{"id":"dfc48efc19c234b6","repo":"gofiber/fiber","slug":"failed-to-write-string-w","errorCode":null,"errorMessage":"failed to write string: %w","messagePattern":"failed to write string: %w","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"router.go","lineNumber":246,"sourceCode":"// to ensure consistent URL generation behavior across APIs.\n//\n// Parameter resolution uses a deterministic three-step lookup:\n//  1. Exact key match on segment.ParamName\n//  2. Case-insensitive fallback picking the lexicographically-smallest matching key (when !caseSensitive)\n//  3. Greedy parameter fallback for wildcard (*) and plus (+) parameters\nfunc buildRouteURL(route *Route, params Map) (string, error) {\n\tif len(route.routeParser.segs) == 0 {\n\t\treturn route.Path, nil\n\t}\n\n\tbuf := bytebufferpool.Get()\n\tdefer bytebufferpool.Put(buf)\n\n\tfor _, segment := range route.routeParser.segs {\n\t\tif !segment.IsParam {\n\t\t\t_, err := buf.WriteString(segment.Const)\n\t\t\tif err != nil {\n\t\t\t\treturn \"\", fmt.Errorf(\"failed to write string: %w\", err)\n\t\t\t}\n\t\t\tcontinue\n\t\t}\n\n\t\tvar (\n\t\t\tval   any\n\t\t\tfound bool\n\t\t)\n\n\t\t// Prefer an exact parameter name match\n\t\tif val, found = params[segment.ParamName]; !found && !route.caseSensitive {\n\t\t\t// Fall back to a case-insensitive match using a deterministic winner\n\t\t\tvar matchedKey string\n\t\t\tfoundMatch := false\n\t\t\tfor key := range params {\n\t\t\t\tif utils.EqualFold(key, segment.ParamName) && (!foundMatch || key < matchedKey) {\n\t\t\t\t\tmatchedKey = key\n\t\t\t\t\tfoundMatch = true","sourceCodeStart":228,"sourceCodeEnd":264,"githubUrl":"https://github.com/gofiber/fiber/blob/9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c/router.go#L228-L264","documentation":"Returned by buildRouteURL (router.go:246) when writing a static (constant) path segment into the reusable bytebufferpool buffer fails during URL construction (e.g. via app.Get route reverse-lookup). The underlying io.Writer error is wrapped with %w so callers can inspect the root cause. In practice a bytebufferpool.ByteBuffer WriteString almost never errors, so seeing this indicates a corrupted/stressed buffer or a custom buffer wrapper.","triggerScenarios":"Calling any route-URL builder (e.g. a helper that calls buildRouteURL with a *Route containing constant segments) where the underlying buffer's WriteString returns a non-nil error. This happens only when the pooled bytebufferpool buffer has been replaced/tampered with or the process is in an OOM/panic-recovery state.","commonSituations":"Almost never seen in normal operation because bytebufferpool.ByteString cannot fail. Could surface in test harnesses that swap the pool, in fuzzing runs that exhaust memory, or after a shared buffer is closed concurrently by misuse.","solutions":["Treat the wrapped error as the source of truth: inspect err via errors.Unwrap/is to find the real cause (OOM, closed buffer, etc.).","If using a custom/forked bytebufferpool, restore the upstream version to eliminate the non-standard WriteString failure.","Free memory / reduce concurrency if the failure correlates with allocation pressure."],"exampleFix":"// before: ignoring the returned error\nurl, _ := buildRouteURL(route, params)\n\n// after: surface and log the root cause\nurl, err := buildRouteURL(route, params)\nif err != nil {\n    return fmt.Errorf(\"build url for %s: %w\", route.Path, err)\n}","handlingStrategy":"try-catch","validationCode":null,"typeGuard":null,"tryCatchPattern":"url, err := buildRouteURL(route, params)\nif err != nil {\n    // err already wraps the writer failure; log and degrade\n    log.Printf(\"route url build failed: %v\", err)\n    return fallbackURL\n}","preventionTips":["Always check the error returned by buildRouteURL / URL builders.","Do not replace bytebufferpool with a custom implementation that can fail.","Never use a buffer after bytebufferpool.Put(buf)."],"tags":["routing","buffer","url-building","io"],"analyzedSha":"9a4c7e57fe0b080a04235d28a4b0d2b4b353d58c","analyzedAt":"2026-08-04T21:44:03.395Z","schemaVersion":2}