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Eelco Dolstra 2019-05-26 15:59:50 +02:00 committed by Nick Van den Broeck
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doc/flakes/design.md
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@ -2,92 +2,83 @@
## Goals
* To provide Nix repositories with an easy and standard way to
reference other Nix repositories.
* Standard and easy way for Nix repos to reference other Nix repos as
dependencies
* To allow such references to be queried and updated automatically.
* Discoverability: Be able to query and update these references to Nix repos
automatically
* To provide a replacement for `nix-channel`, `NIX_PATH` and Hydra
jobset definitions.
* To provide a replacement for `nix-channel`, `NIX_PATH` and Hydra jobset
definitions
* To enable reproducible, hermetic evaluation of packages and NixOS
configurations.
* Reproducibility: Evaluate packages and NixOS configurations hermetic by
default
Things that we probably won't do in the initial iteration:
Upcoming but not yet implemented:
* Sophisticated flake versioning, such as the ability to specify
version ranges on dependencies.
* Sophisticated flake versioning, such as the ability to specify version ranges
on dependencies.
* A way to specify the types of values provided by a flake. For the
most part, flakes can provide arbitrary Nix values, but there will
be some standard attribute names (e.g. `packages` must be a set of
installable derivations).
* A way to specify the types of values provided by a flake. For the most part,
flakes can provide arbitrary Nix values, but there will be some standard
attribute names (e.g. `packages` must be a set of installable derivations).
## Overview
* A flake is (usually) a Git repository that contains a file named
`flake.nix` at top-level.
* A flake is (usually) a Git repository that contains a file named `flake.nix`
at top-level
* Flakes *provide* an attribute set of values, such as packages,
Nixpkgs overlays, NixOS modules, library functions, Hydra jobs,
`nix-shell` definitions, etc.
* A flake *provides* an attribute set of values, such as packages, Nixpkgs
overlays, NixOS modules, library functions, Hydra jobs, `nix-shell`
definitions, etc.
* Flakes can *depend* on other flakes.
* Flakes can *depend* on other flakes or other repositories which aren't flakes
* Flakes are referred to using a *flake reference*, which is either a
URL specifying its repository's location
(e.g. `github:NixOS/nixpkgs/release-18.09`) or an identifier
(e.g. `nixpkgs`) looked up in a *lock file* or *flake
registry*. They can also specify revisions,
e.g. `github:NixOS/nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f`.
* Flakes are referred to using a *flake reference*, which is either a URL
specifying its repository's location or an identifier looked up in a *lock
file* or *flake registry*.
* The *flake registry* is a centrally maintained mapping (on
`nixos.org`) from flake identifiers to flake locations
(e.g. `nixpkgs -> github:NixOS/nixpkgs/release-18.09`).
* A *flake registry* is a mapping from flake identifiers to flake locations
(e.g. `nixpkgs -> github:NixOS/nixpkgs/release-18.09`). There is a centrally
maintained flake registry on `nixos.org`.
* A flake can contain a *lock file* (`flake.lock`) used when resolving
the dependencies in `flake.nix`. It maps flake references to
references containing revisions (e.g. `nixpkgs ->
* A flake can contain a *lock file* (`flake.lock`) used when resolving the
dependencies in `flake.nix`. It maps mutable flake references
(e.g. `github:NixOS/nixpkgs/release-18.09`) to references containing revisions
(e.g. `nixpkgs ->
github:NixOS/nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f`).
* The `nix` command uses the flake registry as its default
installation source. For example, `nix build nixpkgs.hello` builds the
`hello` package provided by the `nixpkgs` flake listed in the
registry. `nix` will automatically download/upload the registry and
flakes as needed.
* The `nix` command uses the flake registry as its default installation source.
For example, `nix build nixpkgs.hello` builds the `hello` package provided by
the `nixpkgs` flake listed in the registry. `nix` will automatically
download/upload the registry and flakes as needed.
* `nix build` without arguments will build the flake in the current
directory (or some parent).
* The command `nix flake update` generates/updates `flake.lock` from
`flake.nix`. This should probably also be done automatically when
building from a local flake.
