Release 2.31.0 (2025-08-21)

  • build-cores = 0 now auto-detects CPU cores #13402

    When build-cores is set to 0, Nix now automatically detects the number of available CPU cores and passes this value via NIX_BUILD_CORES, instead of passing 0 directly. This matches the behavior when build-cores is unset. This prevents the builder from having to detect the number of cores.

  • Fix Git LFS SSH issues #13337 #13743

    Fixed some outstanding issues with Git LFS and SSH.

    • Added support for NIX_SSHOPTS.
    • Properly use the parsed port from URL.
    • Better use of the response of git-lfs-authenticate to determine API endpoint when the API is not exposed on port 443.

  • Add support for user@address:port syntax in store URIs #7044 #3425

    It's now possible to specify the port used for SSH stores directly in the store URL in accordance with RFC3986. Previously the only way to specify custom ports was via ssh_config or the NIX_SSHOPTS environment variable, because Nix incorrectly passed the port number together with the host name to the SSH executable.

    This change affects store references passed via the --store and similar flags in CLI as well as in the configuration for remote builders. For example, the following store URIs now work:

    • ssh://127.0.0.1:2222
    • ssh://[b573:6a48:e224:840b:6007:6275:f8f7:ebf3]:22
    • ssh-ng://[b573:6a48:e224:840b:6007:6275:f8f7:ebf3]:22

  • Represent IPv6 RFC4007 ZoneId literals in conformance with RFC6874 #13445

    Prior versions of Nix since #4646 accepted IPv6 scoped addresses in URIs like store references in the textual representation with a literal percent character: [fe80::1%18]. This was ambiguous, because the the percent literal % is reserved by RFC3986, since it's used to indicate percent encoding. Nix now requires that the percent % symbol is percent-encoded as %25. This implements RFC6874, which defines the representation of zone identifiers in URIs. The example from above now has to be specified as [fe80::1%2518].

  • Use WAL mode for SQLite cache databases #13800

    Previously, Nix used SQLite's "truncate" mode for caches. However, this could cause a Nix process to block if another process was updating the cache. This was a problem for the flake evaluation cache in particular, since it uses long-running transactions. Thus, concurrent Nix commands operating on the same flake could be blocked for an unbounded amount of time. WAL mode avoids this problem.

    This change required updating the versions of the SQLite caches. For instance, eval-cache-v5.sqlite is now eval-cache-v6.sqlite.

  • Enable parallel marking in bdwgc #13708

    Previously marking was done by only one thread, which takes a long time if the heap gets big. Enabling parallel marking speeds up evaluation a lot, for example (on a Ryzen 9 5900X 12-Core):

    • nix search nixpkgs from 24.3s to 18.9s.
    • Evaluating the NixOS/nix/2.21.2 flake regression test from 86.1s to 71.2s.

  • New command nix flake prefetch-inputs #13565

    This command fetches all inputs of a flake in parallel. This can be a lot faster than the serialized on-demand fetching during regular flake evaluation. The downside is that it may fetch inputs that aren't normally used.

  • Add warn-short-path-literals setting #13489

    This setting, when enabled, causes Nix to emit warnings when encountering relative path literals that don't start with . or /, for instance suggesting that foo/bar should be rewritten to ./foo/bar.

  • When updating a lock, respect the input's lock file #13437

    For example, if a flake has a lock for a and a/b, and we change the flakeref for a, previously Nix would fetch the latest version of b rather than using the lock for b from a.

  • Implement support for Git hashing with SHA-256 #13543

    The experimental support for Git-hashing store objects now also includes support for SHA-256, not just SHA-1, in line with upstream Git.

Contributors

This release was made possible by the following 34 contributors: