GitHub Actions backend doesn't respect `jobs`, resulting in lack of build parallelism

[RyanGlScott]

In the haskell-ci–generated GitHub Actions workflow for transformers-compat, I noticed that despite settings jobs: 2, the dependencies were being built sequentially. On the other hand, this doesn't happen with the Travis backend:

haskell-ci/src/HaskellCI/Travis.hs

Lines 193 to 199 in 13af816

	-- Cabal jobs
	for_ (cfgJobs >>= cabalJobs) $ \n ->
	sh $ "echo 'jobs: " ++ show n ++ "' >> $CABALHOME/config"
	-- GHC jobs + ghc-options
	for_ (cfgJobs >>= ghcJobs) $ \m -> do
	shForJob (Range $ C.orLaterVersion (C.mkVersion [7,8])) $ "GHCJOBS=-j" ++ show m

The GitHub Actions backend should do something similar.

[phadej]

looks like we didn't fix #421?

[RyanGlScott]

This is a slightly different issue. The cabal.haskell-ci file for transformers-compat specifies both of these options:

jobs:                   2
install-dependencies:   False

We explicitly pass -j2 to cabal build when building the dependencies, but not to the subsequent invocation of cabal build. However, transformers-compat is a special case where libraries in its test suite depend on transformers-compat itself, so as a result, one can't build its dependencies beforehand (hence the use of install-dependencies: False). In order to recover some amount of package-level build parallelism, I'm using jobs: 2, which affects the subsequent invocation of cabal build.

[phadej]

It feels that we want to have granular jobs control for different steps. That is doable, but I won't have time to think about this now. Especially if this is only "bad" for install-dependencies: False setups.

[RyanGlScott]

In the meantime, would you accept a PR that makes the behavior of jobs consistent across both the Travis and GitHub Actions backends?

[phadej]

Btw, please comment on haskell/cabal#6106 so Cabal folks could prioritize the issue. Whole install-dependencies: False is a workaround for it.

In the meantime, would you accept a PR that makes the behavior of jobs consistent across both the Travis and GitHub Actions backends?

No, I won't have time to review it any time soon, I'm sorry.

[RyanGlScott]

No, I won't have time to review it any time soon, I'm sorry.

I'm a little unclear. Are you philosophically opposed to the current design of the jobs flag, or are you just pressed for time? If it's the latter, I'd be happy to wait until you do have time to review it (and I don't expect the patch to be that large).

[phadej]

just pressed for time?

That.

[RyanGlScott]

Sounds good. When you're ready, I've submitted a fix at #552.