* `nix flake update` generates `flake.lock` from `flake.nix`, ignoring the old
lockfile.
* `nixos-rebuild` will build a configuration from a (locked)
flake. Evaluation will be done in pure mode to ensure there are no
unaccounted inputs. Thus the NixOS configuration can be reproduced
unambiguously from the top-level flake.
* `nixos-rebuild` will build a configuration from a (locked) flake. Evaluation
is done in pure mode to ensure there are no unaccounted inputs. Thus the
NixOS configuration can be reproduced unambiguously from the top-level flake.
* Nix code can query flake metadata such as `commitHash` (the Git
revision) or `date` (the date of the last commit). This is useful
for NixOS to compute the NixOS version string (which will be the
revision of the top-level configuration flake, uniquely identifying
the configuration).
* Nix code can query flake metadata such as `commitHash` (the Git revision) or
`epoch` (the date of the last commit). This is useful for NixOS to compute
the NixOS version string (which will be the revision of the top-level
configuration flake, uniquely identifying the configuration).
* Hydra jobset configurations will consist of a single flake
reference. Thus we can get rid of jobset inputs; any other needed
repositories can be fetched by the top-level flake. The top-level
flake can be locked or unlocked; if some dependencies are unlocked,
then Nix will fetch the latest revision for each.
* Hydra jobset configurations will consist of a single flake reference. Thus we
can get rid of jobset inputs; any other needed repositories can be fetched by
the top-level flake. The top-level flake can be locked or unlocked; if some
dependencies are unlocked, then Nix will fetch the latest revision for each.
## Example flake
A flake is a Git repository that contains a file named
`flake.nix`. For example, here is the `flake.nix` for `dwarffs`, a
small repository that provides a single package and a single NixOS
module.
Let us look at an example of a `flake.nix` file, here for `dwarffs`, a small
repository that provides a single package and a single NixOS module.
```nix
{
@ -101,23 +92,26 @@ module.
# Some other metadata.
description = "A filesystem that fetches DWARF debug info from the Internet on demand";
# A list of flake references denoting the flakes that this flake
# depends on. Nix will resolve and fetch these flakes and pass them
# as a function argument to `outputs` below.
# The flake dependencies. Nix will resolve and fetch these flakes and pass
# them as a function argument to `outputs` below.
#
# `flake:nixpkgs` denotes a flake named `nixpkgs` which is looked up
# "nixpkgs" denotes a flake named `nixpkgs` which is looked up
# in the flake registry, or in `flake.lock` inside this flake, if it
# exists.
inputs = [ flake:nixpkgs ];
# An attribute set listing dependencies which aren't flakes, also to be passed as
# a function argument to `provides`.
nonFlakeRequires = {};
# The stuff provided by this flake. Flakes can provide whatever they
# want (convention over configuration), but some attributes have
# special meaning to tools / other flakes: for example, `packages`
# special meaning to tools / other flakes. For example, `packages`
# is used by the `nix` CLI to search for packages, and
# `nixosModules` is used by NixOS to automatically pull in the
# modules provided by a flake.
#
# `outputs` takes a single argument named `deps` that contains
# `outputs` takes a single argument (`deps`) that contains
# the resolved set of flakes. (See below.)
outputs = deps: {
@ -153,7 +147,11 @@ module.
nixosModules.dwarffs = import ./module.nix deps;
# Provide a single Hydra job (`hydraJobs.dwarffs`).
hydraJobs = deps.this.packages;
hydraJobs.build.x86_64-linux = packages.dwarffs;
# A bunch of things which can be checked (through `nix flake check`) to
# make sure the flake is well-defined.
checks.build = packages.dwarffs;
};
}
```
@ -170,8 +168,11 @@ Similarly, a minimal `flake.nix` for Nixpkgs:
outputs = deps:
let pkgs = import ./. {}; in
let pkgs = import ./. { system = "x86_64-linux"; }; in
{
lib = import ./lib;
lib = (import ./lib) // {
nixosSystem = import ./nixos/lib/eval-config.nix;
};
builders = {
inherit (pkgs) stdenv fetchurl;
@ -180,145 +181,124 @@ Similarly, a minimal `flake.nix` for Nixpkgs:
packages = {
inherit (pkgs) hello nix fuse nlohmann_json boost;
};
legacyPkgs = pkgs;
};
}
```
Note that `packages` is an unpolluted set of packages: non-package
values like `lib` or `fetchurl` are not part of it.
