[GHC] #15890: Provide a way for hadrian users to always pass some options to hadrian itself

[GHC]

#15890: Provide a way for hadrian users to always pass some options to hadrian
itself
-------------------------------------+-------------------------------------
        Reporter:  alpmestan         |                Owner:  alpmestan
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Build System      |              Version:  8.7
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5454
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by alpmestan):

 * owner:  (none) => alpmestan
 * differential:   => Phab:D5454


Comment:

 I have one part done: the ability to override the default flavour (not the
 flavour whose name is `default`, I instead mean the flavour that we use
 when no `--flavour=...` argument is passed to hadrian). See
 https://phabricator.haskell.org/D5454, feedback welcome!

 Regarding `buildThreads`, I'm finding it a little hard to get a chance to
 set the parallelism (through the `shakeThreads` field of `ShakeOptions`)
 when 1/ it's not overriden on the command line with `-j` 2/ using the
 default value specified by that `buildThreads` field I'm adding, since I
 don't have a handle on the flavour just yet. The same applies to the build
 root, I think, even though I haven't looked into that one a lot yet.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15890#comment:7>
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