[tor-bugs] #15593 [Pluggable transport]: Bring sanity to the tor side of the PT shutdown process.

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#15593: Bring sanity to the tor side of the PT shutdown process.
---------------------------------+------------------------------------
 Reporter:  yawning              |          Owner:  asn
     Type:  enhancement          |         Status:  new
 Priority:  normal               |      Milestone:  Tor: 0.2.7.x-final
Component:  Pluggable transport  |        Version:
 Keywords:  pt-wants             |  Actual Points:
Parent ID:                       |         Points:
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 This is the final phase of my great PT shutdown process cleanup as a
 follow up to #15545.

 Now that there's a portable mechanism to signal termination to PTs (close
 the stdin), we should change the PT shutdown process to allow graceful
 termination to look like this:

  1. Close stdin (and on U*IX, send a SIGTERM, PT behavior here is
 equivalent).
  2. Wait for a grace period (~1 sec?)
  3. If the child still is not dead, send a SIGKILL/TerminateProcess().
 (Failsafe)

 This fixes #9330 in that, PTs that wish to trap a graceful shutdown on
 Windows have a way to do so, despite the final stage of the process
 killing the PT in the most violent way possible as a failsafe
 (realistically, PTs should exit shortly after step 1).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15593>
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