Convenience libraries

[ezyang]

Implement "convenience libraries", fixes #269.

Convenience libraries are package-private libraries
that can be used as part of executables, libraries, etc
without being exposed to the external world. Private
libraries are signified using the

library foo

stanza. Within a Cabal package, the name convenience library
shadows the conventional meaning of package name in
build-depends, so that references to "foo" do not indicate
foo in Hackage, but the convenience library defined in the
same package. (So, don't shadow Hackage packages!)

This commit implements convenience libraries such that they
ARE installed the package database (this prevents us from
having to special case dynamically linked executables);
in GHC 7.10 and later they are installed under the same
package name as the package that contained them, but have
a distinct "component ID" (one pay off of making the distinction
between component IDs and installed package IDs.)

There is a "default" library which is identified by the fact
that its library name coincides with the package name. There
are some new convenience functions to permit referencing this.

TODO: if we REALLY don't want to install a convenience library,
we could add a "installable:" flag which toggles whether or
not copy/register should install it. Extra sanity checking
is necessary in this case.

TODO: docs

TODO: cabal-install must ignore convenience libraries when
dep solving

NB: there are a few extra commits in this PR which should be split into their own PRs. Annoying to do slightly.

Signed-off-by: Edward Z. Yang ezyang@cs.stanford.edu

[dcoutts]

In principle this is a great feature.

From brief discussion we think not installing the convenience library when it's not needed is a good improvement, and we can determine exactly when it's needed or not so no user visible flag is needed.

[dcoutts]

For other people reviewing this: the important point is that these libs are not available for other packages to depend on, so it's still the case that one package provides at most one library. And the installed package id for private libs is picked such that it can never clash with primary public libs from any other package.

[23Skidoo]

@ezyang Do you want this to go into 1.24? Otherwise I think I'll postpone merging this PR until the 1.24 branch is created.

[ezyang]

Yeah, I'm fine with postponing this after 1.24.

[ezyang]

Found a bug: internal libraries and the main libraries all get installed to the same directory.

[ezyang]

So, I fixed all those bugs, and along the way also fixed #1893 and also did a bit of refactoring. The patchset got longer but these are all self-contained commits.

[ezyang]

Blah, I broke buildinfo parsing. I'll fix it and add a test.

[ezyang]

(NB: this branch has been rebased on top of #3047)

[23Skidoo]

@ezyang

Can you please create tickets for the remaining TODOs?

As far as I can tell (it's a little difficult since when I rebased we lost the old comments) all comments have been addressed. I'll merge.

I meant the TODOs you mentioned in the top comment, i.e. docs, a change to the solver and the "installable" flag. Though I see that docs have now been added.

[ezyang]

Oh yeah, those comments are out of date. The "installable" flag is irrelevant now, since Cabal just "does the right thing" now to determine if a library should be installed or not. Solver changes are being tracked at #3264/#3257.

[23Skidoo]

Great.