SoftwareSerial.flush() returns too early

[gicking]

Bug Description
SoftwareSerial.flush() returns before the last byte is sent, i.e. after sending 2nd last byte. This is unexpected and different from HardwareSerial.flush() and other core implementations

Reproduction

1. flash this sketch SoftwareSerial_flush.zip
2. measure pins D11(=TxD) and D3(=testpin) with logic analyzer

Expected behavior
SoftwareSerial.flush() should return after the last byte from SoftwareSerial.write(buf, len) has been sent. This is the behavior of HardwareSerial.flush() and also other core implementations

Screenshots

Expected behavior (from HardwareSerial.flush()):

Actual behavior of SoftwareSerial.flush()):

Desktop:

• OS: Linux / Ubuntu
• STM32 core v2.12.0
• Arduino IDE version: 2.3.7
• STM32 core version: 2.12.0

Board:

• Name: Nucleo-STM32L432KC

[gicking]

PS: I just added a pull request which addresses this issue