Zero Hydra Failures towards 26.05 NixOS release

NixOS-26.05 is around the corner! The schedule says it should happen around 25 May.

Today the release process entered ZHF phase. There most packages with build failures are expected to be addresses by the release date. https://github.com/NixOS/nixpkgs/issues/516381 contains detailed step-by-step to identify interesting packages.

This time nixpkgs is full of build failures related to gcc-15 changes (-std=gnu23 switch).

An example package fix

I usually try to fix at least one package during ZHF. Let’s do it again.

I picked ior package. hydra says it is broken since January 2026. Most recent https://hydra.nixos.org/build/327774029 build failure has the build log:

  CC       libaiori_a-option.o
option.c: In function 'option_parse_token':
option.c:325:19: error: too many arguments to function 'fp'; expected 0, have 1
  325 |                   fp(arg);
      |                   ^~ ~~~
make[3]: *** [Makefile:1281: libaiori_a-option.o] Error 1

Looks like a typical gcc-15 failure. Trying to reproduce it locally:

$ git clone --depth 1 https://github.com/NixOS/nixpkgs
$ cd nixpkgs
$ nix build --no-link -f. ior -L
...
ior> option.c:325:19: error: too many arguments to function 'fp'; expected 0, have 1
ior>   325 |                   fp(arg);
ior>       |                   ^~ ~~~
ior> make[3]: *** [Makefile:1281: libaiori_a-option.o] Error 1

Fails just the same! Before trying to fix it ourselves let’s see if upstream already happens to have a fix. I looked at the history of our failed option.c file at https://github.com/hpc/ior/commits/main/src/option.c and I was lucky! https://github.com/hpc/ior/pull/525 is the committed fix for our issue!

I applied it locally as:

--- a/pkgs/by-name/io/ior/package.nix
+++ b/pkgs/by-name/io/ior/package.nix
@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchFromGitHub,
+  fetchpatch,
   mpi,
   perl,
   autoreconfHook,
@@ -19,6 +20,16 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-WsfJWHHfkiHZ+rPk6ck6mDErTXwt6Dhgm+yqOtw4Fvo=";
   };
 
+  patches = [
+    # Fix gcc-15 build:
+    #   https://github.com/hpc/ior/pull/525
+    (fetchpatch {
+      name = "gcc-15.patch";
+      url = "https://github.com/hpc/ior/commit/526c5ad06695a91a27163c520ce3305109f50bef.patch";
+      hash = "sha256-HvbRMt2EcuO7kxLL9qKpozpNKEOmWuHkKQTSUhfU7/w=";
+    })
+  ];
+
   nativeBuildInputs = [
     autoreconfHook
     pkg-config

Tested the build:

$ nix build --no-link -f. ior -L
...
<all good>

And proposed the fix as https://github.com/NixOS/nixpkgs/pull/516449.

Parting Words

If you never contributed to nixpkgs and are wondering how hard it is do give ZHF a try! Chances are you can quickly grab a broken package and make it work.

Have fun!