Sys_error("Broken pipe") printed to console on opam list

[craigfe]

On 2.1.0~alpha, piping opam list into a process that breaks the pipe causes a "Fatal error" message to be printed to the console. The exit code is 0 regardless.

ᐅ opam list | head -n 5
# Packages matching: installed
# Name                  # Installed  # Synopsis
asetmap                 0.8.1        Alternative, compatible, OCaml standard library Sets and Maps
astring                 0.8.3        Alternative String module for OCaml
base                    v0.13.2      Full standard library replacement for OCaml
Fatal error: exception Sys_error("Broken pipe")

ᐅ echo $?
0

I would expect opam to handle a SIGPIPE here with no error message.

[xavierleroy]

This issue was initially submitted on the ocaml/ocaml/ bug tracker, but it's really related to how OPAM chooses to handle SIGPIPE.

[craigfe]

Indeed, apologies. Thanks for taking care of it.

[AltGr]

Ha, funnily enough, the reason the error code is 0 is that in this case the exception happens in the finaliser, which runs flush stdout — after the main try with and the exit code has been decided.
So the good news is that this wouldn't happen if anything actually failed because of the broken pipe.

I am not sure what the better fix is: ensure to flush earlier ? Or just ignore the error once the main is done ? I guess that in this case, it would be best to exit with 0 and no error...

[avsm]

I think we should flush earlier before hitting the finaliser. We should avoid doing anything in the finalisers that might potentially raise. Also in this case, the flush stdout is a core part of what the point of the command is -- to list packages. That doesn't seem like a task for a finaliser :-)

[kit-ty-kate]

This issue is still there in 2.1.0~rc2

[AltGr]

From my tests the return value is actually 2, which is what OCaml returns by default on uncaught exceptions ?

(you need to set -o pipefail or run ( ./opam list; echo $? >&2 ) | head -n 5 instead or you'll get the exit code from head)