Note that `packages` is an unpolluted set of packages: non-package values like
`lib` or `fetchurl` are not part of it.
## Flake registries
## Flake identifiers
A flake has an identifier (e.g. `nixpkgs` or `dwarffs`).
Note: If a flake registry contains an entry `nixpkgs -> github:NixOS/nixpkgs`,
then `nixpkgs/release-18.09` will match to become
`github:NixOS/nixpkgs/release-18.09`. This is referred to as "fuzzymatching".
## Flake references
Flake references are a URI-like syntax to specify the physical
location of a flake (e.g. a Git repository) or to denote a lookup in
the flake registry or lock file.
Flake references are a URI-like syntax to specify the physical location of a
flake (e.g. a Git repository) or to denote a lookup in the flake registry or
lock file. There are four options for the syntax:
* `(flake:)?<flake-id>(/rev-or-ref(/rev)?)?`
* Flake aliases
A flake alias is a name which requires a lookup in a flake
registry or lock file.
Look up a flake by ID in the flake lock file or in the flake
registry. These must specify an actual location for the flake using
the formats listed below. Note that in pure evaluation mode, the
flake registry is empty.
Example: "nixpkgs"
Optionally, the `rev` or `ref` from the dereferenced flake can be
overriden. For example,
* GitHub repositories
A repository which is stored on GitHub can easily be fetched using this type.
Note:
* Only the code in this particular commit is downloaded, not the entire repo
* By default, the commit to download is the last commit on the `master` branch.
See later for how to change this.
> nixpkgs/19.09
Example: `github:NixOS/nixpkgs`
uses the `19.09` branch of the `nixpkgs` flake's GitHub repository,
while
> nixpkgs/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f
uses the specified revision. For Git (rather than GitHub)
repositories, both the rev and ref must be given, e.g.
> nixpkgs/19.09/98a2a5b5370c1e2092d09cb38b9dcff6d98a109f
* `github:<owner>/<repo>(/<rev-or-ref>)?`
A repository on GitHub. These differ from Git references in that
they're downloaded in a efficient way (via the tarball mechanism)
and that they support downloading a specific revision without
specifying a branch. `rev-or-ref` is either a commit hash (`rev`)
or a branch or tag name (`ref`). The default is `master` if none is
specified. Note that in pure evaluation mode, a commit hash must be
used.
Flakes fetched in this manner expose `rev` and `date` attributes,
but not `revCount`.
Examples:
> github:edolstra/dwarffs
> github:edolstra/dwarffs/unstable
> github:edolstra/dwarffs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553
* > https://<server>/<path>.git(\?attr(&attr)*)?
> ssh://<server>/<path>.git(\?attr(&attr)*)?
> git://<server>/<path>.git(\?attr(&attr)*)?
> file:///<path>(\?attr(&attr)*)?
where `attr` is one of `rev=<rev>` or `ref=<ref>`.
A Git repository fetched through https. Note that the path must end
in `.git`. The default for `ref` is `master`.
Examples:
> https://example.org/my/repo.git
> https://example.org/my/repo.git?ref=release-1.2.3
> https://example.org/my/repo.git?rev=e72daba8250068216d79d2aeef40d4d95aff6666
* > /path.git(\?attr(&attr)*)?
Like `file://path.git`, but if no `ref` or `rev` is specified, the
(possibly dirty) working tree will be used. Using a working tree is
not allowed in pure evaluation mode.
* `ssh/https/git/file`
These are generic `FlakeRef`s for downloadding git repositories or tarballs.
Examples:
- https://example.org/my/repo.git
- ssh://git@github.com:NixOS/nix.git
- git://github.com/edolstra/dwarffs.git
- file:///home/my-user/some-repo/some-repo.git
- https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz
- file:///<path>.tar.xz
> /path/to/my/repo
* Local, dirty paths
This `FlakeRef` is the equivalent of `file://<path>` used for dirty paths.
