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Give Futures clearer start/end and exception semantics

Authored by epriestley on Jul 23 2020, 5:07 PM.



Ref T13555. Currently:

  • If an exception is raised in "start()", the exception state is not set on the future.
  • Futures do not always call "startFuture()" before starting, and do not always call "endFuture()" once they become resolvable.
  • If you start an ExecFuture which immediately fails and then call "getPID()" on it, you get an unclear exception.

Simplify these behaviors:

  • In FutureIterator, only start futures which have not already started.
  • When starting a future on any pathway, run start code.
  • When a future becomes resolvable on any pathway, run end code.
  • Raise a more clear exception when calling "getPID()" on a future with no subprocess.
Test Plan

Faked a failing subprocess with "$proc = null", ran "bin/phd debug taskmaster" etc. Got clearer errors and more consistent future lifecycle workflows.

Diff Detail

rARC Arcanist
Lint Not Applicable
Tests Not Applicable