If we want to be careful about hitting the stack protector page, we should use `-fstack-check` instead.
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
This removes a dynamic stack allocation, making the derivation
unparsing logic robust against overflows when large strings are
added to a derivation.
Overflow behavior depends on the platform and stack configuration.
For instance, x86_64-linux/glibc behaves as (somewhat) expected:
$ (ulimit -s 20000; nix-instantiate tests/lang/eval-okay-big-derivation-attr.nix)
error: stack overflow (possible infinite recursion)
$ (ulimit -s 40000; nix-instantiate tests/lang/eval-okay-big-derivation-attr.nix)
error: expression does not evaluate to a derivation (or a set or list of those)
However, on aarch64-darwin:
$ nix-instantiate big-attr.nix ~
zsh: segmentation fault nix-instantiate big-attr.nix
This indicates a slight flaw in the single stack protection page
approach that is not encountered with normal stack frames.
Rather than having them plain strings scattered through the whole
codebase, create an enum containing all the known experimental features.
This means that
- Nix can now `warn` when an unkwown experimental feature is passed
(making it much nicer to spot typos and spot deprecated features)
- It’s now easy to remove a feature altogether (once the feature isn’t
experimental anymore or is dropped) by just removing the field for the
enum and letting the compiler point us to all the now invalid usages
of it.
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.
In particular, this now works:
$ nix path-info --eval-store auto --store https://cache.nixos.org nixpkgs#hello
Previously this would fail as it would try to upload the hello .drv to
cache.nixos.org. Now the .drv is instantiated in the local store, and
then we check for the existence of the outputs in cache.nixos.org.
Once a build is done, get back to the original derivation, and register
all the newly built outputs for this derivation.
This allows Nix to work properly with derivations that don't have all
their build inputs available − thus allowing garbage collection and
(once it's implemented) binary substitution
PRs #4370 and #4348 had a bad interaction in that the second broke the fist
one in a not trivial way.
The issue was that since #4348 the logic for detecting whether a
derivation output is already built requires some logic that was specific
to the `LocalStore`.
It happens though that most of this logic could be upstreamed to any `Store`,
which is what this commit does.
Rather than storing the derivation outputs as `drvPath!outputName` internally,
store them as `drvHashModulo!outputName` (or `outputHash!outputName` for
fixed-output derivations).
This makes the storage slightly more opaque, but enables an earlier
cutoff in cases where a fixed-output dependency changes (but keeps the
same output hash) − same as what we already do for input-addressed
derivations.
If we resolve using the known path of a derivation whose output we
didn't have, we previously blew up. Now we just fail gracefully,
returning the map of all outputs unknown.
