Serial1.available() returns wrong value

In a tight timing loop (without delays, such as Serial.print()), Serial1.available() returns a lower value than is correct.

For example Serial1.readBytes(buf, Serial1.available())); does not read the whole buffer.

If you use int a=Serial.readBytes(buf, SERIAL_BUFFER_SIZE); then a will report correctly how many bytes were read, this is a different value than Serial1.available(); - usually Serial1.available(); is one byte less.

This leads to the bytes in buf being truncated.

If there is a slight delay in the loop, the problem goes away, so it is hard to debug.

Reproduce the issue with Serial1.available() in a tight timing loop, comparing its result with Serial1.readBytes(buf, Serial1.available()) and the reported read count from Serial.readBytes(buf, SERIAL_BUFFER_SIZE). Done means Serial1.available() reports the expected available byte count without requiring an incidental delay, and the full buffer is read.

Written by the indexing model from the issue text.