Re: [Xen-devel] [OSSTEST PATCH 04/11] mg-debug-fail: New utility script for debugging

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("Re: [OSSTEST PATCH 04/11] mg-debug-fail: New utility 
script for debugging"):
> On Fri, 2015-12-04 at 19:35 +0000, Ian Jackson wrote:
> > diff --git a/mg-debug-fail b/mg-debug-fail
> > new file mode 100755
> > index 0000000..64fa235
> > --- /dev/null
> > +++ b/mg-debug-fail
> > @@ -0,0 +1,13 @@
> > +#!/bin/sh
> > +#
> > +# This script can be provided anywhere an executable or command name is
> > +# wanted.  It prints its arguments, and its stdin, to its stderr, and
> > +# then exits nonzero.
> > +#
> > +# When using this it may be useful to provide </dev/null as a
> > +# redirection for the whole program under test.  Otherwise things
> > +# can mysteriously hang.
> 
> "egrep . - /dev/null" is too noisy, it adds a "(standard input):" prefix
> which I don't think you want. But "egrep -h . - /dev/null" seems to remedy
> this without the possibility of these mysterious hangs.

This is confusing to me.  The problem is that mg-debug-fail does not
know whether to try to read and print its stdin.  Sometimes its stdin
will be the stdin for some utility that it is standing in for, and
should be dumped.  Whereas sometimes its stdin is the user's tty,
which should be left alone.

This difficulty is fixed in the next patch.  None of your suggestions
seem to address it.  What problem do you think they are solving ?

> Also, what does egrep give us here over just cat?

I meant to write   egrep ''
and its advantage is that it sanitises missing trailing newline.

But maybe we want to preserve missing trailing newline.

An alternative to replace the egrep would be
   sed 's/^/mg-debug-fail-stdin: /' >&2
which preserves missing trailing newline but annotates the output.

Ian.

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