Re: Hadrian Transitive Dependencies

https://gitlab.haskell.org/dashboard/issues?scope=all&utf8=%E2%9C%93&state=opened&search=%22ghc+-M%22 seems to suggest this never made it into a ticket. I searched in the Hadrian github repo as well, no luck there either. It certainly came up in various discussions I've had though. On 27/03/2019 16:05, Andrey Mokhov wrote:

Hi David,

We had a discussion about this with Neil some time ago, and I think we had the following list of progressively more complex invariants for different types of build systems:

·Non-cloud build systems: **all direct inputs must be declared**. If you miss a direct input dependency then a build may complete successfully but with an incorrect result.

·Cloud build systems: **all direct inputs and direct outputs must be declared**. If you miss a direct output then a build may fail because the cloud will not be able to restore the corresponding output.

·Cloud build systems with shallow (deferred) materialisation of build artefacts: **all transitive inputs and direct outputs must be declared**. Let’s say you’d like to download the resulting GHC binary directly, without materialising any intermediate artefacts. Then you’ll need to know GHC’s ultimate transitive inputs.

I think for now we are really keen to make Hadrian a cloud build system, but whether shallow builds are valuable enough is not clear. Maybe not. Therefore, I’d say we don’t need to track transitive inputs right now. Furthermore, if we were to track all transitive inputs, we would lose the desirable early cutoff property, which prevents rebuilding after adding a comment in a file on which a lot of other files transitively depend on.

Having said that, if we really access a file during compilation, then I think it is **not** a transitive dependency by definition! Any file which is accessed during a build rule is a direct dependency.

...

GHC is reading *.hi files that are not reported as dependencies by

...

`ghc -M  -include-pkg-deps`. This is because they are not direct, but transitive

...

dependencies!

So, here I’m confused. If we read a file A when compiling a file B, then it’s by definition a direct dependency. Perhaps we just read too much? Maybe the solution is to switch to fine-grained `ghc -M` mode, to analyse import dependencies for a single module instead of doing it transitively, which I believe was discussed in a ticket some time ago? I can’t find this ticket, but I think Alp was looking into it at some point. Alp: do you remember it?

Thank you for all your work on Hadrian!

Cheers,

Andrey

*From:*David Eichmann [mailto:davide@well-typed.com] *Sent:* 27 March 2019 12:54 *To:* Neil Mitchell ; Andrey Mokhov ; GHC developers *Subject:* Hadrian Transitive Dependencies

Hello Shake/Hadrian contributors and the like,

Recently I've been putting Hadrian's fsatrace linting feature to good use, tracking down missing dependencies in Hadrian. Ultimately, we want to use shake's cloud build / shared cache feature and ensure it works across CI builds. Unfortunately the feature isn't working smoothly with Hadrian: see #16295 https://gitlab.haskell.org/ghc/ghc/issues/16295. This is very desirable to improve CI build times. It is my understanding that in order to get caching to work:

1. All accessed files must declared with `need` AND 2. All created files must be declared with `produces` (or be the target of the build rule)

Is my understanding correct? Or is there a weaker condition (perhaps only 2 is necessary)?

If I'm correct, this amounts to fixing all fsatrace lint errors. See here https://gitlab.haskell.org/ghc/ghc/issues/16400#note_188901 for a breakdown of lint errors / missing dependencies. A large portion of these are Haskell interface files (i.e. *.hi files). Before building a Haskell object file, dependencies are discovered via `ghc` using the `-M -include-pkg-deps` options. Unfortunately, shake's fsatrace linting complains about other *.hi files being accessed! For example when building `stage1/libraries/mtl/build/Control/Monad/RWS/Class.o` we get the following dependencies from ghc:

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : libraries/mtl/Control/Monad/RWS/Class.hs _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Prelude.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Data/Monoid.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/RWS/Strict.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/RWS/Lazy.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Identity.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Maybe.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Except.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Error.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Class.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/Writer/Class.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/State/Class.hi _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/Reader/Class.hi

And shake complains of the following missing deps:

_build/stage0/bin/ghc -Wall -hisuf hi -osuf o -hcsuf hc -static -hide-all-packages -no-user-package-db '-package-db _build/stage1/lib/package.conf.d' '-this-unit-id mtl-2.2.2' '-package-id base-4.13.0.0' '-package-id transformers-0.5.5.0' -i -i_build/stage1/libraries/mtl/build -i_build/stage1/libraries/mtl/build/autogen -ilibraries/mtl/. -Iincludes -I_build/generated -I_build/stage1/libraries/mtl/build -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/include -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/integer-gmp-1.0.2.0/include -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/rts-1.0/include -I_build/generated -optc-I_build/generated -optP-include -optP_build/stage1/libraries/mtl/build/autogen/cabal_macros.h -outputdir _build/stage1/libraries/mtl/build -Wnoncanonical-monad-instances -optc-Werror=unused-but-set-variable -optc-Wno-error=inline -c libraries/mtl/Control/Monad/RWS/Class.hs -o _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o -O2 -H32m -Wall -fno-warn-unused-imports -fno-warn-warnings-deprecations -Wcompat -Wnoncanonical-monad-instances -Wnoncanonical-monadfail-instances -XHaskell2010 -XSafe -ghcversion-file=/home/david/MEGA/File_Dump/Well-Typed/GHC/_nosync_git/ghc/_build/generated/ghcversion.h -Wno-deprecated-flags Lint checking error - _build/HEAD_default/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o - 22 values were used but not depended upon:   Used:  _build/HEAD_default/stage0/lib/settings   Used:  _build/HEAD_default/stage0/lib/platformConstants   Used:  _build/HEAD_default/stage0/lib/llvm-targets   Used:  _build/HEAD_default/stage0/lib/llvm-passes   Used:  _build/HEAD_default/stage0/lib/package.conf.d/package.cache   Used:  _build/HEAD_default/stage1/lib/package.conf.d/package.cache   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Float.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Base.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/ghc-prim-0.5.3/GHC/Types.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Maybe.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Writer/Lazy.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Writer/Strict.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/State/Lazy.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/State/Strict.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Reader.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/List.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Cont.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/ghc-prim-0.5.3/GHC/Tuple.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/IO/Exception.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/integer-gmp-1.0.2.0/GHC/Integer/Type.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Data/Either.hi   Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Natural.hi

GHC is reading *.hi files that are not reported as dependencies by `ghc -M  -include-pkg-deps`. This is because they are not direct, but /transitive/ dependencies! How do we fix these lint errors (again with the goal of using shakes shared cache feature)? Some ideas:

* Wildly over approximate dependencies. This may be easier to implement but cause unneeded recompilation (when a false dependency changes). Either:     * `need` all dependent packages' interface files recursively as well as transitive dependencies reported by `ghc -M  -include-pkg-deps` within the current package. OR     * OR `need` all transitive dependencies reported by `ghc -M  -include-pkg-deps`. This will likely result in fewer dependencies but requires a bit more work in recovering dependent packages' dependency graphs. * Perhaps transitive dependencies are not important for shared caching to work. Change shakes linting feature to allow (untracked?) transitive dependencies to be accessed.

Feed back would be greatly appreciated.

David Eichmann

-- David Eichmann, Haskell Consultant Well-Typed LLP,http://www.well-typed.com Registered in England & Wales, OC335890 118 Wymering Mansions, Wymering Road, London W9 2NF, England

-- Alp Mestanogullari, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England and Wales, OC335890 118 Wymering Mansions, Wymering Road, London, W9 2NF, England