nix-super/doc/manual/src/release-notes/rl-1.2.md

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# Release 1.2 (2012-12-06)
This release has the following improvements and changes:
- Nix has a new binary substituter mechanism: the *binary cache*. A
binary cache contains pre-built binaries of Nix packages. Whenever
Nix wants to build a missing Nix store path, it will check a set of
binary caches to see if any of them has a pre-built binary of that
path. The configuration setting `binary-caches` contains a list of
URLs of binary caches. For instance, doing
$ nix-env -i thunderbird --option binary-caches http://cache.nixos.org
will install Thunderbird and its dependencies, using the available
pre-built binaries in <http://cache.nixos.org>. The main advantage
over the old “manifest”-based method of getting pre-built binaries
is that you dont have to worry about your manifest being in sync
with the Nix expressions youre installing from; i.e., you dont
need to run `nix-pull` to update your manifest. Its also more
scalable because you dont need to redownload a giant manifest file
every time.
A Nix channel can provide a binary cache URL that will be used
automatically if you subscribe to that channel. If you use the
Nixpkgs or NixOS channels (<http://nixos.org/channels>) you
automatically get the cache <http://cache.nixos.org>.
Binary caches are created using `nix-push`. For details on the
operation and format of binary caches, see the `nix-push` manpage.
More details are provided in [this nix-dev
posting](https://nixos.org/nix-dev/2012-September/009826.html).
- Multiple output support should now be usable. A derivation can
declare that it wants to produce multiple store paths by saying
something like
outputs = [ "lib" "headers" "doc" ];
This will cause Nix to pass the intended store path of each output
to the builder through the environment variables `lib`, `headers`
and `doc`. Other packages can refer to a specific output by
referring to `pkg.output`, e.g.
buildInputs = [ pkg.lib pkg.headers ];
If you install a package with multiple outputs using `nix-env`, each
output path will be symlinked into the user environment.
- Dashes are now valid as part of identifiers and attribute names.
- The new operation `nix-store --repair-path` allows corrupted or
missing store paths to be repaired by redownloading them. `nix-store
--verify --check-contents
--repair` will scan and repair all paths in the Nix store.
Similarly, `nix-env`, `nix-build`, `nix-instantiate` and `nix-store
--realise` have a `--repair` flag to detect and fix bad paths by
rebuilding or redownloading them.
- Nix no longer sets the immutable bit on files in the Nix store.
Instead, the recommended way to guard the Nix store against
accidental modification on Linux is to make it a read-only bind
mount, like this:
$ mount --bind /nix/store /nix/store
$ mount -o remount,ro,bind /nix/store
Nix will automatically make `/nix/store` writable as needed (using a
private mount namespace) to allow modifications.
- Store optimisation (replacing identical files in the store with hard
links) can now be done automatically every time a path is added to
the store. This is enabled by setting the configuration option
`auto-optimise-store` to `true` (disabled by default).
- Nix now supports `xz` compression for NARs in addition to `bzip2`.
It compresses about 30% better on typical archives and decompresses
about twice as fast.
- Basic Nix expression evaluation profiling: setting the environment
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variable `NIX_COUNT_CALLS` to `1` will cause Nix to print how many
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times each primop or function was executed.
- New primops: `concatLists`, `elem`, `elemAt` and `filter`.
- The command `nix-copy-closure` has a new flag `--use-substitutes`
(`-s`) to download missing paths on the target machine using the
substitute mechanism.
- The command `nix-worker` has been renamed to `nix-daemon`. Support
for running the Nix worker in “slave” mode has been removed.
- The `--help` flag of every Nix command now invokes `man`.
- Chroot builds are now supported on systemd machines.
This release has contributions from Eelco Dolstra, Florian Friesdorf,
Mats Erik Andersson and Shea Levy.