bug#50441: Wrong build directory number shown in environment-variables file

[Sarah Morgensen <iskarian@mgsn.dev>]

Hello Christine,

Christine Lemmer-Webber <cwebber@dustycloud.org> writes:

> I was doing guix build with "--keep-missing" and looking around in a few
> of the output directories.  I was peeking in:
>
>   /tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-4
>
> But wait... the "environment-variables" file says:
>
> #+BEGIN_SRC sh
> export TEMP=\
> "/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
> export TEMPDIR=\
> "/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
> export TMP=\
> "/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
> export TMPDIR=\
> "/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
> #+END_SRC
>
> Now wait a minute.  Look at that last number.  What the hell is going on
> here?  Is this a bug in Guix?  Why is it pointing at -0 in the -4 build
> directory?
>
> Does this point at a more serious issue?  What's going on?

I've definitely noticed this before as well, and had to work around it
(I needed to inspect temp caches).  Thanks for actually sending a bug
report :)

I'm not at all familiar with the nix code, but it looks like this might
be related:

nix/libstore/build.cc:1654
--8<---------------cut here---------------start------------->8---
    /* In a sandbox, for determinism, always use the same temporary
       directory. */
    tmpDirInSandbox = useChroot ? canonPath("/tmp", true) + "/guix-build-" + drvName + "-0" : tmpDir;

    /* For convenience, set an environment pointing to the top build
       directory. */
    env["NIX_BUILD_TOP"] = tmpDirInSandbox;

    /* Also set TMPDIR and variants to point to this directory. */
    env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDirInSandbox;

    /* Explicitly set PWD to prevent problems with chroot builds.  In
       particular, dietlibc cannot figure out the cwd because the
       inode of the current directory doesn't appear in .. (because
       getdents returns the inode of the mount point). */
    env["PWD"] = tmpDirInSandbox;
--8<---------------cut here---------------end--------------->8---

I'm not entirely sure what's going on.  I think we build in chroot by
default, which would explain why the temp vars get set to "-0" even
though everything else seems to work okay.

--
Sarah