Fix #1747

Author: yhirose

httplib.h

@@ -8,7 +8,7 @@
 #ifndef CPPHTTPLIB_HTTPLIB_H
 #define CPPHTTPLIB_HTTPLIB_H
 
-#define CPPHTTPLIB_VERSION "0.14.3"
+#define CPPHTTPLIB_VERSION "0.15.0"
 
 /*
  * Configuration
@@ -4637,28 +4637,31 @@ inline std::string to_lower(const char *beg, const char *end) {
   return out;
 }
 
-inline std::string make_multipart_data_boundary() {
+inline std::string random_string(size_t length) {
   static const char data[] =
       "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 
   // std::random_device might actually be deterministic on some
   // platforms, but due to lack of support in the c++ standard library,
   // doing better requires either some ugly hacks or breaking portability.
-  std::random_device seed_gen;
+  static std::random_device seed_gen;
 
   // Request 128 bits of entropy for initialization
-  std::seed_seq seed_sequence{seed_gen(), seed_gen(), seed_gen(), seed_gen()};
-  std::mt19937 engine(seed_sequence);
+  static std::seed_seq seed_sequence{seed_gen(), seed_gen(), seed_gen(), seed_gen()};
 
-  std::string result = "--cpp-httplib-multipart-data-";
+  static std::mt19937 engine(seed_sequence);
 
-  for (auto i = 0; i < 16; i++) {
+  std::string result;
+  for (size_t i = 0; i < length; i++) {
     result += data[engine() % (sizeof(data) - 1)];
   }
-
   return result;
 }
 
+inline std::string make_multipart_data_boundary() {
+  return "--cpp-httplib-multipart-data-" + detail::random_string(16);
+}
+
 inline bool is_multipart_boundary_chars_valid(const std::string &boundary) {
   auto valid = true;
   for (size_t i = 0; i < boundary.size(); i++) {
@@ -5133,20 +5136,6 @@ inline bool parse_www_authenticate(const Response &res,
   return false;
 }
 
-// https://stackoverflow.com/questions/440133/how-do-i-create-a-random-alpha-numeric-string-in-c/440240#answer-440240
-inline std::string random_string(size_t length) {
-  auto randchar = []() -> char {
-    const char charset[] = "0123456789"
-                           "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-                           "abcdefghijklmnopqrstuvwxyz";
-    const size_t max_index = (sizeof(charset) - 1);
-    return charset[static_cast<size_t>(std::rand()) % max_index];
-  };
-  std::string str(length, 0);
-  std::generate_n(str.begin(), length, randchar);
-  return str;
-}
-
 class ContentProviderAdapter {
 public:
   explicit ContentProviderAdapter(