Function assertUniqueLabel

http/unstable_message_signatures.ts:872–891

(headers: Headers, label: string)

// that silently corrupt prior signatures on parse (last value wins per
// RFC 8941).
function assertUniqueLabel(headers: Headers, label: string): void {
  for (const name of ["Signature-Input", "Signature"] as const) {
    const value = headers.get(name);
    if (value === null) continue;
    let dict: Dictionary;
    try {
      dict = parseDictionary(value);
    } catch (cause) {
      throw new TypeError(
        `Cannot parse existing "${name}" header on message to check label uniqueness`,
        { cause },
      );
    }
    if (dict.has(label)) {
      throw new TypeError(
        `Signature label "${label}" is already present in the "${name}" header on the message; choose a unique label (RFC 9421 section 4)`,
      );
    }
  }
}

// =============================================================================
// verifyMessage