Nix wipes top-level $TEMPDIR, disallowing other processes to create temporary files

[iggy-da]

The hook _nix_shell_clean_tmpdir may execute rm -rf $TEMPDIR. Observed on Ubuntu 24.04 and MacOS when using nix with direnv.

This happens when this tmpDir variable is assigned the $TEMPDIR path itself, rather than a temp subdirectory.

The $TEMPDIR is then rm -rfed here

nix/src/nix-build/nix-build.cc

Line 601 in 32becc8

(R"(_nix_shell_clean_tmpdir() { command rm -rf %1%; };)"s

This means processes trying to create temporary files inside $TEMPDIR (such as, for example, docker), will fail.

Fix: This tmpDir variable should never be allowed to be the top-level $TEMPDIR.