[Nix-dev] Multiple outputs and hierarchical Nix store paths

[Vladimír Čunát]

On 05/21/2016 03:53 PM, Bjørn Forsman wrote:
> In Nix:
> I'm actually unsure about the details here, but I *think* the standard
> builder simply takes the first output of each package in buildInputs. [...]

Yes, the first but that one typically propagates other outputs - by
default the ones including headers, libraries and binaries. You can
choose which by setting propagatedBuildOutputs = [ "foo" "bar" "out" ];

> [...]
> First I tried to figure out a way to union Nix outputs into one "all"
> output. The idea is that we need more finely split packages, but that
> isn't really feasible unless we can easily bundle them together first,
> for builds. But I couldn't come up with anything that wouldn't end up
> referencing the "all" output, which would prevent garbage collection.

BTW, there is also myPackage.all - a list of all output paths.

> [...]
> $ nix-build -A pkgconfig
> /nix/store/wndacaca5dq5cpyfa41v1fv4sfhr0y77-pkg-config-0.29
> $ nix-build -A pkgconfig.share
> /nix/store/wndacaca5dq5cpyfa41v1fv4sfhr0y77-pkg-config-0.29/share
> $ nix-build -A pkgconfig.share.doc
> /nix/store/wndacaca5dq5cpyfa41v1fv4sfhr0y77-pkg-config-0.29/share/doc
> 
> (Note that all outputs have the same store path prefix.)
> [...]

That is similar to what regular distros use... only they drop the hashed
prefixes completely. The problem is with detecting runtime dependencies.
First, we only scan for the hash part AFAIK. We might change that to
scan for the whole prefix, but if someone gets a reference to
pkgconfig.share.doc, one can do ../../lib/foo and get to other outputs
(!) Sure it would actually solve some problems due to packages assuming
everything is in the same output, but we would sacrifice part of purity,
e.g. GC could break reachable stuff.

--Vladimir


-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 3771 bytes
Desc: S/MIME Cryptographic Signature
Url : http://lists.science.uu.nl/pipermail/nix-dev/attachments/20160522/4ba869cb/attachment-0001.bin