[parrot/parrot] 27e36b: [pmc] Clarify dead coroutines

[GitHub]

  Branch: refs/heads/rurban/coros-gh564
  Home:   https://github.com/parrot/parrot
  Commit: 27e36bfac601dce57fe9afd0083af02fd2c2b586
      https://github.com/parrot/parrot/commit/27e36bfac601dce57fe9afd0083af02fd2c2b586
  Author: Reini Urban <rurban at cpanel.net>
  Date:   2014-10-06 (Mon, 06 Oct 2014)

  Changed paths:
    M docs/book/pir/ch06_subroutines.pod
    M docs/pmc/subs.pod
    M src/pmc/coroutine.pmc
    M t/pmc/coroutine.t

  Log Message:
  -----------
  [pmc] Clarify dead coroutines

Coros are not resumable when all yield states are exhausted.
This is consistent with the LUA design, which we follow, and all other
coro implementations I know of. Just the parrot book argues that after a
return the state should be automatically reset. Technically no problem, even
without the quirks described in https://github.com/parrot/parrot/issues/564#issuecomment-3503597
or via a seperate reset() method, but semantically an auto-reset is problematic.

Added testcases for all the relevant tickets: #564, #585 and there is on
bug remaining: TT #1003 leading to Null PMC access in get_string on a dead coro