free(): invalid pointer

Describe the bug

I'm getting a core dump when running nix-instantiate:

$ nix-instantiate foo.nix -A some.attribute
free(): invalid pointer
Aborted (core dumped)

Unfortunately I can't share the code, but it seems to be happening when running a derivation that's then imported (IFD) but I'm not sure if that actually matters. -vv doesn't show anything suspicious. nix-store --realizeing the .drv works fine so I guess the problem is not in the drv's build.

After tweaking the Nix code a bit, I get a few more errors:

$ nix-instantiate foo.nix -A some-attribute
munmap_chunk(): invalid pointer
Aborted (core dumped)

and sometimes

Segmentation fault (core dumped)

(got that one in the repl)

Happy to share as much info as I can, I'll try to boil it down to a snippet I can share.

$ nix --version
nix (Nix) 2.4pre20200721_ff314f1

I'll try to reproduce.

Start by reducing the reported IFD case to a shareable snippet that reproduces the crash with nix-instantiate, noting whether the same .drv succeeds with nix-store --realize. Compare the invalid-pointer, segmentation-fault, and REPL cases; the work is done when the cause is identified and the reproducible invocation no longer aborts.