bug#66338: 30.0.50; grep-commnd set and using an old fish results in empty Copyright files

[Alex Schroeder]

My directories are littered with empty “Copyright” files. I think the
problem is that I use fish version 3.0.2 (current is 3.6.1) and I have
grep-command set.

So, using fish, start Emacs -Q.

(getenv "SHELL") ;; "/usr/bin/fish"
(setq grep-command "grep --color -niH -e ")
M-x grep

You don’t have to actually use grep. Invoking grep calls
grep-compute-defaults which – if grep-command is set – calls grep-probe
which calls process-file-shell-command. The resulting command:

/usr/bin/fish -c "grep --color -niH -e ^Copyright \
/usr/local/share/emacs/30.0.50/etc/HELLO"

If I run this in the shell, it hangs. I terminate with C-d and notice
that the empty file “Copyright” was created. If I quote the regular
expression, no problem:

/usr/bin/fish -c "grep --color -niH -e '^Copyright' \
/usr/local/share/emacs/30.0.50/etc/HELLO"

(This correctly produces a line of output.)

This seems to be a feature of fish. The following command generates an
empty file called “XXX”.

echo ^XXX

In the documentation I see the following regarding the caret, in a
section about feature flags:

“stderr-nocaret was introduced in fish 3.0 (and made the default in
3.3). It makes ^ an ordinary character instead of denoting an stderr
redirection, to make dealing with quoting and such easier. Use 2>
instead. This can no longer be turned off since fish 3.5. The flag can
still be tested for compatibility, but a no-stderr-nocaret value will
simply be ignored.”
https://fishshell.com/docs/current/language.html#featureflags

So this is the problem and it’s already improved in newer versions of
fish.

In my case, however:

alex@melanobombus ~> status features
stderr-nocaret  off     3.0     ^ no longer redirects stderr
qmark-noglob    off     3.0     ? no longer globs

I have since changed the stderr-nocaret to “on” for my system (and as
seen above, it is going to be mandatory going forward). This seems to do
the job. No change to Emacs was required.

I’m not quite sure what to suggest, here. All I can say is that I was
very angry about these empty Copyright files appearing every now and
then all over my directories. I looked at hooks, I grepped the sources
for "Copyright" (which didn’t match "^Copyright"), I looked at packages,
I just couldn’t find it, and I also couldn’t reliably reproduce it,
since it effectively only created an empty Copyright file the first time
I called grep in an Emacs session.

Given the difficulty in finding the source of the problem, perhaps some
sort of warning message or workaround is appropriate?

Cheers
Alex



In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5,
 cairo version 1.16.0) of 2023-10-04 built on melanobombus
Repository revision: fbfdd1e0e3e67f765c6dbf9f61b5f913e55e004e
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: PureOS

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LCMS2 LIBOTF LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
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