stack path --stack-root wants GHC installed; vicious cycle with CI caching

[andreasabel]

All that stack path --stack-root should do is to print out the stack root directory.
Nothing more. Because everything else is just making it behave worse.

Currently stack path --stack-root does tons of things before printing the stack root, and these things can fail, e.g. if no GHC is installed:
https://github.com/agda/agda-language-server/actions/runs/6930512530/job/18850319673

echo STACK_ROOT="$(stack --no-install-ghc path --stack-root)" >> "${GITHUB_ENV}"

Warning: Continuing despite missing tool: msys2
Error: [S-pointlessnumber]
No compiler found, expected minor version match or newer with ghc-8.10.7 (x86_64) (based on resolver setting in D:\a\agda-language-server\agda-language-server\stack.yaml).
To install the correct GHC into C:\Users\runneradmin\AppData\Local\Programs\stack\x86_64-windows\, try running 'stack setup' or use the '--install-ghc' flag. To use your system GHC installation, run 'stack config set system-ghc --global true', or use the '--system-ghc' flag.

What is stack doing here? I has given away the stack root in an error message:

To install the correct GHC into C:\Users\runneradmin\AppData\Local\Programs\stack\x86_64-windows ...

But it can print it for me in a regular fashion? Instead, it insists on a operational system.

Why is it problematic that stack path --stack-root does its full "system operational" check at the beginning?
Because one may need the stack root location to restore the "operational system" from a cache.
Before restoring, the system is simply not operational...

Stack needlessly constructs a vicious cycle here between needing the stack root to restore a cached GHC and needing a GHC to print the stack root. (Not sure if I should laugh or cry here.)

Request: make stack path --stack-root simply print the stack root and do nothing else.
This will also bring the runtime of this command down by 3 orders of magnitude; it currently takes >1000ms on my system.

Here is a dump of what stack path --stack-root does on a specific project of mine:

Version 2.13.1, Git revision 8102bb8afce90fc954f48efae38b87f37cabc988 x86_64 hpack-0.36.0
2023-11-20 21:24:45.647127: [debug] Checking for project config at: .../stack.yaml
2023-11-20 21:24:45.651389: [debug] Loading project config file stack.yaml
2023-11-20 21:24:45.681590: [debug] Use of Casa server enabled: (CasaRepoPrefix "https://casa.stackage.org", 1280).
2023-11-20 21:24:45.707139: [debug] (SQL) SELECT COUNT(*) FROM "last_performed" WHERE ("action"=?) AND ("timestamp">=?); [PersistInt64 1,PersistUTCTime 2023-11-19 13:24:45.706403 UTC]
2023-11-20 21:24:45.713999: [debug] Using package location completions from a lock file
2023-11-20 21:24:45.729765: [debug] Loaded snapshot from Pantry database.
2023-11-20 21:24:46.042721: [debug] Running Hpack on .../package.yaml
2023-11-20 21:24:46.054086: [debug] Hpack output unchanged in .../agda-language-server.cabal
2023-11-20 21:24:46.054701: [debug] Prefetching git repos: []
2023-11-20 21:24:46.054765: [debug] []
2023-11-20 21:24:46.055803: [debug] Parsing cabal file for Agda (from Hackage)
2023-11-20 21:24:46.057210: [debug] Got blob from Pantry database for Agda-2.6.2.2@sha256:b69c2f317db2886cb387134af00a3e42a06fab6422686938797924d034255a55,37047
2023-11-20 21:24:46.105154: [debug] Asking for a supported GHC version
2023-11-20 21:24:46.105429: [debug] Getting system compiler version
2023-11-20 21:24:46.138231: [debug] (SQL) SELECT "id","actual_version","arch","ghc_path","ghc_size","ghc_modified","ghc_pkg_path","runghc_path","haddock_path","cabal_version","global_db","global_db_cache_size","global_db_cache_modified","info","global_dump" FROM "compiler_cache" WHERE "ghc_path"=?; [PersistText "/usr/local/bin/ghc-8.10.7"]
2023-11-20 21:24:46.199671: [debug] Loaded compiler information from cache
2023-11-20 21:24:46.199829: [debug] Asking for a supported GHC version
2023-11-20 21:24:46.199888: [debug] Resolving package entries
2023-11-20 21:24:46.199954: [debug] Parsing the targets
2023-11-20 21:24:46.203941: [debug] Checking flags
2023-11-20 21:24:46.204312: [debug] SourceMap constructed
2023-11-20 21:24:46.221763: [debug] Run process: /usr/local/bin/ghc-pkg-8.10.7 init /Users/abel/.stack/snapshots/x86_64-osx/63d15aee5b7a1bf5deccdd3e187d8717ed023d339e15844489a96429439b2c15/8.10.7/pkgdb/
2023-11-20 21:24:46.419287: [debug] Process finished in 197216896000ms: /usr/local/bin/ghc-pkg-8.10.7 init /Users/abel/.stack/snapshots/x86_64-osx/63d15aee5b7a1bf5deccdd3e187d8717ed023d339e15844489a96429439b2c15/8.10.7/pkgdb/
2023-11-20 21:24:46.420442: [debug] Run process: /usr/local/bin/ghc-pkg-8.10.7 init /.../.stack-work/install/x86_64-osx/63d15aee5b7a1bf5deccdd3e187d8717ed023d339e15844489a96429439b2c15/8.10.7/pkgdb/
2023-11-20 21:24:46.470519: [debug] Process finished in 49993980000ms: /usr/local/bin/ghc-pkg-8.10.7 init .../.stack-work/install/x86_64-osx/63d15aee5b7a1bf5deccdd3e187d8717ed023d339e15844489a96429439b2c15/8.10.7/pkgdb/
2023-11-20 21:24:46.470839: [debug] Starting to execute command inside EnvConfig
2023-11-20 21:24:46.471269: [debug] Checking for project config at: .../stack.yaml
2023-11-20 21:24:46.471352: [debug] Loading project config file stack.yaml
2023-11-20 21:24:46.472616: [debug] Use of Casa server enabled: (CasaRepoPrefix "https://casa.stackage.org", 1280).
2023-11-20 21:24:46.474439: [debug] (SQL) SELECT COUNT(*) FROM "last_performed" WHERE ("action"=?) AND ("timestamp">=?); [PersistInt64 1,PersistUTCTime 2023-11-19 13:24:46.474404 UTC]
2023-11-20 21:24:46.474849: [debug] Using package location completions from a lock file
2023-11-20 21:24:46.476795: [debug] Loaded snapshot from Pantry database.
2023-11-20 21:24:46.734981: [debug] Running Hpack on .../package.yaml
2023-11-20 21:24:46.740440: [debug] Hpack output unchanged in ....cabal
2023-11-20 21:24:46.740858: [debug] Prefetching git repos: []
2023-11-20 21:24:46.740915: [debug] []
2023-11-20 21:24:46.741878: [debug] Parsing cabal file for Agda (from Hackage)
2023-11-20 21:24:46.742208: [debug] Got blob from Pantry database for Agda-2.6.2.2@sha256:b69c2f317db2886cb387134af00a3e42a06fab6422686938797924d034255a55,37047
2023-11-20 21:24:46.753838: [debug] Asking for a supported GHC version
2023-11-20 21:24:46.753931: [debug] Getting system compiler version
2023-11-20 21:24:46.755844: [debug] (SQL) SELECT "id","actual_version","arch","ghc_path","ghc_size","ghc_modified","ghc_pkg_path","runghc_path","haddock_path","cabal_version","global_db","global_db_cache_size","global_db_cache_modified","info","global_dump" FROM "compiler_cache" WHERE "ghc_path"=?; [PersistText "/usr/local/bin/ghc-8.10.7"]
2023-11-20 21:24:46.815015: [debug] Loaded compiler information from cache
2023-11-20 21:24:46.815166: [debug] Asking for a supported GHC version
2023-11-20 21:24:46.815250: [debug] Resolving package entries
2023-11-20 21:24:46.815310: [debug] Parsing the targets
2023-11-20 21:24:46.818406: [debug] Checking flags
2023-11-20 21:24:46.818498: [debug] SourceMap constructed
2023-11-20 21:24:46.825414: [debug] Starting to execute command inside EnvConfig
/Users/abel/.stack

[mpilgrem]

@andreasabel, I think that should be relatively easy to change. Currently, all the path options happen within:

runHaddock :: Bool -> RIO EnvConfig () -> RIO Runner ()
runHaddock x action = local modifyConfig $
  withConfig YesReexec $
    withDefaultEnvConfig action
 where
  modifyConfig = set
    (globalOptsL.globalOptsBuildOptsMonoidL.buildOptsMonoidHaddockL) (Just x)

but some only need the information in the RIO Config environment and --stack-root only needs the information in the RIO Runner environment.