I had started the trend of doing `std::visit` by value (because a type
error once mislead me into thinking that was the only form that
existed). While the optomizer in principle should be able to deal with
extra coppying or extra indirection once the lambdas inlined, sticking
with by reference is the conventional default. I hope this might even
improve performance.
That doesn’t really make sense with CA derivations (and wasn’t even
really correct before because of FO derivations, though that probably
didn’t matter much in practice)
Move the `closure` logic of `computeFSClosure` to its own (templated) function.
This doesn’t bring much by itself (except for the ability to properly
test the “closure” functionality independently from the rest), but it
allows reusing it (in particular for the realisations which will require
a very similar closure computation)
This avoids an ambiguity where the `StorePathWithOutputs { drvPath, {}
}` could mean "build `brvPath`" or "substitute `drvPath`" depending on
context.
It also brings the internals closer in line to the new CLI, by
generalizing the `Buildable` type is used there and makes that
distinction already.
In doing so, relegate `StorePathWithOutputs` to being a type just for
backwards compatibility (CLI and RPC).
In particular, this means that derivations can output derivations. But
that ramification isn't (yet!) useful as we would want, since there is
no way to have a dependent derivation that is itself a dependent
derivation.
I’m not 100% sure this is wanted since it kind of makes everything
have to know about ca even if they don’t really want to. But it also
make things easier in dealing with looking up ca.
Originally, the test was only checking for different “real” storeDir.
That’s an easy case to handle, but the much harder one is if different
virtual store dirs are used. To do this, we need the SubstitutionGoal
to know about the ca, so it can recalculate the path to copy it over.
An important note here is that the store path passed to copyStorePath
needs to be one for srcStore - so that queryPathInfo works properly.
This also adds an error message when the store path from queryPathInfo
is different from the one we requested.
Substituters can substitute from one store dir to another with a
little bit of help. The store api just needs to have a CA so it can
recompute the store path based on the new store dir. We can only do
this for fixed output derivations with no references, though.
Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.
Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.
Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).
