Fix #1616 - randomize mqtt will and client id for parallel test

src/util.c

@@ -21,6 +21,19 @@ void mg_random(void *buf, size_t len) {
 }
 #endif
 
+char *mg_random_str(char *buf, size_t len) {
+  size_t i;
+  mg_random(buf, len);
+  for (i = 0; i < len; i++) {
+    uint8_t c = ((uint8_t *) buf)[i] % 62;
+    buf[i] = i == len - 1 ? '\0'                   // 0-terminate last byte
+             : c < 26     ? (char) ('a' + c)       // lowercase
+             : c < 52     ? (char) ('A' + c - 26)  // uppercase
+                          : (char) ('0' + c - 52);     // numeric
+  }
+  return buf;
+}
+
 uint32_t mg_ntohl(uint32_t net) {
   uint8_t data[4] = {0, 0, 0, 0};
   memcpy(&data, &net, sizeof(data));

src/util.h

@@ -5,6 +5,7 @@
 #include "str.h"
 
 void mg_random(void *buf, size_t len);
+char *mg_random_str(char *buf, size_t len);
 uint16_t mg_ntohs(uint16_t net);
 uint32_t mg_ntohl(uint32_t net);
 uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len);