> /path/to/my/repo?ref=develop
Example: /path/to/my/repo
> /path/to/my/repo?rev=e72daba8250068216d79d2aeef40d4d95aff6666
* > https://<server>/<path>.tar.xz(?hash=<sri-hash>)
> file:///<path>.tar.xz(?hash=<sri-hash>)
A flake distributed as a tarball. In pure evaluation mode, an SRI
hash is mandatory. It exposes a `date` attribute, being the newest
file inside the tarball.
Example:
> https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz
> https://releases.nixos.org/nixos/unstable/nixos-19.03pre167858.f2a1a4e93be/nixexprs.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c
Note: currently, there can be only one flake per Git repository, and
it must be at top-level. In the future, we may want to add a field
(e.g. `dir=<dir>`) to specify a subdirectory inside the repository.
Notes:
- Each FlakeRef (except for the Path option) allows for a Git revision (i.e.
commit hash) and/or referenceo(i.e. git branch name) to be added. For
tarbals, an SRI hash needs to be added.
Examples:
* `"nixpkgs/release-18.09"`
* `github:NixOS/nixpkgs/1e9e709953e315ab004951248b186ac8e2306451`
* `git://github.com/edolstra/dwarffs.git?ref=flake&rev=2efca4bc9da70fb001b26c3dc858c6397d3c4817`
* file:///<path>.tar.xz(?hash=<sri-hash>)
- In full pure mode, no mutable `FlakeRef`s can be used
* No aliases, because they need to be looked up
* `github` requires a specified `rev`
* `ssh/https/git/file` require a specified `ref` _and_ `rev`
* `path` is always mutable
- Flakes don't need to be top-level, but can also reside in a subdirectory. This is shown by adding `dir=<subdir>` to the `FlakeRef`.
Example: `./foo?dir=bar`
## Flake lock files
This is a JSON file named `flake.lock` that maps flake identifiers
used in the corresponding `flake.nix` to "immutable" flake references;
that is, flake references that contain a revision (for Git
repositories) or a content hash (for tarballs).
A lockfile is a JSON file named `flake.lock` which contains a forrest of
entries mapping `FlakeRef`s to the immutable `FlakeRef` they were resolved to.
Example:
```json
{
"nixpkgs": "github:NixOS/nixpkgs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553",
"foo": "https://example.org/foo.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c"
"nixpkgs": {
"uri": "github:NixOS/nixpkgs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553",
"content-hash": "sha256-vy2UmXQM66aS/Kn2tCtjt9RwxfBvV+nQVb5tJQFwi8E="
},
"foo": {
"uri": "https://example.org/foo.tar.xz?hash=sha256-56bbc099995ea8581ead78f22832fee7dbcb0a0b6319293d8c2d0aef5379397c",
"content-hash": "sha256-vy2UmXQM66aS/Kn2tCtjt9RwxfBvV+nQVb5tJQFwi8E="
}
}
```
Lockfiles are used to help resolve the dependencies of a flake.
- `nix build github:<..>` uses the remote lockfile and update it
- `nix build /home/user/dwarffs` uses the local lockfile, updates it and writes the result to file
- `nix flake update <flakeref>` recreates the lockfile from scratch and writes it to file
- `--no-registries` makes the command pure, also when fetching dependencies
- `--no-save-lock-file`: Several commands will update the lockfile (e.g. `nix
build`). This flag prevents the updated lockfile to be written to file.
- `--recreate-lock-file` makes prevents the current lockfile from being used
## `outputs`
The flake attribute `outputs` is a function that takes an argument
named `deps` and returns a (mostly) arbitrary attrset of values. Some
of the standard result attributes:
The function argument `deps` is an attrset containing all dependencies listed
in `requires` and `nonFlakeRequires` as well as `path` (for the flake's source
code) and an attribute `meta` with:
- `description`
- `commitHash` (not for tarball flakes): The Git commit hash.
- `date`: The timestamp of the most recent commit (for Git repos), or of the
most recently modified file (for tarballs)
- `revCount` (for Git flakes, but not GitHub flakes): The number of ancestors
of the revision. Useful for generating version strings.
