Fixes#20776. At some point, the call to rebuild the sycoca database at startup
was removed, but it will not be rebuilt automatically because the timestamps in
the Nix store never change.
This patch add a new argument to Nixpkgs default expression named "overlays".
By default, the value of the argument is either taken from the environment variable `NIXPKGS_OVERLAYS`,
or from the directory `~/.nixpkgs/overlays/`. If the environment variable does not name a valid directory
then this mechanism would fallback on the home directory. If the home directory does not exists it will
fallback on an empty list of overlays.
The overlays directory should contain the list of extra Nixpkgs stages which would be used to extend the
content of Nixpkgs, with additional set of packages. The overlays, i-e directory, files, symbolic links
are used in alphabetical order.
The simplest overlay which extends Nixpkgs with nothing looks like:
```nix
self: super: {
}
```
More refined overlays can use `super` as the basis for building new packages, and `self` as a way to query
the final result of the fix-point.
An example of overlay which extends Nixpkgs with a small set of packages can be found at:
https://github.com/nbp/nixpkgs-mozilla/blob/nixpkgs-overlay/moz-overlay.nix
To use this file, checkout the repository and add a symbolic link to
the `moz-overlay.nix` file in `~/.nixpkgs/overlays` directory.
Linux 4.9 includes experimental amdgpu support for AMD Southern Islands
chipsets. (By default, only Sea Islands and newer chipsets are supported.)
Southern Islands chips will still use radeon by default, but daring users may
set `services.xserver.videoDrivers = [ "amdgpu" ];` to try the experimental
driver.
This reverts commit 986dba716f.
Fixes:
error: anonymous function at /home/travis/build/NixOS/nixpkgs/pkgs/development/node-packages/node-env.nix:3:1 called with unexpected argument ‘python’, at /home/travis/build/NixOS/nixpkgs/pkgs/development/node-packages/composition-v6.nix:8:13
This commit is doing a lot more than removing the yarn package, it also
upgrades a ton of other packages.
