Possible reasons why meld cannot access /dev/fd/* when run as unprivileged user?

Question

I suspected meld needed them to be also writable. However, it does say Could not read file (notice "read"); plus then the implication would be that they are writable for root, because there was no such error for root.

~$ diff <(echo foo) <(echo bar)
1c1
< foo
---
> bar

~$ meld <(echo foo) <(echo bar)  # not working, see comment below
~$ sudo -s
~# meld <(echo foo) <(echo bar)  # works just fine

The first one returns Could not read file and [Errno 2] No such file or directory: '/dev/fd/63'.

What do you suspect is the reason for this behavior?

Unprivileged user above, root below.

Answer 1

This happens when a Meld window is already open. In that case, running meld again tries to use the existing Meld process; but that process can’t access the /dev/fd files which are used for the substitution...

There doesn’t seem to be an option to force Meld to use the “new” process, ignoring all others.