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Does Nix's break from FHS cause problems?
(lemy.lol)
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this post was submitted on 25 Jan 2024
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Why doesn't nix use fhs again?
Nix installs derivations into separate folders. A derivation can be a package, but can also be other things like configuration files, scripts or sources for packages. Nix doesn't distinguish between these derivations by a name but rather by a hash created from their build instructions.
For example two instances of the same package with a different version are two different derivations and thus nix can have both package versions installed without them interfering with each other. But this goes beyond just a package version. It is e.g. possible to have the same package with the same version but different patches applied, or relying on different versions of dependencies. Since their build instructions differ both can be installed simultaneously.
This approach grants a variety of advantages. For example upgrading your NixOS system just installs new derivations of packages and configuration files that have changed, while keeping previous derivations until they're garbage collected at a later time. This allows you to switch freely between both iterations of your system, for example if an update causes issues you can just revert back to before an update easily. Another advantage is that an unprivileged user can install packages they need without interfering with the rest of the system, for example an older python version or a newer one, or some software they want but the system does not provide.
The price of having this kind of isolation between packages is that nixos cannot install binaries and libraries into common locations. Effectively /usr/bin only contains the env binary. If you're familiar with shell scripting you might have run into lines such as
#!/usr/bin/env bash. This env util will essentially search bash in your PATH variable and start it. Lines like#!/bin/bashhowever will not work, because there's no bash installed in that location.Another case where a missing fhs is a problem is when using pre-compiled binaries. In contrast to binaries built through nix, which have their required libraries hardcoded as absolute paths, pre-compiled binaries you download usually only contain the name of the library they need, which works in a conventional fhs environment, because these libraries tend to be found in
/libor/usr/lib. On NixOS neither of those are present. There two solutions to this. Either you create an fhs environment by listing the set of derivations to be symlinked into a chroot environment which mimics an FHS. Or you can install https://github.com/Mic92/nix-ld which automatically finds the required libraries the nix way if you start such a binary. There's also steam-run which installs an fhs with most of the dependencies necessary to start Linux games from Steam.Why isn't this done on the actual system and by default? That would make it fhs compliant, no?
If you put an FHS on the actual system you wouldn't be able to install multiple versions of the same package, updates wouldn't be atomic - you wouldn't get the big selling points of Nix.
If your system uses 3 different Pythons as dependencies of different packages, which one gets to be /usr/bin/python?
The most recent one by default unless another is manually chosen. The nix packages can keep using their specific versions directly
Now you'll have a zillion users trying to install software in ways that violate all the assumptions that NixOS operates on, but which are still tightly coupled to your NixOS config. Now updates to your system, or even seemingly unrelated config changes (through some transitive dependency chain) can easily break that software.
So now we've basically removed half the advantages that motivate Nix/OS in the first place, and when stuff breaks it will look like it's Nix's fault, even if it isn't.
On the other hand, nixpkgs is already the most comprehensive repository of system software out there, and for 99% of packages Nixifying it is pretty trivial. Hell, my NixOS config does that for 3 different GitHub repos right inline in my config.nix
Choosing the most recent one might be impossible if you have multiple installations of the same package with same version but different features enabled during the configure step.
Conflict resolution is not an impossible task. You just need to have a sensible algorithm. I get that they don't want to do it lest people abuse it instead of using nix but there isn't a technical challenge that can't be overcome.
Conflict resolution was not my point. Rather the question which the "most recent" between two almost identical installations...
That's what I meant. How the default is chosen is irrelevant and is not my point. (You can pick the earliest installed among the latest version for example) The point is, it can be done and isn't a technical challenge.
It's not that it's hard to do. It's that it goes directly against the idea of NixOS since it breaks the separation. With NixOS I can start a shell in a different iteration of my system without switching over the whole system. If I had all my software installed into standard places, that shell might find things it's not supposed to find.
Bottom line is: Most things work on NixOS out of the box. The PATH variable is adjusted accordingly to what a program is supposed to find, which in my opinion is perfectly reasonable and enough for software to find other software. The dynamic library paths are hardcoded as absolute paths, so software can find it's libraries. There's a special dynamic loader for binaries that don't adhere to this. And if you really need an FHS compliant environment NixOS gives you the tools to create one in a sandbox.
You can either have the perks of NixOS or use an FHS compliant distro. That's your choice.
That's what I said:
Imho there's a difference between "people abuse it" and "it is possible for programs to use software that they shouldn't even find". Anyway I noticed just now you weren't the one to actually ask the initial question of whether it's technically possible, so I apologize for not noticing this earlier. However I think it's a meaningless endeavor to ponder whether or not it's possible when that fact is irrelevant.
So that it's possible for different versions of software/libs to coexist on the same system
Because the nix package manager places all system packages under
/nix/store/uniquehash-packagename-version/Where the unique hash is obtained via a Merkel tree of all the inputs. So in particular, binaries and libraries exist underneath those directories, not in the places you would expect from FHS.
In order to make the system actually work, environment variables are set up and executables are patched to refer to specific paths within the Nix Store.
A lot of features of the Nix package manager, such as having multiple versions of a package installed, don't work with FHS.