Available yield

[Makuna]

#540

NOTE: There is bleed over in the changes from a previous pull request, don't know how to clean that out other than revert it on my branch; BUT I DO NOT WANT TO REVERT THOSE CHANGES if they get merged in.

[igrr]

Since there isn't a way to handle this yield issue in one single place, i would propose to reduce the amount of code duplication by refactoring this as follows.

• Introduce a global function, e.g. maybe_yield():

void maybe_yield() {
  if (micros() - esp_micros_at_task_start() > 10000) {
    yield();
  }
}

• Update available methods to use it instead of esp_micros_at_task_start (which was a quick and dirty hack on my side):

bool isAvailable = ... ; // depending on the class
if (!isAvailable) {
  maybe_yield();
  isAvailable = ...; // perhaps update isAvailable, depending on the class?
}
return isAvailable;

[igrr]

Also i would like to merge your fixes in 4d3ef20. Would you mind if I rebase that change directly right now and then we go with a clean pull related to isAvailable?

[Makuna]

Go ahead and take the other, I will refactor this stuff and put out a new pull

[Makuna]

Note that #534 already contains the changes you want, just merge that.

[Links2004]

will the yield brake the code if i call it for example in the Ticker or the IO interrupt handler?

[igrr]

That's right, yield is non-reentrant, so calling it from Ticker or GPIO interrupt will cause an exception.
Doing while(!Serial.available()){} from an interrupt is not good either, but i think it would be good not to yield on the first invocation of available(). That was the rationale behind the existing code — so that you could call available() at least once from system context.

[Links2004]

yes while(!Serial.available()){} is a bad idea,
but a simple int i = Serial.available(); need to work otherwise we get more problems again.

[igrr]

My suggestion here is to yield only if available() was called more than once with zero result.
This way we can still read all available data from both sketch and system contexts, and handle the case of while(!available()); loop.

[Makuna]

this is replaced by #551