paths (e.g., `/nix/store/...random-hash...-aterm'), which are
subsequently rewritten to actual content-addressable store paths
(i.e., the hash part of the store path equals the hash of the
contents).
A complication is that the temporary output paths have to be passed
to the builder (e.g., in $out). Likewise, other environment
variables and command-line arguments cannot contain fixed store
paths because their names are no longer known in advance.
Therefore, we now put placeholder store paths in environment
variables and command-line arguments, which we *rewrite* to the
actual paths prior to running the builder.
TODO: maintain the mapping of derivation placeholder outputs
("output path equivalence classes") to actual output paths in the
database. Right now the first build succeeds and all its
dependencies fail because they cannot find the output of the first.
TODO: locking is no longer an issue with random temporary paths, but
at the cost of having no blocking if we build the same thing twice
in parallel. Maybe the "random" path should actually be a hash of
the placeholder and the name of the user who started the build.
This simplifies garbage collection and `nix-store --query
--requisites' since we no longer need to treat derivations
specially.
* Better maintaining of the invariants, e.g., setReferences() can only
be called on a valid/substitutable path.
promise :-) This allows derivations to specify on *what* output
paths of input derivations they are dependent. This helps to
prevent unnecessary downloads. For instance, a build might be
dependent on the `devel' and `lib' outputs of some library
component, but not the `docs' output.
`derivations.cc', etc.
* Store the SHA-256 content hash of store paths in the database after
they have been built/added. This is so that we can check whether
the store has been messed with (a la `rpm --verify').
* When registering path validity, verify that the closure property
holds.
