by setting the ‘outputs’ attribute. For example:
stdenv.mkDerivation {
name = "aterm-2.5";
src = ...;
outputs = [ "out" "tools" "dev" ];
configureFlags = "--bindir=$(tools)/bin --includedir=$(dev)/include";
}
This derivation creates three outputs, named like this:
/nix/store/gcnqgllbh01p3d448q8q6pzn2nc2gpyl-aterm-2.5
/nix/store/gjf1sgirwfnrlr0bdxyrwzpw2r304j02-aterm-2.5-tools
/nix/store/hp6108bqfgxvza25nnxfs7kj88xi2vdx-aterm-2.5-dev
That is, the symbolic name of the output is suffixed to the store
path (except for the ‘out’ output). Each path is passed to the
builder through the corresponding environment variable, e.g.,
${tools}.
The main reason for multiple outputs is to allow parts of a package
to be distributed and garbage-collected separately. For instance,
most packages depend on Glibc for its libraries, but don't need its
header files. If these are separated into different store paths,
then a package that depends on the Glibc libraries only causes the
libraries and not the headers to be downloaded.
The main problem with multiple outputs is that if one output exists
while the others have been garbage-collected (or never downloaded in
the first place), and we want to rebuild the other outputs, then
this isn't possible because we can't clobber a valid output (it
might be in active use). This currently gives an error message
like:
error: derivation `/nix/store/1s9zw4c8qydpjyrayxamx2z7zzp5pcgh-aterm-2.5.drv' is blocked by its output paths
There are two solutions: 1) Do the build in a chroot. Then we don't
need to overwrite the existing path. 2) Use hash rewriting (see the
ASE-2005 paper). Scary but it should work.
This is not finished yet. There is not yet an easy way to refer to
non-default outputs in Nix expressions. Also, mutually recursive
outputs aren't detected yet and cause the garbage collector to
crash.
write ‘attrs ? a.b’ to test whether ‘attrs’ has an attribute ‘a’
containing an attribute ‘b’. This is more convenient than ‘attrs ?
a && attrs.a ? b’.
Slight change in the semantics: it's no longer an error if the
left-hand side of ‘?’ is not an attribute set. In that case it just
returns false. So, ‘null ? foo’ no longer throws an error.
checked too soon whether substitutes are available. That is, it did
so for every available package, rather than those matching installed
packages. This was very slow and subject to assertion failures. So
do the check much later. Idem for `nix-env -qab' and `nix-env -ib'.
little RAM. Even if the memory isn't actually used, it can cause
problems with the overcommit heuristics in the kernel. So use a VM
space of 25% of RAM, up to 384 MB.
while checking the contents, since this operation can take a very
long time to finish. Also, fill in missing narSize fields in the DB
while doing this.
even with a very long busy timeout, because SQLITE_BUSY is also
returned to resolve deadlocks. This should get rid of random
"database is locked" errors. This is kind of hard to test though.
* Fix a horrible bug in deleteFromStore(): deletePathWrapped() should
be called after committing the transaction, not before, because the
commit might not succeed.
tree). This saves a lot of memory. The vector should be sorted so
that names can be looked up using binary search, but this is not the
case yet. (Surprisingly, looking up attributes using linear search
doesn't have a big impact on performance.)
Memory consumption for
$ nix-instantiate /etc/nixos/nixos/tests -A bittorrent.test --readonly-mode
on x86_64-linux with GC enabled is now 185 MiB (compared to 946
MiB on the trunk).
improves GC effectiveness a bit more (because a live value doesn't
keep other values in the environment plus the parent environments
alive), and removes the need for copy nodes.
a pointer to a Value, rather than the Value directly. This improves
the effectiveness of garbage collection a lot: if the Value is
stored inside the set directly, then any live pointer to the Value
causes all other attributes in the set to be live as well.
