nix is not in binary cache for aarch64

[Gerschtli]

Describe the bug

When building nix on aarch64, it should be fetched from binary cache. Instead I need to rebuild it, which is really time consuming because of recompiling nlohmann_json.

To Reproduce
Steps to reproduce the behavior:

1. Run nix-build test.nix with test.nix:

with import <nixpkgs> { };

stdenv.mkDerivation {
  name = "test";

  buildInputs = [
    nix
  ];
}

2. I see the following output which shows that the packages are going to be rebuilt.

these derivations will be built:
  /nix/store/cnfnd4dm74i4cydcsqchs0plc74b56hf-nlohmann_json-3.6.1.drv
  /nix/store/qdhngkjj1lb9589dwa9psvfbqzyy3dfx-nix-2.3.drv
  /nix/store/pzz87q87cwwc357wm1ikf9q5jihjabjy-test.drv
building '/nix/store/cnfnd4dm74i4cydcsqchs0plc74b56hf-nlohmann_json-3.6.1.drv'...
unpacking sources
unpacking source archive /nix/store/5xagvh9xb69njglwpdbzq814l4dv6xgi-source

Expected behavior

nix and all its dependencies should always be fetchable via the official binary cache.

Additional context

nlohmann_json is failing sometimes in hydra, which might cause the issue, see https://hydra.nixos.org/job/nixos/release-19.09-aarch64/nixpkgs.nlohmann_json.aarch64-linux. But as nix depends on it, we should provide more stability in its build.

Metadata

• system: "aarch64-linux"
• host os: Linux 5.3.7, NixOS, 19.09.976.c75de8bc12c (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3
• channels(root): "home-manager, nixos-19.09.1046.a1b46bd1252, nur-gerschtli, unstable-20.03pre199897.471869c9185"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
  - nix
  - nlohmann_json
  # probably all versions of the nix derivation
# a list of nixos modules affected by the problem
module:

[prusnak]

Seems that nlohmann_json timeouts in test test-unicode_all which checks 8860608 test cases and this does not fit into the 1500 second limit (log).

Suggestion: increase the time limit for this particular test or decrease the number of cases for this test.

Oddly enough this package has no maintainer, which is not ideal as this is a critical package because it's a direct dependency of nix.

CCing @dtzWill as he did most of the package changes recently.

[dtzWill]

• Wonder if 3.7.0 (on master, not 19.09) has the same behavior?
• Nix includes a vendored copy of nlohmann_json, providing this as a buildInput seems redundant and potentially problematic (although mostly redundant)

Perhaps it can just be dropped? Looks like all tags of 2.0 have the file, so maybe only needed for nix1? Only limits this issue to nix1, and kinda dodges the real issue of the tests taking too long on aarch64.

[dtzWill]

Upstream: nlohmann/json#1816 . Shall we just drop the test on aarch64, or....?

[dtzWill]

https://hydra.nixos.org/job/nixpkgs/trunk/nlohmann_json.aarch64-linux <-- limited data for 3.7.0 on trunk :).

[grahamc]

19.09 is blocked on this as well #72890

[vcunat]

Workaround merged; reopen in case this reappears.