Temporary Writable Directory for Builds

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1

Some package builds need a temporary writable directory, be it because $HOME must be accessible, it relies on some caching or any other reason.

I went looking around on the nixpkgs repo and found at least three different solutions:

  1. mktemp — e. g., r2modman
  2. $TMPDIR — e. g., build-support/go/module.nix
  3. $NIX_BUILD_TOP — e. g., hare

Would there be a correct solution?

Regarding the ones above, I’ve managed to found at least an issue in regards to $NIX_BUILD_TOP:

https://github.com/NixOS/nixpkgs/issues/189691#issue-1361062317

However, its stated harmfulness is related to the use of nix-shell, not the build process.

2

mktemp -d is definitely the way to go! Most notably mktemp -d ensures that it’s a fresh directory. So you don’t have to worry about the path potentially being polluted already.

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3

Does Nix do anything to ensure that mktemp -d directories are cleaned up after a build? Or are package authors responsible for putting a trap 'rm -rf $whatever' EXIT in the relevant phase script?

4

Only the build directory ($NIX_BUILD_TOP, also the same as $TMPDIR) is writable by the build, and it’s definitely being cleaned up after the build :slight_smile:

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5

nix-shell doesn’t have any clean-up phase like nix-build, so I think it’s best if temporary directories are created inside the build directory/current working directory and/or using trap 'rm -rf...' EXIT.

6

Thanks for the clarification!, @Infinisil.

7

Do nix-shell and nix-build have the same value for TMPDIR? Because if nix-shell sets it to a tmpfs partition — e. g., /run —, the created directory would be cleaned up after a reboot and, thus, there would be no need for the trap call.

8

Sorry for digging this up, but it still seems like the most relevant issue, and I’ve been thinking about how to deal with this.

It looks like nix-shell respects TMPDIR and if it’s set, will use it for both TMPDIR and NIX_BUILD_TOP inside the shell.

nix develop goes a step further and invokes the shell with a fresh mktemp -d set as both TMPDIR and NIX_BUILD_TOP, which it cleans up when the shell exits.

I don’t think traps are really an option because you need some way to compose them, and stdenv already sets an EXIT trap. It does expose failureHook and exitHook, but only calls one or the other, and I don’t see anything in nixpkgs using these for cleanup.

Based on all that, I think the best option is just to use mktemp and either use nix develop, or be careful about the TMPDIR used with nix-shell.

Expecting NIX_BUILD_TOP (or TMPDIR) to be empty seems like a bad idea, given how nix-shell works. So I would avoid doing things like:

            install -m 0600 /dev/null "$NIX_BUILD_TOP/env-vars" &&
            export 2>/dev/null >| "$NIX_BUILD_TOP/env-vars"

    PGDATA="$NIX_BUILD_TOP/postgresql"

Unfortunately there are tons of places that do this.

Edit: Actually I was wrong about nix develop cleaning up the temp dir. It doesn’t seem to, at least not in all cases.

9

I don’t have the best sense of whether working around trap’s limitations here is enough to meet your needs, but here’s a post showing how to work around this with a trap-shimming shell library I wrote: