gofiber/fiber · error

failed to write cached response while locked: %w

Error message

failed to write cached response while locked: %w

What it means

Wrapping error returned by the middleware (idempotency.go:135-136) when the SECOND maybeWriteCachedResponse call — performed AFTER acquiring Lock(key) — fails. Same underlying causes as [167] (read) and [168] (unmarshal), but the 'while locked' label indicates another request may currently hold or have just released the same key.

Source

Thrown at middleware/idempotency/idempotency.go:136

		// 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
		}

		// Execute the request handler
		if err := c.Next(); err != nil {
			// If the request handler returned an error, return it and skip idempotency
			return err
		}

		// Construct response
		res := &response{
			StatusCode: c.Response().StatusCode(),
			Body:       c.Response().Body(),
		}
		{
			headers := make(map[string][]string)
			if err := c.Bind().RespHeader(headers); err != nil {

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Unwrap to find the underlying read ([167]) or unmarshal ([168]) cause and apply those fixes.
2. Ensure the Storage is healthy under concurrent access — the locked re-read amplifies load.
3. Investigate key collisions if unmarshal failures cluster here.

// same fail-open pattern as fastpath failures
if err := mw(c); err != nil {
    log.Printf("idempotency locked read failed: %v", err)
    return c.Next()
}

When it happens

Trigger: Storage read or unmarshal failure after lock acquisition: typically a transient storage error coinciding with contention on the same idempotency key, or another writer having stored a corrupt payload between your fastpath check and your locked check.

Common situations: Two retries for the same key racing; another instance wrote a corrupt payload (see [168]); storage degraded under load.

Related errors

• failed to write cached response at fastpath: %w
• failed to read response: %w
• failed to unmarshal response: %w
• failed to lock: %w
• failed to save response: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/677869483f33b176.json. Report an issue: GitHub.