ipfs/kubo · warning

err.Error()

Error message

err.Error()

What it means

The mutex-fraction endpoint parses the request body with r.ParseForm before reading the 'fraction' parameter. If the request body is malformed for form parsing (bad Content-Type, truncated body, invalid encoding), it returns HTTP 400 with the raw parse error text in the body.

Source

Thrown at core/corehttp/mutex_profile.go:22

	"net"
	"net/http"
	"runtime"
	"strconv"

	core "github.com/ipfs/kubo/core"
)

// MutexFractionOption allows to set runtime.SetMutexProfileFraction via HTTP
// using POST request with parameter 'fraction'.
func MutexFractionOption(path string) ServeOption {
	return func(_ *core.IpfsNode, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
		mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
			if r.Method != http.MethodPost {
				http.Error(w, "only POST allowed", http.StatusMethodNotAllowed)
				return
			}
			if err := r.ParseForm(); err != nil {
				http.Error(w, err.Error(), http.StatusBadRequest)
				return
			}

			asfr := r.Form.Get("fraction")
			if len(asfr) == 0 {
				http.Error(w, "parameter 'fraction' must be set", http.StatusBadRequest)
				return
			}

			fr, err := strconv.Atoi(asfr)
			if err != nil {
				http.Error(w, err.Error(), http.StatusBadRequest)
				return
			}
			log.Infof("Setting MutexProfileFraction to %d", fr)
			runtime.SetMutexProfileFraction(fr)
		})

View on GitHub (pinned to 329838acdf)

Solutions

1. Send the parameter as a proper form body: `curl -X POST -d 'fraction=5' <debug-path>` (curl sets application/x-www-form-urlencoded).
2. Ensure the Content-Type is application/x-www-form-urlencoded or multipart/form-data when POSTing manually.
3. URL-encode parameter values correctly; avoid raw special characters in the fraction field.
4. Verify no proxy/middleware is truncating or rewriting the request body.

Example fix

// before: JSON body cannot be form-parsed
curl -X POST -H 'Content-Type: application/json' -d '{"fraction":5}' <debug-path>  // 400

// after
curl -X POST -d 'fraction=5' <debug-path>

form := url.Values{"fraction": {"5"}}
req, _ := http.NewRequest(http.MethodPost, debugURL, strings.NewReader(form.Encode()))
req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
// body is guaranteed parseable by r.ParseForm

resp, err := http.DefaultClient.Do(req)
if err == nil && resp.StatusCode == http.StatusBadRequest {
    body, _ := io.ReadAll(resp.Body)
    // body contains the ParseForm error; fix Content-Type/encoding and resend
    return fmt.Errorf("form rejected: %s", body)
}

When it happens

Trigger: POSTing to the mutex-fraction debug path with a body that cannot be parsed as a form — e.g. sending JSON without application/x-www-form-urlencoded or multipart/form-data, a corrupted/chunked body, or invalid URL encoding in the query/body.

Common situations: Scripts sending `--data` with JSON to the endpoint; clients setting the wrong Content-Type header; URL-encoded values containing invalid percent sequences; proxies mangling the request body.

Related errors

• only POST allowed
• parameter 'fraction' must be set
• unexpected redirect
• GitHub API returned HTTP %d for %s
• download returned HTTP %d

AI-assisted analysis of ipfs/kubo@329838acdf (2026-09-03). Data as JSON: /api/errors/d00e8530201583f3. Report an issue: GitHub.