* `packages`: A set of installable derivations used by the `nix`
command. That is, commands such as `nix install` ignore all other
flake attributes.
The flake attribute `outputs` is a function that takes an argument named `deps`
and returns an attribute set. Some of the members of this set have protected
names:
* `packages`: A set of installable derivations used by the `nix` command. That
is, commands such as `nix install` ignore all other flake attributes. It
cannot be a nested set.
* `hydraJobs`: Used by Hydra.
@ -329,213 +309,155 @@ of the standard result attributes:
we need to avoid a situation where `nixos-rebuild` needs to fetch
its own `nixpkgs` just to do `evalModules`.)
* `devShell`: A specification of a development environment in some TBD
format.
* `devShell`: A derivation to create a development environment
The function argument `flakes` is an attrset that contains an
attribute for each dependency specified in `inputs`. (Should it
contain transitive dependencies? Probably not.) Each attribute is an
attrset containing the `outputs` of the dependency, in addition to
the following attributes:
* `path`: The path to the flake's source code. Useful when you want to
use non-Nix artifacts from the flake, or if you want to *store* the
source code of the dependency in a derivation. (For example, we
could store the sources of all flake dependencies in a NixOS system
configuration, as a generalization of
`system.copySystemConfiguration`.)
* `meta`: An attrset containing the following:
* `description`
* `commitHash` (or `rev`?) (not for tarball flakes): The Git commit
hash.
* `date`: The timestamp of the most recent commit (for Git
repositories), or the timestamp of the most recently modified file
(for tarballs).
* `revCount` (for Git flakes, but not GitHub flakes): The number of
ancestors of the revision. Useful for generating version strings.
## Non-flake dependencies
It may be useful to pull in repositories that are not flakes
(i.e. don't contain a `flake.nix`). This could be done in two ways:
* Allow flakes not to have a `flake.nix` file, in which case it's a
flake with no inputs and no outputs. The downside of this
approach is that we can't detect accidental use of a non-flake
repository. (Also, we need to conjure up an identifier somehow.)
* Add a flake attribute to specifiy non-flake dependencies, e.g.
> nonFlakeInputs.foobar = github:foo/bar;
* `self`: The result of the flake's output which is passed to itself
Example: `self.outputs.foo` works.
## Flake registry
The flake registry maps flake IDs to flake references (where the
latter cannot be another indirection, i.e. it must not be a
`flake:<flake-id>` reference).
The default registry is kept at
`https://nixos.org/flake-registry.json`. It looks like this:
A flake registry is a JSON file mapping flake references to flake references.
The default/global registry is kept at
`https://github.com/NixOS/flake-registry/blob/master/flake-registry.json` and
looks like this:
```json
{
"version": 1,
"flakes": {
"dwarffs": {
"uri": "github:edolstra/dwarffs/flake"
},
"nix": {
"uri": "github:NixOS/nix/flakes"
},
"nixpkgs": {
"uri": "github:NixOS/nixpkgs/release-18.09"
"uri": "github:edolstra/nixpkgs/release-19.03"
},
"hydra": {
"uri": "github:NixOS/hydra/flake"
},
"patchelf": {
"uri": "github:NixOS/patchelf"
}
}
},
"version": 1
}
```
Nix automatically (re)downloads the registry. The downloaded file is a
GC root so the registry remains available if nixos.org is unreachable.
TBD: when to redownload?
Nix automatically (re)downloads this file whenever you have network access. The
downloaded file is a GC root so the registry remains available if nixos.org is
unreachable.
In addition to a global registry, there is also a user registry stored in
`~/.config/nix/registry.json`.
## Nix UI
Commands for registry / user flake configuration:
There is a list of new commands added to the `nix` CLI:
* `nix flake list`: Show all flakes in the registry.
* `nix flake list`: Show all flakes in the registry
* `nix flake add <flake-ref>`: Add or override a flake to/in the
user's flake configuration (`~/.config/nix/flakes.nix`). For
example, `nix flake add nixpkgs/nixos-18.03` overrides the `nixpkgs`
flake to use the `nixos-18.03` branch. There should also be a way to
add multiple branches/revisions of the same flake by giving them a
different ID, e.g. `nix flake add --id nixpkgs-ancient
nixpkgs/nixos-16.03`).
