Recognizing Existing or Prior Nix Installs (Curing)

[Hoverbear]

Some users have reported issues when trying to use this package on a system that had previously had Nix installed on it.

Currently, users will ideally get a planner error (for example, that a file or volume #217 already exists), but in some cases may get an install-time error.

We'd like to address this and make it easier.

This can break down into three major chunks of work:

• Giving better error messages in cases where there are easily fixable problems the user can take. For example in Verify the apfs volume doesn't already exist before trying to create it #217 we tell the user the exact command to run.
• Recognizing artifacts from previous installs, for example, an existing /nix/nix.conf with existing configuration which we can just modify.
  • In particular with /nix/nix.conf we would ideally have a way to 'merge' or 'upsert' the config to include features we toggle on.
  • There is an open question of what to do with these files that on uninstall, as the user might want us to remove /etc/nix.conf if they uninstall.
• Offering some reasonable way to "cure" an existing install.

The last point is a bit wishy-washy so let's consider what that might look like:

• A recognize subcommand which creates receipts for existing (possibly partial) installs and allows for use of uninstall later.
• Some sort of doctor subcommand which essentially follows the recommended remedy steps codified in the first point
• A "Cure" phase that happens between the plan and install phases, where the user is interactively prompted about running remedy steps.
• Allowing actions to add "Cure" steps to the plan and presenting those inside of the plan summary for users.

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TODO:

• Cure APFS/fstab on mac: Cure APFS/Fstabs on Mac #246
• Cure nix.conf: Integrate nix-config-parser #263
• Cure service manager configurations (i.e. systemd's nix-daemon.service/socket, macos's nix-daemon LaunchDaemon)