Re: ignored SIGPIPE (Darwin vs. cat)

[Bruno Haible]

Eric Blake wrote:
> When running test-closein.sh, I'm getting spurious output on Darwin:
> 
> cat: standard output: Bad file descriptor
> PASS: test-closein.sh

How to reproduce? On Darwin 7 and 9 (MacOS X 10.3.x and 10.9.x) I reproduce an
error from
  $ cat foo | :
or
  $ { sleep 1; cat foo; } | :
only if I previously issued the command
  $ trap '' SIGPIPE

By default, or by using
  $ trap '-' SIGPIPE
or
  $ trap 'echo caught' SIGPIPE
I don't see an error.

> -# Test for lack of error on pipe
> -cat ${p}in.tmp | ./test-closein${EXEEXT} || exit 1
> +# Test for lack of error on pipe.  Ignore any EPIPE failures from cat.
> +cat ${p}in.tmp 2>/dev/null | ./test-closein${EXEEXT} || exit 1

Changes like this reduce the reliability and debuggability of shell scripts,
because you throw away *all* kinds of error output. If there was a typo
in the word 'cat', or the program did not find its shared libraries, or
is symlinks to an /etc/alternatives/cat which is misconfigured, or similar
general errors, we *want* to see the error message.

Is the situation where SIGPIPE is ignored a recurrent one? Sam Steingold
reported apparently this situation, now you too. I would prefer to add a
catch-all clause to the beginning of all tests which use pipes:

  #!/bin/sh
  if trap | grep "^trap -- ['\"]['\"] SIGPIPE\$" > /dev/null; then
    echo "Skipping test: SIGPIPE is ignored"
    exit 77
  fi

Now to the Darwin specific part of your mail:
> $ echo hi > foo
> $ cat foo | :
> cat: standard output: Bad file descriptor
> $ cat foo | { :; sleep 0.01; }
> $
> 
> It seems like the Darwin kernel is failing to give SIGPIPE/EPIPE failures,
> and instead gives EBADF when the read end of the pipe is gone.

On MacOS X 10.3.x and 10.5.x with bash 2 or bash 3 I see:
  $ trap ''  SIGPIPE
  $ { sleep 1; cat foo; } | :
  cat: write error: Broken pipe

What's the version of your shell? The problem might lie in the ':' built-in
command of the shell.

Bruno