importPaths incompatibility with latest nix

[ivan]

I am having trouble building Hydra on nixpkgs master. Bumping hydra-unstable to 6318866 and building shows:

g++ -DHAVE_CONFIG_H -I. -I../..    -std=c++17 -I/nix/store/2xhb4hlskn33pbyph36v4wxcan56dnrw-boehm-gc-8.0.4-dev/include -I/nix/store/5rjgsqjdm71cflfb68q7m771a1rqcsk3-nix-2.4pre20200719_a79b6dd-dev/include/nix -Wall -I ../libhydra -Wno-deprecated-declarations -g -O2 -std=c++17 -include nix/config.h -c -o hydra_queue_runner-build-remote.o `test -f 'build-remote.cc' || echo './'`build-remote.cc
build-remote.cc: In member function 'void State::buildRemote(nix::ref<nix::Store>, Machine::ptr, Step::ptr, unsigned int, unsigned int, unsigned int, RemoteResult&, std::shared_ptr<State::ActiveStep>, std::function<void(StepState)>)':
build-remote.cc:447:70: error: no matching function for call to 'nix::Store::importPaths(nix::FdSource&, std::shared_ptr<nix::FSAccessor>&, nix::CheckSigsFlag)'
  447 |             destStore->importPaths(from, result.accessor, NoCheckSigs);
      |                                                                      ^
In file included from /nix/store/5rjgsqjdm71cflfb68q7m771a1rqcsk3-nix-2.4pre20200719_a79b6dd-dev/include/nix/parsed-derivations.hh:1,
                 from state.hh:13,
                 from build-remote.cc:9:
/nix/store/5rjgsqjdm71cflfb68q7m771a1rqcsk3-nix-2.4pre20200719_a79b6dd-dev/include/nix/store-api.hh:628:16: note: candidate: 'nix::StorePaths nix::Store::importPaths(nix::Source&, nix::CheckSigsFlag)'
  628 |     StorePaths importPaths(Source & source, CheckSigsFlag checkSigs = CheckSigs);
      |                ^~~~~~~~~~~
/nix/store/5rjgsqjdm71cflfb68q7m771a1rqcsk3-nix-2.4pre20200719_a79b6dd-dev/include/nix/store-api.hh:628:16: note:   candidate expects 2 arguments, 3 provided
make[3]: *** [Makefile:546: hydra_queue_runner-build-remote.o] Error 1

nixpkgs hydra-unstable depends on nixFlakes which now includes Remove 'accessor' from addToStore() NixOS/nix@545bb2e, but Hydra was not updated for that change.

[edolstra]

It's best to stick with the version of nix recorded in the lock file (1ab9da915422405452118ebb17b88cdfc90b1e10) for now.

[Ma27]

@ivan I'm well-aware of that. Unfortunately I don't have the time and knowledge atm to implement a proper fix. See also https://logs.nix.samueldr.com/nixos-dev/2020-07-21#1595348242-1595348822;

Another option I can think of is to add a third Nix package which is proven to work with Hydra (i.e. the nixUnstable from Hydra's flake.lock). While this isn't the prettiest solution, it fixes the situation in nixpkgs until we can get rid of all of this when flakes become stable.