* `nix flake add <alias FlakeRef> <resolved FlakeRef>`: Add or override a flake
to/in the user flake registry.
* `nix flake remove <flake-id>`: Remove a flake from the user's flake
configuration. Any flake with the same ID in the registry remains
available.
* `nix flake remove <alias FlakeRef>`: Remove a FlakeRef from the user flake
registry.
* `nix flake lock <flake-id>`: Lock a flake. For example, `nix flake
lock nixpkgs` pins `nixpkgs` to the current revision.
* `nix flake pin <alias FlakeRef>`: Look up to which immutable FlakeRef the
alias FlakeRef maps to currently, and store that map in the user registry.
Example: `nix flake pin github:NixOS/nixpkgs` will create an entry
`github:NixOS/nixpkgs ->
github:NixOS/nixpkgs/444f22ca892a873f76acd88d5d55bdc24ed08757`.
Commands for creating/modifying a flake:
* `nix flake init`: Create a `flake.nix` in the current directory
* `nix flake init`: Create a `flake.nix` in the current directory.
* `nix flake update`: Update the lock file for the `flake.nix` in the
current directory. In most cases, this should be done
automatically. (E.g. `nix build` should automatically update the
lock file is a new dependency is added to `flake.nix`.)
* `nix flake update`: Recreate the lock file from scratch, from the `flake.nix`.
* `nix flake check`: Do some checks on the flake, e.g. check that all
`packages` are really packages.
* `nix flake clone`: Do a Git clone of the flake repository. This is a
convenience to easily start hacking on a flake. E.g. `nix flake
clone dwarffs` clones the `dwarffs` GitHub repository to `./dwarffs`.
TODO: maybe the first set of commands should have a different name
from the second set.
* `nix flake clone`: `git clone` the flake repo
Flags / configuration options:
* `--flakes (<flake-id>=<flake-ref>)*`: add/override some flakes.
* `--flakes (<alias FlakeRef>=<resolved FlakeRef>)*`: add/override some
FlakeRef
* (In `nix`) `--flake <flake-ref>`: set the specified flake as the
installation source. E.g. `nix build --flake ./my-nixpkgs hello`.
* `--flake <flake-ref>`: set the specified flake as the installation source
E.g. `nix build --flake ./my-nixpkgs hello`.
The default installation source in `nix` is the `packages` from all
flakes in the registry, that is:
The default installation source in `nix` is the `packages` from all flakes in
the registry, that is:
```
builtins.mapAttrs (flakeName: flakeInfo:
(getFlake flakeInfo.uri).${flakeName}.outputs.packages or {})
builtins.flakeRegistry
```
(where `builtins.flakeRegistry` is the global registry with user
overrides applied, and `builtins.getFlake` downloads a flake and
resolves its dependencies.)
It may be nice to extend the default installation source with the
`packages` from the flake in the current directory, so that
> nix build hello
does something similar to the old
> nix-build -A hello
Specifically, it builds `packages.hello` from the flake in the current
directory. Of course, this creates some ambiguity if there is a flake
in the registry named `hello`.
Maybe the command
> nix dev-shell
should do something like use `outputs.devShell` to initialize the
shell, but probably we should ditch `nix shell` / `nix-shell` for
direnv.
where `builtins.flakeRegistry` is the global registry with user overrides
applied, and `builtins.getFlake` downloads a flake and resolves its
dependencies.
## Pure evaluation and caching
Flake evaluation should be done in pure mode. Thus:
Flake evaluation is done in pure mode. Thus:
* Flakes cannot do `NIX_PATH` lookups via the `<...>` syntax.
* Flakes cannot use `NIX_PATH` via the `<...>` syntax.
* They can't read random stuff from non-flake directories, such as
* Flakes cannot read random stuff from non-flake directories, such as
`~/.nix/config.nix` or overlays.
