time: Make std::string version of strftime() avoid runaway memory allocations (#5348)

esphome/core/time.cpp

@@ -49,6 +49,11 @@ std::string ESPTime::strftime(const std::string &format) {
   struct tm c_tm = this->to_c_tm();
   size_t len = ::strftime(&timestr[0], timestr.size(), format.c_str(), &c_tm);
   while (len == 0) {
+    if (timestr.size() >= 128) {
+      // strftime has failed for reasons unrelated to the size of the buffer
+      // so return a formatting error
+      return "ERROR";
+    }
     timestr.resize(timestr.size() * 2);
     len = ::strftime(&timestr[0], timestr.size(), format.c_str(), &c_tm);
   }

esphome/core/time.h

@@ -45,6 +45,10 @@ struct ESPTime {
    *
    * @warning This method uses dynamically allocated strings which can cause heap fragmentation with some
    * microcontrollers.
+   *
+   * @warning This method can return "ERROR" when the underlying strftime() call fails, e.g. when the
+   * format string contains unsupported specifiers or when the format string doesn't produce any
+   * output.
    */
   std::string strftime(const std::string &format);