Patching <nixpkgs>

When you use unstable open-source code, you get what you paid for. Well, I suppose you paid for nothing, but what you’re getting can be unstable. When you’re on the bleeding-edge, you get first-swing at all the newest fun code, but things are going to break. There’s no real way around it. You can pack-up and head back to the stable branch, but let’s barrel-forward instead!

In this article, I’ll use, as an example, a specific bug I ran into yesterday. That being said, you should be able to follow this strategy to bypass any number of bugs you encounter in nixpkgs. In short, give this a spin if the following holds true:

• You’re experiencing a bug, and suspect it to be from nixpkgs.
• Someone’s submitted a GitHub pull-request that (probably) fixes this bug.
• This pull-request may or may-not have been accepted yet, but either way it hasn’t been merged into the branch your nixpkgs is sourced from. (nixos-unstable, in my case).
• You don’t want to wait!

Yesterday I was writing a flake.nix for a new project, and as soon as I ran nix develop for the first time, I was hit with this really peculiar error (don’t bother read it):

error: [json.exception.parse_error.101] parse error at line 57, column 178:
syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last
read: '" \n    local fn=\"$1\";\n    local fd;\n    local magic;\n    exec
{fd}< \"$fn\";\n    read -r -n 4 -u \"$fd\" magic;\n    exec {fd}>&-;\n
if [[ \"$magic\" = ''

My project has nothing to do with JSON, and my flake.nix isn’t even 57 lines long, so I was pretty sure this error wasn’t my doing. Fortunately, the error message was unique enough that searching for "nixos""json.exception.parse_error.101" quickly lead me to a relevant GitHub Issue: “get-env.sh interprets hex literals in function bodies.”

I have no idea what this issue is about, or even what these smart programmers are talking about. To be honest, I just don’t have the time right now to figure it out. However, if we scroll down the page, we can see the original poster, winterqt, has already submitted a pull-request will a potential fix: “stdenv: remove isMachO helper function”. What more, it’s already been approved and merged. Thanks for your hard work, winterqt!

Now, just because this fix has been approved and merged, that doesn’t necessarily mean that it’s available in our channel of nixpkgs, even when we’re using nixos-unstable! We can use a neat tool written by Alyssa Ross to check the status of this PR: Nixpkgs Pull Request Tracker: 138186.

It will get to our channel eventually, but we want to work on our project right now!

In our example scenario, we’re just starting a new project, so our flake.nix is still pretty light:

 1: {
 2:   description = "A totally fine flake.nix with a buggy nixpkgs";
 3: 
 4:   # TODO: uncomment the next line to reproduce the example bug described in this article.
 5:   # inputs.nixpkgs.url = github:nixos/nixpkgs?rev=79c444b5bdeaba142d128afddee14c89ecf2a968;
 6: 
 7:   outputs = { self, nixpkgs }:
 8:     let
 9:       system = "x86_64-linux";
10:       pkgs = import nixpkgs { inherit system; };
11:     in {
12:       devShell.${system} = pkgs.mkShell {
13:         buildInputs = with pkgs; [ ddate ];
14:         shellHook = ''
15:           echo "Welcome to your project. Today is $(ddate)."
16:         '';
17:       };
18:     };
19: }

The above flake.nix is totally fine and on a normal day, running nix develop would present you with something like:

$ nix develop

Welcome to your project. Today is Today is Setting Orange, the 46th day of Bureaucracy in the YOLD 3187.

[jon@ertt.ca:~/my-project]$

But today we get error: [json.exception.parse_error.101] parse error at line 57, column 178: .... So let’s fix it!

Days, weeks, or months later, when upstream nixpkgs is working fine again, all you have to do is revert your flake.nix back to its original version and delete the .patch file from your project repo. You may also want to run nix store gc to recover a bit of disk space.