{"id":"151ddd6b8a31d17e","repo":"gofiber/fiber","slug":"failed-to-write-cached-response-at-fastpath-w","errorCode":null,"errorMessage":"failed to write cached response at fastpath: %w","messagePattern":"failed to write cached response at fastpath: %w","errorType":"http","errorClass":null,"httpStatus":null,"severity":"error","filePath":"middleware/idempotency/idempotency.go","lineNumber":120,"sourceCode":"\t\t// Don't execute middleware if Next returns true\n\t\tif cfg.Next != nil && cfg.Next(c) {\n\t\t\treturn c.Next()\n\t\t}\n\n\t\t// Don't execute middleware if the idempotency key is empty\n\t\tif c.Get(cfg.KeyHeader) == \"\" {\n\t\t\treturn c.Next()\n\t\t}\n\n\t\t// Validate key\n\t\tkey := utils.CopyString(c.Get(cfg.KeyHeader))\n\t\tif err := cfg.KeyHeaderValidate(key); err != nil {\n\t\t\treturn err\n\t\t}\n\n\t\t// First-pass: if the idempotency key is in the storage, get and return the response\n\t\tif ok, err := maybeWriteCachedResponse(c, key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to write cached response at fastpath: %w\", err)\n\t\t} else if ok {\n\t\t\treturn nil\n\t\t}\n\n\t\tif err := cfg.Lock.Lock(key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to lock: %w\", err)\n\t\t}\n\t\tdefer func() {\n\t\t\tif err := cfg.Lock.Unlock(key); err != nil {\n\t\t\t\tlog.Errorf(\"[IDEMPOTENCY] failed to unlock key %q: %v\", maskKey(key), err)\n\t\t\t}\n\t\t}()\n\n\t\t// Lock acquired. If the idempotency key now is in the storage, get and return the response\n\t\tif ok, err := maybeWriteCachedResponse(c, key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to write cached response while locked: %w\", err)\n\t\t} else if ok {\n\t\t\treturn nil","sourceCodeStart":102,"sourceCodeEnd":138,"githubUrl":"https://github.com/gofiber/fiber/blob/9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c/middleware/idempotency/idempotency.go#L102-L138","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Inspect errors.Is / errors.As on the wrapped chain to reach the underlying storage or unmarshal cause.","Apply the fixes for [167] (storage connectivity) or [168] (corrupt cached data).","Log the underlying error verbosely; the 'fastpath' label is just localization."],"exampleFix":"// before — generic log on error\nif err := app.Test(req); err != nil { log.Println(err) }\n\n// after — unwrap to classify\nvar target *fiber.Error\nif errors.As(err, &target) {\n    log.Printf(\"idempotency fastpath: code=%d\", target.Code)\n} else {\n    log.Printf(\"idempotency fastpath underlying: %v\", errors.Unwrap(err))\n}","handlingStrategy":"fallback","validationCode":null,"typeGuard":"// classify the wrapped error\nfunc isStorageErr(err error) bool {\n    var p *fiber.Error\n    if errors.As(err, &p) { return true }\n    unwrapped := errors.Unwrap(err)\n    return unwrapped != nil && strings.Contains(unwrapped.Error(), \"storage\")\n}","tryCatchPattern":"// fail open on fastpath read; log for investigation\nif err := mw(c); err != nil {\n    log.Printf(\"idempotency fastpath: %v\", err)\n    return c.Next()\n}","preventionTips":["Unwrap chained errors to reach the underlying cause before logging.","Distinguish fastpath (pre-lock) failures from locked failures when triaging.","Apply fixes for the underlying [167]/[168]."],"tags":["idempotency","storage","fastpath","wrapper"],"analyzedSha":"9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c","analyzedAt":"2026-08-04T21:44:03.395Z","schemaVersion":2}