Install cabal-install, firstly run cabal install --lib base, and vanilla GHC stops working

[gksato]

Describe the bug

If you execute cabal install --lib base as the first command for the installation of cabal-install, vanilla GHC stops working.

The command neither installs nor builds any package, but will create ~/.ghc/<arch>-<os>-<ghcver>/environments/default. If this is before any build of packages, the file ~/.cabal/store/ghc-<ghcver>/package.db is and continues to be non-existent, but it gets pointed to by the newly created ~/.ghc/<arch>-<os>-<ghcver>/environments/default. The env file is in an invalid state! As a result, vanilla GHC stops working until you delete the env file or you build something with cabal-install.

To Reproduce

Steps to reproduce the behavior:

# create vanilla installation
$ rm -rf ~/.cabal
$ curl --proto '=https' --tlsv1.2 -sSf https://get-ghcup.haskell.org | BOOTSTRAP_HASKELL_NONINTERACTIVE=1 BOOTSTRAP_CABAL_VERSION=3.8.1.0 BOOTSTRAP_GHC_VERSION=9.4.4 sh

$ cabal v2-update
Downloading the latest package list from hackage.haskell.org
Package list of hackage.haskell.org has been updated.
The index-state is set to 2023-03-01T23:54:23Z.
To revert to previous state run:
    cabal v2-update 'hackage.haskell.org,2023-02-02T05:57:26Z'

$ cabal v2-install --lib base
Resolving dependencies...
Up to date

$ ghci
Loaded package environment from $HOME/.ghc/x86_64-linux-9.4.4/environments/default
GHCi, version 9.4.4: https://www.haskell.org/ghc/  :? for help
ghc-9.4.4: can't find a package database at $HOME/.cabal/store/ghc-9.4.4/package.db

Expected behavior

Vanilla GHC should work after cabal v2-install --lib base. We have, at least, three options:

• cabal v2-install --lib base should not create ~/.ghc/<arch>-<os>-<ghcver>/environments/default.
• cabal v2-install --lib base should ensure that the cabal-managed package database exists.
• cabal v2-install --lib base should not write the dependency on the cabal-managed package database in an env file until an actual dependency appears.

I have no idea which is the best among these three options or whether there is a better option.

System information

• OS: Ubuntu 22.10 on Docker on MacOS Ventura 13.2.1
• cabal-install: 3.8.1.0
• GHC: 9.4.4

[gbaz]

Good catch. I think the correct and general answer would be cabal v2-install --lib ensures that the database exists before writing an env file pointing to it.

[Mikolaj]

Would it ensure by failing or by creating it?

[gksato]

What I meant is by creating it. "By failing" would mean doing nothing, so I considered it included in the first option.

[gbaz]

Good catch mikolaj, my proposal would actually be "ensure by failing" not creating -- as I think creating a vacuous empty packagedb is a bit silly -- and further, sort of invasive since it would involve mucking around with packagedb logic.

On the other hand, simply not creating an env file but emitting a warning can be done entirely as a local change to one function: https://github.com/haskell/cabal/blob/master/cabal-install/src/Distribution/Client/CmdInstall.hs#L697

[gksato]

@gbaz Sounds completely sensible. And you say "ensure" to mean cabal v2-install --lib base should create an env fille if cabal's package database exists and do nothing with a warning only in the case the db does not exist, right? After all, some users might actually want to work in an environment where only base is available, and after #8607 the command should create such an environment.