bug#25242: Cannot build source derivations with a custom TMPDIR

[Leo Famulari <leo@famulari.name>]

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On Wed, Dec 21, 2016 at 03:22:40AM -0500, Leo Famulari wrote:
> I ran the guix-daemon with strace, and I see these relevant lines:
> 
> 15337 [pid 30675] mkdir("/home/leo/tmp/guix-build/guix-build-nmap-7.40.tar.bz2.drv-0", 0700) = 0
> 15338 [pid 30675] getegid()                   = 0
> 15339 [pid 30675] chown("/home/leo/tmp/guix-build/guix-build-nmap-7.40.tar.bz2.drv-0", -1, 0) = 0

I believe this corresponds to the use of createTempDir() at
nix/libstore/build.cc:1718. The path of the new directory is saved in
the tmpDir variable.

> 15438 [pid 30693] chdir("/tmp/guix-build-nmap-7.40.tar.bz2.drv-0") = -1 ENOENT (No such file or directory)

And then later, at nix/libstore/build.cc:2204, we do this:

2204         if (chdir(tmpDirInSandbox.c_str()) == -1)
2205             throw SysError(format("changing into `%1%'") % tmpDir);

It tries to change to the wrong directory (the new "out of band"
downloader doesn't use a chroot, IIUC), and then prints a misleading
error message. This explains the discrepancy between what we see in
strace and on stderr.

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