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Implement -jsem: parallelism controlled by semaphores

sheaf requested to merge wip/jsem into master
Implement -jsem: parallelism controlled by semaphores

See for a
complete description for the motivation for this feature.

The `-jsem` option allows a build tool to pass a semaphore to GHC which
GHC can use in order to control how much parallelism it requests.

GHC itself acts as a client in the GHC jobserver protocol.

GHC Jobserver Protocol

This proposal introduces the GHC Jobserver Protocol. This protocol allows
a server to dynamically invoke many instances of a client process,
while restricting all of those instances to use no more than <n> capabilities.
This is achieved by coordination over a system semaphore (either a POSIX
semaphore [6]_  in the case of Linux and Darwin, or a Win32 semaphore [7]_
in the case of Windows platforms).

There are two kinds of participants in the GHC Jobserver protocol:

- The *jobserver* creates a system semaphore with a certain number of
  available tokens.

  Each time the jobserver wants to spawn a new jobclient subprocess, it **must**
  first acquire a single token from the semaphore, before spawning
  the subprocess. This token **must** be released once the subprocess terminates.

  Once work is finished, the jobserver **must** destroy the semaphore it created.

- A *jobclient* is a subprocess spawned by the jobserver or another jobclient.

  Each jobclient starts with one available token (its *implicit token*,
  which was acquired by the parent which spawned it), and can request more
  tokens through the Jobserver Protocol by waiting on the semaphore.

  Each time a jobclient wants to spawn a new jobclient subprocess, it **must**
  pass on a single token to the child jobclient. This token can either be the
  jobclient's implicit token, or another token which the jobclient acquired
  from the semaphore.

  Each jobclient **must** release exactly as many tokens as it has acquired from
  the semaphore (this does not include the implicit tokens).

Build tools such as cabal act as jobservers in the protocol and are
responsibile for correctly creating, cleaning up and managing the

Fixes #19349
Edited by Matthew Pickering

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