSpace(string(out)))
    return nil
}

// FromContext already narrows safely — use it instead of digging in the pool.
func mustSession(c fiber.Ctx) *session.Middleware {
    if m := session.FromContext(c); m != nil {
        return m
    }
    return nil // no active session middleware for this request
}

// This error is a panic, not a returned error. Wrap risky handlers with recover
// middleware so a stray panic doesn't kill the process.
app.Use(recoverware.New())
app.Use(session.New())

When it happens

Trigger: Effectively unreachable through the public API. Would require something to put a non-*Middleware value into the unexported middlewarePool, or a memory-corruption/data-race bug. Surfaces as a server panic (not a returned error) at the start of every request through session.New().

Common situations: Reports of this in practice trace to: a fork that mutated pool internals, an unsafe third-party package corrupting the pool, or a sync.Pool race exposed under -race. Not something end users hit with stock Fiber.

Related errors

• [CORS] Invalid origin format after normalization:${maskedOri
• [session] AbsoluteTimeout must be greater than or equal to I
• runtime.Goexit() called in handler or server panic
• session ID cannot be empty
• session ID not found in session store

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/02e2c9910583f304.json. Report an issue: GitHub.