Nix Enhancements: Parallel Builds, User Environment Management, Garbage Collection, and More

application, this is easier than subscribing to a channel. - `nix-store -r PATHS` now builds all the derivations PATHS in parallel. Previously it did them sequentially (though exploiting possible parallelism between subderivations). This is nice for build farms. - `nix-channel` has new operations `--list` and `--remove`. - New ways of installing components into user environments: - Copy from another user environment: $ nix-env -i --from-profile .../other-profile firefox - Install a store derivation directly (bypassing the Nix expression language entirely): $ nix-env -i /nix/store/z58v41v21xd3...-aterm-2.3.1.drv (This is used to implement `nix-install-package`, which is therefore immune to evolution in the Nix expression language.) - Install an already built store path directly: $ nix-env -i /nix/store/hsyj5pbn0d9i...-aterm-2.3.1 - Install the result of a Nix expression specified as a command-line argument: $ nix-env -f .../i686-linux.nix -i -E 'x: x.firefoxWrapper' The difference with the normal installation mode is that `-E` does not use the `name` attributes of derivations. Therefore, this can be used to disambiguate multiple derivations with the same name. - A hash of the contents of a store path is now stored in the database after a successful build. This allows you to check whether store paths have been tampered with: `nix-store --verify --check-contents`. - Implemented a concurrent garbage collector. It is now always safe to run the garbage collector, even if other Nix operations are happening simultaneously. However, there can still be GC races if you use `nix-instantiate` and `nix-store --realise` directly to build things. To prevent races, use the `--add-root` flag of those commands. - The garbage collector now finally deletes paths in the right order (i.e., topologically sorted under the “references” relation), thus making it safe to interrupt the collector without risking a store that violates the closure invariant. - Likewise, the substitute mechanism now downloads files in the right order, thus preserving the closure invariant at all times. - The result of `nix-build` is now registered as a root of the garbage collector. If the `./result` link is deleted, the GC root disappears automatically. - The behaviour of the garbage collector can be changed globally by setting options in `/nix/etc/nix/nix.conf`. - `gc-keep-derivations` specifies whether deriver links should be followed when searching for live paths. - `gc-keep-outputs` specifies whether outputs of derivations should be followed when searching for live paths. - `env-keep-derivations` specifies whether user environments should store the paths of derivations when they are added (thus keeping the derivations alive). - New `nix-env` query flags `--drv-path` and `--out-path`. - `fetchurl` allows SHA-1 and SHA-256 in addition to MD5. Just specify the attribute `sha1` or `sha256` instead of `md5`. - Manual updates.

This section details several enhancements to Nix, including parallel building of derivations using `nix-store -r`, new operations for `nix-channel`, and improved ways to install components into user environments. It covers copying from other profiles, directly installing store derivations or paths, and using Nix expressions. Additionally, it highlights the implementation of a concurrent garbage collector, ensuring safety during simultaneous operations, and improvements to path deletion and substitute mechanisms to maintain closure invariants. The garbage collector's behavior is now configurable via `/nix/etc/nix/nix.conf`, and `fetchurl` now supports SHA-1 and SHA-256.