Commits on Jun 2, 2017

1. Add a GNU make jobserver implementation to Cargo …

   This commit adds a GNU make jobserver implementation to Cargo, both as a client
   of existing jobservers and also a creator of new jobservers. The jobserver is
   actually just an IPC semaphore which manifests itself as a pipe with N bytes
   of tokens on Unix and a literal IPC semaphore on Windows. The rough protocol
   is then if you want to run a job you read acquire the semaphore (read a byte on
   Unix or wait on the semaphore on Windows) and then you release it when you're
   done.
   
   All the hairy details of the jobserver implementation are housed in the
   `jobserver` crate on crates.io instead of Cargo. This should hopefully make it
   much easier for the compiler to also share a jobserver implementation
   eventually.
   
   The main tricky bit here is that on Unix and Windows acquiring a jobserver token
   will block the calling thread. We need to either way for a running job to exit
   or to acquire a new token when we want to spawn a new job. To handle this the
   current implementation spawns a helper thread that does the blocking and sends a
   message back to Cargo when it receives a token. It's a little trickier with
   shutting down this thread gracefully as well but more details can be found in
   the `jobserver` crate.
   
   Unfortunately crates are unlikely to see an immediate benefit of this once
   implemented. Most crates are run with a manual `make -jN` and this overrides the
   jobserver in the environment, creating a new jobserver in the sub-make. If the
   `-jN` argument is removed, however, then `make` will share Cargo's jobserver and
   properly limit parallelism.
   
   Closes rust-lang#1744

   

   alexcrichton committed Jun 2, 2017
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