Hello, l...@gnu.org (Ludovic Courtès) writes:
> Hi, > > Alex Vong <alexvong1...@gmail.com> skribis: > >> Mark H Weaver <m...@netris.org> writes: >> >>> Alex Vong <alexvong1...@gmail.com> writes: >>> >>>> I get the following error when running ``guix --version | head -n 1''. I >>>> can get similar after replacing ``--version'' with ``--help''. Also, the >>>> error is nondeterministic. Any idea? >>> >>> Attempts to write to a pipe that has already been closed on the other >>> end results in EPIPE. From the write(2) man page: >>> >>> EPIPE fd is connected to a pipe or socket whose reading end is closed. >>> When this happens the writing process will also receive a >>> SIGPIPE signal. (Thus, the write return value is seen only if >>> the program catches, blocks or ignores this signal.) >>> >>> In this case, there's a race condition. The result depends on whether >>> "head -n 1" closes its end of the pipe before or after "guix --version" >>> is finished writing all of its output. If "head -n 1" closes the pipe >>> first, then "guix --version" will receive EPIPE while attempting to >>> write to it. >>> >>> What normally happens is that the sending process receives SIGPIPE, >>> which simply causes it to exit prematurely without ever receiving this >>> error. However, since Guix arranges to ignore SIGPIPE in >>> 'initialize-guix' in guix/ui.scm, we receive EPIPE. >>> >>> That's what's happening here. I'll need to think on how best to fix it. >>> >>> Regards, >>> Mark >> >> Nice explaination as always! I forget to mention that I reported a bug >> of similar flavour before <http://bugs.gnu.org/27017>. I agree that >> thought is needed to fix all instances of this type of bug. > > Not sure! We specifically ignore EPIPE in cases where it matters, such > as for the output of ‘guix package --search’, ‘guix package -A’, etc. > In other cases, it’s probably an error, so it’s worth reporting. > > WDYT? > > In C such errors are usually ignored, which is nice for shell hackery > but otherwise not so great. > > Ludo’. Do you mean there are use-cases where the EPIPE signal really means there is an error? What I think is that the 'guix' command is meant to be used in a shell script, so it should work nice with other shell tools in a pipe, including head & tail. But maybe it will cause other problems if we always ignore EPIPE, I don't know...
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