nix develop nix makes clangd fill up XDG_RUNTIME_DIR tmpfs

[roberth]

Describe the bug

nix develop changes TMPDIR to point to /run/user/1000 (typically), which is a tmpfs of limited size.

When programs, such as clangd, which we use for hacking on Nix itself, dump non-trivial amounts of information into TMPDIR, it causes various issues with not just the nix shell, but desktop programs as well.

Steps To Reproduce

1. Use clangd from .#native-clangStdenvPackages shell, on NixOS
2. Hack around a bit. clangd may generate a bunch of large .pch files, and put them in the undersized /run/user/1000 directory.

Expected behavior

Shell uses the normal tmpdir, which is larger.

nix-env --version output

Additional context

Priorities

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[whentze]

The behaviour of setting TMPDIR to $XDG_RUNTIME_DIR is simply busted, and has continued to cause all kinds of problems for nearly a decade now.

I have tried to summarize my feelings about this in #2957 (comment).

For now, you can work around this by

• Exporting TMPDIR=/tmp before entering nix develop or
• Increasing the size and inode limits on $XDG_RUNTIME_DIR.