{"record":{"id":"a3bc7d33d9f10a68","repo":"cloudflare/cloudflared","slug":"error-piping-traceroute-s-output-w","errorCode":null,"errorMessage":"error piping traceroute's output: %w","messagePattern":"error piping traceroute's output: %w","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"diagnostic/network/collector_utils.go","lineNumber":16,"sourceCode":"package diagnostic\n\nimport (\n\t\"bufio\"\n\t\"bytes\"\n\t\"fmt\"\n\t\"io\"\n\t\"os/exec\"\n)\n\ntype DecodeLineFunc func(text string) (*Hop, error)\n\nfunc decodeNetworkOutputToFile(command *exec.Cmd, decodeLine DecodeLineFunc) ([]*Hop, string, error) {\n\tstdout, err := command.StdoutPipe()\n\tif err != nil {\n\t\treturn nil, \"\", fmt.Errorf(\"error piping traceroute's output: %w\", err)\n\t}\n\n\tif err := command.Start(); err != nil {\n\t\treturn nil, \"\", fmt.Errorf(\"error starting traceroute: %w\", err)\n\t}\n\n\t// Tee the output to a string to have the raw information\n\t// in case the decode call fails\n\t// This error is handled only after the Wait call below returns\n\t// otherwise the process can become a zombie\n\tbuf := bytes.NewBuffer([]byte{})\n\ttee := io.TeeReader(stdout, buf)\n\thops, err := Decode(tee, decodeLine)\n\t// regardless of success of the decoding\n\t// consume all output to have available in buf\n\t_, _ = io.ReadAll(tee)\n\n\tif werr := command.Wait(); werr != nil {","sourceCodeStart":1,"sourceCodeEnd":34,"githubUrl":"https://github.com/cloudflare/cloudflared/blob/2253eeeb25a44a713a4b60b8ba1e1b3f377d1a0f/diagnostic/network/collector_utils.go#L1-L34","documentation":"decodeNetworkOutputToFile attaches a stdout pipe to the traceroute command before starting it. If exec.Cmd.StdoutPipe fails (typically because the pipe could not be created, or was already set), this wrapped error is returned and the traceroute is never started. It indicates the transport for reading traceroute output could not be established.","triggerScenarios":"Calling decodeNetworkOutputToFile (via network Collect) when StdoutPipe fails — e.g. the command's Stdout field was already set before StdoutPipe was called, or OS-level pipe/fd exhaustion (too many open files).","commonSituations":"Process fd limit (ulimit -n) exhausted on hosts with many concurrent connections; accidental reconfiguration of exec.Cmd causing 'exec: Stdout already set'; highly constrained containers.","solutions":["Check open file descriptor limits (ulimit -n) and raise them if exhausted.","Ensure nothing sets command.Stdout before StdoutPipe is called (library-internal; check for forks/modifications).","Close other resources to free fds; look for fd leaks in the process (lsof).","Retry the diagnostic collection once fds are available."],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":"// check fd headroom before collecting\nvar lim syscall.Rlimit\nsyscall.Getrlimit(syscall.RLIMIT_NOFILE, &lim)\n// if usage near lim.Cur, expect StdoutPipe failures","typeGuard":null,"tryCatchPattern":"hops, _, err := network.Collect(ctx, cfg)\nif err != nil {\n\tvar oe *os.PathError\n\tif errors.As(err, &oe) && errors.Is(oe.Err, syscall.EMFILE) {\n\t\t// too many open files: raise ulimit or free fds, then retry\n\t}\n}","preventionTips":["Raise RLIMIT_NOFILE on hosts with many connections","Audit for fd leaks (lsof | wc -l vs ulimit -n)","Do not pre-assign exec.Cmd.Stdout in code that wraps the collector","Serialize diagnostic collections rather than running many concurrently"],"tags":["diagnostics","network","process","exec"],"backgroundTag":"command-not-found","analyzedSha":"2253eeeb25a44a713a4b60b8ba1e1b3f377d1a0f","analyzedAt":"2026-09-06T04:14:33.757Z","contentChangedAt":"2026-09-06T04:14:33.757Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}