This enables aggressive caching or precomputation of Nixpkgs package
sets. For example, for a particular Nixpkgs flake closure (as
identified by, say, a hash of the fully-qualified flake references
after dependency resolution) and system type, an attribute like
`packages.hello` should always evaluate to the same derivation. So we
can:
This enables aggressive caching or precomputation of Nixpkgs package sets. For
example, for a particular Nixpkgs flake closure (as identified by, say, a hash
of the fully-qualified flake references after dependency resolution) and system
type, an attribute like `packages.hello` should always evaluate to the same
derivation. So we can:
* Keep a local evaluation cache (say `~/.cache/nix/eval.sqlite`)
* Keep a local evaluation cache (say `~/.cache/nix/eval-cache-v1.sqlite`)
mapping `(<flake-closure-hash, <attribute>) -> (<drv-name>,
<drv-output-paths>, <whatever other info we want to cache>)`.
* Download a precomputed cache
(e.g. `https://releases.nixos.org/eval/<flake-closure-hash>.sqlite`). So
a command like `nix search` could avoid evaluating Nixpkgs entirely.
* Download a precomputed cache, e.g.
`https://releases.nixos.org/eval/<flake-closure-hash>.sqlite`. So a command
like `nix search` could avoid evaluating Nixpkgs entirely.
Of course, this doesn't allow overlays. With pure evaluation, the only
way to have these is to define a top-level flake that depends on the
Nixpkgs flake and somehow passes in a set of overlays.
TODO: in pure mode we have to pass the system type explicitly!
Of course, this doesn't allow overlays. With pure evaluation, the only way to
have these is to define a top-level flake that depends on the Nixpkgs flake and
somehow passes in a set of overlays.
## Hydra jobset dependencies
Hydra can use the flake dependency resolution mechanism to fetch
dependencies. This allows us to get rid of jobset configuration in the
web interface: a jobset only requires a flake reference. That is, *a
jobset is a flake*. Hydra then just builds the `hydraJobs` attrset
`provide`d by the flake. (It omitted, maybe it can build `packages`.)
Hydra can use the flake dependency resolution mechanism to fetch dependencies.
This allows us to get rid of jobset configuration in the web interface: a
jobset only requires a flake reference. That is, a jobset *is* a flake. Hydra
then just builds the `hydraJobs` attrset
## NixOS system configuration
NixOS currently contains a lot of modules that really should be moved
into their own repositories. For example, it contains a Hydra module
that duplicates the one in the Hydra repository. Also, we want
reproducible evaluation for NixOS system configurations. So NixOS
system configurations should be stored as flakes in (local) Git
repositories.
NixOS currently contains a lot of modules that really should be moved into
their own repositories. For example, it contains a Hydra module that duplicates
the one in the Hydra repository. Also, we want reproducible evaluation for
NixOS system configurations. So NixOS system configurations should be stored as
flakes in (local) Git repositories.
`my-system/flake.nix`:
```nix
{
name = "my-system";
epoch = 201906;
inputs =
[ "nixpkgs/nixos-18.09"
"dwarffs"
"hydra"
... lots of other module flakes ...
];
outputs = flakes: {
nixosSystems.default =
flakes.nixpkgs.lib.evalModules {
@ -549,13 +471,6 @@ repositories.
];
};
};
inputs =
[ "nixpkgs/nixos-18.09"
"dwarffs"
"hydra"
... lots of other module flakes ...
];
}
```
@ -563,5 +478,5 @@ We can then build the system:
```
nixos-rebuild switch --flake ~/my-system
```
This performs dependency resolution starting at `~/my-system/flake.nix`
and builds the `system` attribute in `nixosSystems.default`.
This performs dependency resolution starting at `~/my-system/flake.nix` and
builds the `system` attribute in `nixosSystems.default`.

8
src/nix/flake.cc
View file

@ -128,14 +128,6 @@ static void printNonFlakeInfo(const NonFlake & nonFlake)
printSourceInfo(nonFlake.sourceInfo);
}
static nlohmann::json nonFlakeToJson(const NonFlake & nonFlake)
{
nlohmann::json j;
j["id"] = nonFlake.alias;
sourceInfoToJson(nonFlake.sourceInfo, j);
return j;
}
// FIXME: merge info CmdFlakeInfo?
struct CmdFlakeDeps : FlakeCommand
{