Only substitute wanted outputs of a derivation

If a derivation has multiple outputs, then we only want to download
those outputs that are actuallty needed.  So if we do "nix-build -A
openssl.man", then only the "man" output should be downloaded.
Likewise if another package depends on ${openssl.man}.

The tricky part is that different derivations can depend on different
outputs of a given derivation, so we may need to restart the
corresponding derivation goal if that happens.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index 6b7c3e6ab1..703410b925 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -85,7 +85,9 @@ extern DrvHashes drvHashes;
 typedef std::pair<string, std::set<string> > DrvPathWithOutputs;
 DrvPathWithOutputs parseDrvPathWithOutputs(const string & s);
 
-Path makeDrvPathWithOutputs(const Path & drvPath, std::set<string> outputs);
+Path makeDrvPathWithOutputs(const Path & drvPath, const std::set<string> & outputs);
+
+bool wantOutput(const string & output, const std::set<string> & wanted);
 
 
 }