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PostScript Printer Description (ppd) files describe printer features and capabilities. They are usually evaluated by CUPS to convert print jobs into a format suitable for a printer. The conversion is often accomplished by commands or even short shell scripts inside the ppd files. ppd files are included in many printer driver packages. Their scripts sometimes refer to other executables; some of them are more common (like `perl`), others are more exotic (like `rastertohp`). If an executable is called with its name alone, the effects of the ppd file depend on whether the executable is in the PATH of CUPS, and on the executable's version. If an executable is called with an absolut path (like `/usr/bin/perl`), it won't work at all in NixOS. The commit at hand adds a setup hook that uses the `fixupPhase` to substitute certain executable's invocations in pdd files with absolute paths. To use it, add the hook to `nativeBuildInputs` and provide a list of executable names in `ppdFileCommands`. Each executable must be available in the package that is being built, or in `buildInputs`. The setup hook's script then looks for ppd files in `share/cups/model` and `share/ppds` in each output, and replaces executable names with their absolute paths. If ppd files need to be patched in unorthodox locations or the setup hook needs to be invoked manually for other reasons, one may leave the list `ppdFileCommands` empty to avoid automatic processing of ppd files, then call the shell function `patchPpdFileCommands` directly. Details are described in the file `patch-ppd-hook.sh`. Notes on the motivation for this setup hook: Most packages in nixpkgs that provide ppd files do not patch those ppd files at all. This is not fatal when the executables are just called with their names since the user can add packages with the executables to `services.printing.drivers`. E.g. if the user adds `pkgs.perl`, then all ppd files that invoke `perl` will work as expected. Nevertheless, to make these ppd files independent of their execution environment, command invocations should be substituted with absolut paths into the nix store. This is similar to patching shebang lines so scripts can be called independently of having the interpreter in the PATH. The hook script in this commit is meant to support new packages `foomatic-db*` which will generate several thousands of ppd files referencing a plethora of different executables. During development of these packages, I realized that it's quite hard to patch ppd files in a robust way. While binary names like `rastertokpsl` seem to be sufficiently unique to be patched with `sed`, names like `date` or `gs` are hard to patch without producing "false positives", i.e., coincidental occurences of the executable's name that do *not* refer to the executable and should not be patched at all. As this problem also affects other packages, it seems reasonable to put a robust implementation in its own setup hook so that other packages can use it without much effort. Notes on the implementation: The ppd file format is far from trivial. The basic structure are key-value pairs; keys may occur multiple times. Only a small subset of keys may contain executable names or shell scripts in their values. Some values may span multiple lines; a linebreak might even occur in the middle of a token. Some executable names also occur in other keys by accident where they must not be substituted (e.g. `gs` or `date`). It is necessary to provide the list of command names that will be patched for two reasons: ppd files often contain "tokens" that might look like commands (e.g. "file" or "host") but aren't; these would erroneously get patched. Also, looking for everything that might be a command would slow down the patching process considerably. The implementation uses `awk` to detect keys that might contain executable names; only their values are treated for substitution. This avoids most cases of "overzealous" substitutions. Since values may span multiple lines, `sed` alone (while faster than `awk`) cannot focus its substitution capabilities on relevant keys. An elaborate set of regular expressions further helps to minimize the probability of "false positives". Several tricks are employed to speed up `awk`. Notably, relevant files are identified with `grep` before `awk` is applied to those files only. Note that the script probably cannot handle fancy command names (like spaces or backslashes as part of the name). Also, there are still edge cases that the script would mistakenly skip, e.g. if a shell script contains a line break in the middle of an executable's name; although ppd files permit such constellations, I have yet to see one. ppd files may be gzipped. The setup hook accepts gzipped ppd files: It decompresses them, substitutes paths, then recompresses them. However, Nix cannot detect substituted paths as runtime dependencies in compressed ppd files. To ensure substituted paths are propagated as runtime dependencies, the script adds each substituted path to the variable `propagatedBuildInputs`. Since this might not be enough for multi-output packages, those paths are also written directly to `nix-support/propagated-build-inputs`. See the comment in `patch-ppd-hook.sh` for details. Finally, the setup hook comes with a small test that probes some edge cases with an artificial ppd file. References: * https://www.cups.org/doc/spec-ppd.html * general ppd file specification * lists some keys that may contain executable names or shell scripts * https://refspecs.linuxfoundation.org/LSB_4.0.0/LSB-Printing/LSB-Printing/ppdext.html * lists some keys that may contain executable names or shell scripts * https://en.wikipedia.org/wiki/PostScript_Printer_Description#CUPS * lists the usual locations of ppd files |
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Nixpkgs is a collection of over 80,000 software packages that can be installed with the Nix package manager. It also implements NixOS, a purely-functional Linux distribution.
