{"id":"02e2c9910583f304","repo":"gofiber/fiber","slug":"failed-to-type-assert-to-middleware","errorCode":null,"errorMessage":"failed to type-assert to *Middleware","messagePattern":"failed to type-assert to \\*Middleware","errorType":"exception","errorClass":"ErrTypeAssertionFailed","httpStatus":null,"severity":"critical","filePath":"middleware/session/middleware.go","lineNumber":36,"sourceCode":"type Middleware struct {\n\tSession     *Session\n\tctx         fiber.Ctx\n\tconfig      Config\n\tmu          sync.RWMutex\n\tisDestroyed bool\n}\n\n// Context key for session middleware lookup.\ntype middlewareKey int\n\nconst (\n\t// middlewareContextKey is the key used to store the *Middleware in the context locals.\n\tmiddlewareContextKey middlewareKey = iota\n)\n\nvar (\n\t// ErrTypeAssertionFailed occurs when a type assertion fails.\n\tErrTypeAssertionFailed = errors.New(\"failed to type-assert to *Middleware\")\n\n\t// Pool for reusing middleware instances.\n\tmiddlewarePool = &sync.Pool{\n\t\tNew: func() any {\n\t\t\treturn &Middleware{}\n\t\t},\n\t}\n)\n\n// New initializes session middleware with optional configuration.\n//\n// Parameters:\n//   - config: Variadic parameter to override default config.\n//\n// Returns:\n//   - fiber.Handler: The Fiber handler for the session middleware.\n//\n// Usage:","sourceCodeStart":18,"sourceCodeEnd":54,"githubUrl":"https://github.com/gofiber/fiber/blob/9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c/middleware/session/middleware.go#L18-L54","documentation":"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.","triggerScenarios":"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().","commonSituations":"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.","solutions":["Stop mutating or sharing the session package's internal pool; only use session.New/NewWithStore.","Reproduce under `go test -race` to find any data race corrupting pool state.","Ensure exactly one gofiber/fiber/v3 module version is in the build (`go mod graph`) — mixed versions can alias types unexpectedly.","If using a fork, diff middleware/session/middleware.go against upstream to confirm the pool New func is intact."],"exampleFix":null,"handlingStrategy":"validation","validationCode":"// There is no public pre-check; the assertion guards an internal sync.Pool.\n// Validate your environment instead: ensure a single Fiber module version and\n// no third-party mutation of session package internals.\nfunc assertSingleFiberVersion() error {\n    out, err := exec.Command(\"go\", \"list\", \"-m\", \"github.com/gofiber/fiber/v3\").Output()\n    if err != nil { return err }\n    fmt.Println(\"fiber version:\", strings.TrimSpace(string(out)))\n    return nil\n}","typeGuard":"// FromContext already narrows safely — use it instead of digging in the pool.\nfunc mustSession(c fiber.Ctx) *session.Middleware {\n    if m := session.FromContext(c); m != nil {\n        return m\n    }\n    return nil // no active session middleware for this request\n}","tryCatchPattern":"// This error is a panic, not a returned error. Wrap risky handlers with recover\n// middleware so a stray panic doesn't kill the process.\napp.Use(recoverware.New())\napp.Use(session.New())","preventionTips":["Run tests with `go test -race` to catch data races that could corrupt the pool.","Keep a single gofiber/fiber/v3 version in go.mod (run `go mod tidy`).","Use the public session.New() / FromContext API; never touch unexported pool state."],"tags":["session","panic","internal","type-assertion"],"analyzedSha":"9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c","analyzedAt":"2026-08-04T21:44:03.395Z","schemaVersion":2}