Allow ignoring global package db

[duncan]

For sandboxing, users want to be able to use only local package dbs. To handle rts, base etc, they copy those specific packages from the global package db into a local package db.

Currently however there is no sensible way to tell ghc to look only at the listed package dbs, and not the global db. There is a flag -no-user-package-conf to avoid looking at the per-user one, but no equivalent to ignore or to specify the global one.

(Note: as a quirky non-sensible way, you can use GHC_PACKAGE_PATH="" to clear the package db stack, but this should be possible using command line flags.)

The user interface we're now using in cabal is like this:

• --global sets the initial db stack to [GlobalPackageDB]
• --user sets the initial db stack to [GlobalPackageDB, UserPackageDB]
• --package-db=clear clears the db stack to []
• --package-db=global pushes GlobalPackageDB on the top of the stack
• --package-db=user pushes UserPackageDB on the top of the stack
• --package-db=${otherfile} pushes SpecificPackageDB ${otherfile} on the top of the stack

It seems to me we could do something similar with ghc. Currently in ghc we have a -hide-all-packages flag that resets the package set to empty. We could do with an equivalent for the package database stack. I suggest something like:

• -clear-package-conf
• -global-package-conf
• -user-package-conf (and the existing -no-user-package-conf)
• -package-conf file

The default would remain that the global and user dbs are used, but this can be reset using -clear-package-conf.

Personally I prefer the nomenclarture "package db" rather than "package conf" here, but it's not that important.

[simonmar]

Yep, let's do something for 7.6.1.

[pcapriotti]

The attached patch series adds the extra flags described in this ticket (with the addition of no-global-package-conf, for symmetry), and renames them to *-package-db.

You can find the corresponding patch for Cabal in my github fork:  https://github.com/pcapriotti/cabal

[simonmar]

I had a look at the patches. The situation in DynFlags is now a bit strange - we have both the Opt_ReadGlobalPackageConf options and the [PkgConfRef]. I think we should have just the [PkgConfRef] representing the stack, and all the flags should manipulate it directly.

So

• We start with [global,user]
• -no-user-package-db removes user from the list (similarly for global)
• -user-package-db adds user to the end (similarly for global)
• -package-conf file adds file to the end
• -clear-package-db sets the list to empty

[pcapriotti]

I'm not sure how that would interact with GHC_PACKAGE_PATH. For example, if I run:

GHC_PACKAGE_PATH="foo:" ghc --no-user-package-db

I'd expect it to use [global, foo] as db stack, but we can't remove user from the stack originating from GHC_PACKAGE_PATH unless we keep the information somewhere (currently it's in Opt_ReadUserPackageConf).

So at the moment there is an "initial stack", which is [global, user] by default, and is affected by -no-user-package-db, -no-global-package-db, -clear-package-db, and GHC_PACKAGE_PATH. The other flags append to the stack *after* the initial stack has been determined.

[simonmar]

I find it weird that -no-user-package-db affects the "initial stack", but -user-package-db appends the user db to the final stack. So e.g. -user-package-db -no-user-package-db does not do what you expect.

So I see two alternatives. One is to modify my proposal to take into account GHC_PACKAGE_PATH: just initialise the stack from GHC_PACKAGE_PATH before processing the arguments, defaulting to [global,user] if GHC_PACKAGE_PATH is not set.

The other alternative is to keep the concept of the "initial stack", but make -user-package-db and -global-package-db just the inverses of -no-user-package-db and -no-global-package-db respectively. I'd be ok with that too.

[pcapriotti]

The attached patch implements option 1. The list of flags is now maintained as a [PkgConfRef] -> [PkgConfRef], which is later applied to the base stack obtained by GHC_PACKAGE_PATH.

[simonmar]

Yes, that looks ok.

[p.capriotti@…]

commit 6a831be4aa73e86568256813ffa862d7cfd5732d

Author: Paolo Capriotti <p.capriotti@gmail.com>
Date:   Thu May 3 11:29:51 2012 +0100

    Add flags to manipulate package db stack (#5977)
    
    Introduce new flags to allow any package database stack to be set up.
    The `-no-user-package-conf` and `-no-global-package-conf` flags remove
    the corresponding package db from the initial stack, while
    `-user-package-conf` and `-global-package-conf` push it back on top of
    the stack.

 compiler/main/DynFlags.hs  |   24 +++++++++++--
 compiler/main/Packages.lhs |   81 +++++++++++++++++++++-----------------------
 2 files changed, 60 insertions(+), 45 deletions(-)