Manuals
- NixOS Manual - how to install, configure, and maintain a purely-functional Linux distribution
- Nixpkgs Manual - contributing to Nixpkgs and using programming-language-specific Nix expressions
- Nix Package Manager Manual - how to write Nix expressions (programs), and how to use Nix command line tools
Community
- Discourse Forum
- Matrix Chat
- NixOS Weekly
- Community-maintained wiki
- Community-maintained list of ways to get in touch (Discord, Telegram, IRC, etc.)
Other Project Repositories
The sources of all official Nix-related projects are in the NixOS organization on GitHub. Here are some of the main ones:
- Nix - the purely functional package manager
- NixOps - the tool to remotely deploy NixOS machines
- nixos-hardware - NixOS profiles to optimize settings for different hardware
- Nix RFCs - the formal process for making substantial changes to the community
- NixOS homepage - the NixOS.org website
- hydra - our continuous integration system
- NixOS Artwork - NixOS artwork
Continuous Integration and Distribution
Nixpkgs and NixOS are built and tested by our continuous integration system, Hydra.
- Continuous package builds for unstable/master
- Continuous package builds for the NixOS 22.05 release
- Tests for unstable/master
- Tests for the NixOS 22.05 release
Artifacts successfully built with Hydra are published to cache at https://cache.nixos.org/. When successful build and test criteria are met, the Nixpkgs expressions are distributed via Nix channels.
Contributing
Nixpkgs is among the most active projects on GitHub. While thousands of open issues and pull requests might seem a lot at first, it helps consider it in the context of the scope of the project. Nixpkgs describes how to build tens of thousands of pieces of software and implements a Linux distribution. The GitHub Insights page gives a sense of the project activity.
Community contributions are always welcome through GitHub Issues and Pull Requests. When pull requests are made, our tooling automation bot, OfBorg will perform various checks to help ensure expression quality.
The Nixpkgs maintainers are people who have assigned themselves to maintain specific individual packages. We encourage people who care about a package to assign themselves as a maintainer. When a pull request is made against a package, OfBorg will notify the appropriate maintainer(s). The Nixpkgs committers are people who have been given permission to merge.
Most contributions are based on and merged into these branches:
masteris the main branch where all small contributions gostagingis branched from master, changes that have a big impact on Hydra builds go to this branchstaging-nextis branched from staging and only fixes to stabilize and security fixes with a big impact on Hydra builds should be contributed to this branch. This branch is merged into master when deemed of sufficiently high quality
For more information about contributing to the project, please visit the contributing page.
Donations
The infrastructure for NixOS and related projects is maintained by a nonprofit organization, the NixOS Foundation. To ensure the continuity and expansion of the NixOS infrastructure, we are looking for donations to our organization.
You can donate to the NixOS foundation through SEPA bank transfers or by using Open Collective:
License
Nixpkgs is licensed under the MIT License.
Note: MIT license does not apply to the packages built by Nixpkgs, merely to the files in this repository (the Nix expressions, build scripts, NixOS modules, etc.). It also might not apply to patches included in Nixpkgs, which may be derivative works of the packages to which they apply. The aforementioned artifacts are all covered by the licenses of the respective packages.