network: Add nixpkgs attribute for passing custom nixpkgs

[adisbladis]

This is useful to pin nixpkgs without flakes.

[cole-h]

Is using this as simple as setting network.nixpkgs = sources.nixpkgs; or something?

Would be nice if there were some docs to accompany this describing how to use it -- feel free to take inspiration from my suggestion over at #1331.

[adisbladis]

An example of how to use this is:

{
  network.description = "Test deployment";
  network.nixpkgs = import <nixpkgs> {};

  myhost = { config, lib, resources, ... }: {};
}

[cole-h]

On this branch, if I set network.nixpkgs = import sources.nixpkgs { }; and I run NIX_PATH= nixops deploy --dry-activate, I get the following error:

error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)
Traceback (most recent call last):
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/bin/.nixops-wrapped", line 9, in <module>
    sys.exit(main())
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/__main__.py", line 710, in main
    args.op(args)
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/script_defs.py", line 638, in op_deploy
    max_concurrent_activate=args.max_concurrent_activate,
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/deployment.py", line 1418, in deploy
    self.run_with_notify("deploy", lambda: self._deploy(**kwargs))
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/deployment.py", line 1407, in run_with_notify
    f()
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/deployment.py", line 1418, in <lambda>
    self.run_with_notify("deploy", lambda: self._deploy(**kwargs))
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/deployment.py", line 1334, in _deploy
    dry_run=dry_run, repair=repair, include=include, exclude=exclude
  File "/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/deployment.py", line 761, in build_configs
    text=True,
  File "/nix/store/q732h09azy7lf0j30bnnhdl15p4rxpdy-python3-3.7.7/lib/python3.7/subprocess.py", line 411, in check_output
    **kwargs).stdout
  File "/nix/store/q732h09azy7lf0j30bnnhdl15p4rxpdy-python3-3.7.7/lib/python3.7/subprocess.py", line 512, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['nix-instantiate', '--find-file', 'nixpkgs/nixos', '-I', 'nixops=/nix/store/50l5y9imknxf9hx58kmy46ffbfwhf32q-python3.7-nixops-1.8.0/lib/python3.7/site-packages/nixops/../nix']' returned non-zero exit status 1.

This appears to stem from

nixops/nixops/deployment.py

Line 759 in 231e43f

["nix-instantiate", "--find-file", "nixpkgs/nixos"]

(as you'll probably notice in the traceback).

[cole-h]

I fixed my previous issue with the following diff. However, note that it's very brittle -- it relies on self.nix_exprs[0] being the network file that has network.nixpkgs defined in order to get the nixpkgs path, in addition to preventing any other -I nixpkgs= flags from taking effect (because AFAICT the first of a flag wins over all the others). Now I can run NIX_PATH= nixops deploy --dry-activate and it works as expected! Another note is that NIX_PATH= nixops deploy without a network.nixpkgs defined fails as it did before.

diff --git a/nixops/deployment.py b/nixops/deployment.py
index b3ccb40f..bc389237 100644
--- a/nixops/deployment.py
+++ b/nixops/deployment.py
@@ -360,6 +360,18 @@ class Deployment:
             path for paths in get_plugin_manager().hook.nixexprs() for path in paths
         ]
 
+        nixpkgs_path = subprocess.run(
+            [
+                "nix-instantiate",
+                "--eval",
+                # FIXME: self.nix_exprs[0] is not guaranteed to always work
+                "-E" "(import {0}).network.nixpkgs.path".format(self.nix_exprs[0]),
+            ],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.DEVNULL,
+            text=True,
+        ).stdout.rstrip()
+
         flags = (
             list(
                 itertools.chain(
@@ -371,6 +383,9 @@ class Deployment:
             )
             + self.extra_nix_flags
         )
+        flags.extend(
+            ["-I", "nixpkgs=" + nixpkgs_path] if nixpkgs_path is not "" else []
+        )
         flags.extend(["-I", "nixops=" + self.expr_path])
         return flags
 

[lovesegfault]

Is anything blocking this?