Commits on Sep 9, 2019

1. Create a jobserver with N tokens, not N-1 …

   I recently added `jobserver` support to the `cc` crate and ended up
   running afoul of a `jobserver` quirk on Windows. Due to how it's
   implemented, on Windows you can't actually add more than the intial
   number of tokens to the jobserver (it uses an IPC semaphore). On Unix,
   however, you can since you're just writing bytes into a pipe.
   
   In `cc`, however, I found it convenient to control parallelism by simply
   releasing a token before the parallel loop, then reacquiring the token
   after the loop. That way the loop just has to acquire a token for each
   job it wants to spawn and then release it when the job finishes. This is
   a bit simpler than trying to juggle the "implicit token" all over the
   place as well as coordinating its use. It's technically invalid because
   it allows a brief moment of `N+1` parallelism since we release a token
   and then do a bit of work to acquire a new token, but that's hopefully
   not really the end of the world.
   
   In any case this commit updates Cargo's creation of a jobserver to create
   it with `N` tokens instead of `N-1`. The same semantics are preserved
   where Cargo then immediately acquires one of the tokens, but the
   difference is that this "implicit token" can be released back to the
   jobserver pool, unlike before.

   

   alexcrichton committed Sep 9, 2019
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