5b0191f74ab05b187f81ea037623338a615b1619, I eliminated shadowing, on the justification that we should never pick the same component ID (previously installed package ID) for two distinct packages unless the ABIs of the packages were the same, so we could just globally assume that component IDs are unique.
However, I've recently come across some cases where we can't easily actually make this be the case. Here are a few of them:
- GHC's build system assumes that the ghc package will always be a library name of the form HSghc-7.11.a (i.e. that its component ID is ghc-7.11). It's a bit nontrivial to change this assumption, because there are stage1 shenanigans which are rewriting the version number to one without the date. But obviously if you are bootstrapping GHC with itself, the component ID of the GHC you are building will conflict with the component ID of the host GHC.
- A similar situation exist for bootstrapping packages. When compiling GHC, we want to pick fairly deterministic component IDs for it in order to avoid needless rebuilding. But then if you turn around and use one of these builds to bootstrap GHC, if you use the same deterministic scheme you will end up with conflicting component IDs.
- More generally, if you are building a package which is already in the global database (and thus not easily removable), it is sometimes VERY CONVENIENT to be able to pick whatever component ID you want, even if it's in the global database. (1) and (2) are cases of this.
So I think what we want to do is bring back shadowing, but have it operate on a PER-DATABASE basis. For example, if we have the following databases:
pkg.conf.1 foo-0.1-XXX bar-0.2-YYY (depends on foo-0.1-XXX) pkg.conf.2 foo-0.1-XXX baz-0.3-ZZZ (depends on foo-0.1-XXX)
If we stack pkg.conf.2 on top of pkg.conf.1, we invalidate foo-0.1-XXX FROM pkg.conf.1 (which simply means any packages from pkg.conf.1 which refer to it.)
It also occurs to me that we should also be recording the ABIs of packages we depend on, to ensure consistency. Consider:
pkg.conf.1 foo-0.1-XXX (abi is AAA) pkg.conf.2 foo-0.1-XXX (abi is BBB) pkg.conf.3 bar-0.1-YYY (depends on foo-0.1-XXX)
Which is the correct database to use
pkg.conf.3 with? We have no way of telling today! Sure,
pkg.conf.3 is incomplete, but this is a supported mode of operation.