Make UART flush function consistent (#748)

See also 78be9d2937

esphome/components/mhz19/mhz19.cpp

@@ -77,16 +77,17 @@ void MHZ19Component::abc_disable() {
 }
 
 bool MHZ19Component::mhz19_write_command_(const uint8_t *command, uint8_t *response) {
-  this->flush();
+  // Empty RX Buffer
+  while (this->available())
+    this->read();
   this->write_array(command, MHZ19_REQUEST_LENGTH);
   this->write_byte(mhz19_checksum(command));
+  this->flush();
 
   if (response == nullptr)
     return true;
 
-  bool ret = this->read_array(response, MHZ19_RESPONSE_LENGTH);
-  this->flush();
-  return ret;
+  return this->read_array(response, MHZ19_RESPONSE_LENGTH);
 }
 float MHZ19Component::get_setup_priority() const { return setup_priority::DATA; }
 void MHZ19Component::dump_config() {

esphome/components/sds011/sds011.cpp

@@ -94,7 +94,6 @@ float SDS011Component::get_setup_priority() const { return setup_priority::DATA;
 void SDS011Component::set_rx_mode_only(bool rx_mode_only) { this->rx_mode_only_ = rx_mode_only; }
 
 void SDS011Component::sds011_write_command_(const uint8_t *command_data) {
-  this->flush();
   this->write_byte(SDS011_MSG_HEAD);
   this->write_byte(SDS011_COMMAND_ID_REQUEST);
   this->write_array(command_data, SDS011_DATA_REQUEST_LENGTH);

esphome/components/uart/uart.cpp

@@ -316,7 +316,9 @@ uint8_t ESP8266SoftwareSerial::peek_byte() {
     return 0;
   return this->rx_buffer_[this->rx_out_pos_];
 }
-void ESP8266SoftwareSerial::flush() { this->rx_in_pos_ = this->rx_out_pos_ = 0; }
+void ESP8266SoftwareSerial::flush() {
+  // Flush is a NO-OP with software serial, all bytes are written immediately.
+}
 int ESP8266SoftwareSerial::available() {
   int avail = int(this->rx_in_pos_) - int(this->rx_out_pos_);
   if (avail < 0)

esphome/components/uart/uart.h

@@ -61,6 +61,7 @@ class UARTComponent : public Component, public Stream {
 
   int available() override;
 
+  /// Block until all bytes have been written to the UART bus.
   void flush() override;
 
   float get_setup_priority() const override { return setup_priority::BUS; }