gofiber/fiber · error

failed to write cached response at fastpath: %w

Error message

failed to write cached response at fastpath: %w

What it means

Wrapping error returned by the middleware (idempotency.go:119-120) when the FIRST maybeWriteCachedResponse call (the unlocked fast path) fails. It hides nothing — it simply adds 'failed to write cached response at fastpath' context around the underlying [167] read error or [168] unmarshal error.

Source

Thrown at middleware/idempotency/idempotency.go:120

		// Don't execute middleware if Next returns true
		if cfg.Next != nil && cfg.Next(c) {
			return c.Next()
		}

		// Don't execute middleware if the idempotency key is empty
		if c.Get(cfg.KeyHeader) == "" {
			return c.Next()
		}

		// Validate key
		key := utils.CopyString(c.Get(cfg.KeyHeader))
		if err := cfg.KeyHeaderValidate(key); err != nil {
			return err
		}

		// First-pass: if the idempotency key is in the storage, get and return the response
		if ok, err := maybeWriteCachedResponse(c, key); err != nil {
			return fmt.Errorf("failed to write cached response at fastpath: %w", err)
		} else if ok {
			return nil
		}

		if err := cfg.Lock.Lock(key); err != nil {
			return fmt.Errorf("failed to lock: %w", err)
		}
		defer func() {
			if err := cfg.Lock.Unlock(key); err != nil {
				log.Errorf("[IDEMPOTENCY] failed to unlock key %q: %v", maskKey(key), err)
			}
		}()

		// Lock acquired. If the idempotency key now is in the storage, get and return the response
		if ok, err := maybeWriteCachedResponse(c, key); err != nil {
			return fmt.Errorf("failed to write cached response while locked: %w", err)
		} else if ok {
			return nil

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Inspect errors.Is / errors.As on the wrapped chain to reach the underlying storage or unmarshal cause.
2. Apply the fixes for [167] (storage connectivity) or [168] (corrupt cached data).
3. Log the underlying error verbosely; the 'fastpath' label is just localization.

Example fix

// before — generic log on error
if err := app.Test(req); err != nil { log.Println(err) }

// after — unwrap to classify
var target *fiber.Error
if errors.As(err, &target) {
    log.Printf("idempotency fastpath: code=%d", target.Code)
} else {
    log.Printf("idempotency fastpath underlying: %v", errors.Unwrap(err))
}

// classify the wrapped error
func isStorageErr(err error) bool {
    var p *fiber.Error
    if errors.As(err, &p) { return true }
    unwrapped := errors.Unwrap(err)
    return unwrapped != nil && strings.Contains(unwrapped.Error(), "storage")
}

// fail open on fastpath read; log for investigation
if err := mw(c); err != nil {
    log.Printf("idempotency fastpath: %v", err)
    return c.Next()
}

When it happens

Trigger: Any storage read failure or cached-payload unmarshal failure on the pre-lock fast path. See [167] and [168] for the root causes; this error indicates the failure happened before the Lock(key) was acquired.

Common situations: Same as [167]/[168]; the distinguishing signal is timing — the request failed before the lock acquisition line, so no other in-flight request for the same key was involved.

Related errors

• failed to write cached response while locked: %w
• failed to read response: %w
• failed to unmarshal response: %w
• failed to save response: %w
• file: failed to store file

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/151ddd6b8a31d17e.json. Report an issue: GitHub.