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1st chunk of `doc/manual/source/release-notes/rl-2.25.md`
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# Release 2.25.0 (2024-11-07)

- New environment variables to override XDG locations [#11351](https://github.com/NixOS/nix/pull/11351)

  Added new environment variables:

  - `NIX_CACHE_HOME`
  - `NIX_CONFIG_HOME`
  - `NIX_DATA_HOME`
  - `NIX_STATE_HOME`

  Each, if defined, takes precedence over the corresponding [XDG environment variable](@docroot@/command-ref/env-common.md#xdg-base-directories).
  This provides more fine-grained control over where Nix looks for files. It allows having a stand-alone Nix environment that only uses files in a specific directory and that doesn't interfere with the user environment.

- Define integer overflow in the Nix language as an error [#10968](https://github.com/NixOS/nix/issues/10968) [#11188](https://github.com/NixOS/nix/pull/11188)

  Previously, integer overflow in the Nix language invoked C++ level signed overflow, which manifested as wrapping around on overflow. It now looks like this:

  ```
  $ nix eval --expr '9223372036854775807 + 1'
  error: integer overflow in adding 9223372036854775807 + 1
  ```

  Some other overflows were fixed:
  - `builtins.fromJSON` of values greater than the maximum representable value in a signed 64-bit integer will generate an error.
  - `nixConfig` in flakes will no longer accept negative values for configuration options.

- The `build-hook` setting no longer has a useful default when using `libnixstore` as a library [#11178](https://github.com/NixOS/nix/pull/11178)

  *This is an obscure issue that only affects usage of the `libnixstore` library outside of the Nix executable. It is unrelated to the `post-build-hook` settings, which is often used for pushing to a cache.*

  As part the ongoing [rewrite of the build system](https://github.com/NixOS/nix/issues/2503) to use [Meson](https://mesonbuild.com/), we are also switching to packaging individual Nix components separately (and building them in separate derivations).
  This means that when building `libnixstore` we do not know where the Nix binaries will be installed --- `libnixstore` doesn't know about downstream consumers like the Nix binaries at all.

  This has a small adverse affect on remote building --- the `build-remote` executable that is specified from the [`build-hook`](@docroot@/command-ref/conf-file.md#conf-build-hook) setting will not be gotten from the (presumed) installation location, but instead looked up on the `PATH`.
  This means that other applications linking `libnixstore` that wish to use remote building must arrange for the `nix` command to be on the PATH (or manually overriding `build-hook`) in order for that to work.

  Long term we don't envision this being a downside, because we plan to [get rid of `build-remote` and the build hook setting entirely](https://github.com/NixOS/nix/issues/1221).
  There should simply be no need to have an extra, intermediate layer of remote-procedure-calling when we want to connect to a remote builder.
  The build hook protocol did in principle support custom ways of remote building, but that can also be accomplished with a custom service for the ssh or daemon/ssh-ng protocols, or with a custom [store type](@docroot@/store/types/index.md) i.e. `Store` subclass. <!-- we normally don't mention classes, but consider that this release note is about a library use case -->

  The Perl bindings no longer expose `getBinDir` either, since the underlying C++ libraries those bindings wrap no longer know the location of installed binaries as described above.

- Wrap filesystem exceptions more correctly [#11378](https://github.com/NixOS/nix/pull/11378)

  With the switch to `std::filesystem` in different places, Nix started to throw `std::filesystem::filesystem_error` in many places instead of its own exceptions.
  As a result, Nix no longer generated error traces when (for example) listing a non-existing directory. It could also lead to crashes inside the Nix REPL.

  This version catches these types of exception correctly and wraps them into Nix's own exception type.
Title: Nix Release 2.25.0: New Environment Variables, Integer Overflow Fixes, and Build System Changes
Summary
Nix Release 2.25.0 introduces new environment variables (`NIX_CACHE_HOME`, `NIX_CONFIG_HOME`, `NIX_DATA_HOME`, `NIX_STATE_HOME`) to override XDG locations, offering more fine-grained control for Nix file paths. A significant change is that integer overflow in the Nix language (including `builtins.fromJSON` and `nixConfig` for negative values) now results in an error instead of silent wrapping. Additionally, the `build-hook` setting's default behavior is altered when `libnixstore` is used as a library, impacting remote building and requiring the `nix` command on the PATH, a step towards its eventual deprecation. Finally, the release improves error handling by correctly wrapping `std::filesystem::filesystem_error` exceptions into Nix's own exception type, preventing missing error traces and REPL crashes.