ghc-pkg update should fail if dependent packages might break

[simonmar]

If a package is recompiled and re-installed, and other packages depend on it, those packages might be broken; e.g. see #3060.

Hence ghc-pkg update should refuse to perform the update, unless explicitly overridden with --force. However, it's not quite as simple as that:

• The package might already have been installed over the top of the old one, and it's too late to reverse that.

• cabal-install likes to build and install a bunch of packages at the same time, using ghc-pkg update on each one. This will fail.

• The GHC build system uses ghc-pkg update to register packages, and this breaks when packages are re-built (it's ok for a clean build, though).

I've attached a couple of patches to implement it, but let's hold these back until the above issues are sorted out.

[simonmar]

patch for ghc

[simonmar]

patch for testsuite

[duncan]

For cabal-install to update a whole bunch of packages correctly we would need more than just ghc-pkg update, if we implement this check. We would need to be able to register/update a whole batch of packages at once. Or we'd have to unregister each package and all its dependencies (in reverse order) and then re-register them all (assuming we did actually update them all).

[malcolm.wallace@…]

Or cabal-install could use ghc-pkg register --force on all dependent packages (but not on the root package?)

[simonpj]

Closing because

• cabal install already checks
• Sooner or later (sooner we hope) Cabal will be able to install multiple instances of a package without breakage.