[dm-devel] [PATCH] kpartx: print "loop deleted" to stdout, not stderr

[Julian Andres Klode]

On Mon, Feb 05, 2018 at 02:58:46PM +0100, Martin Wilck wrote:
> On Mon, 2018-02-05 at 11:36 +0100, Julian Andres Klode wrote:
> > Commit fa643f5d2590028a59c671b81ab41383806fd258 moved some
> > code around and changed the print for loop deleted from stdout
> > to stderr - but this is not an error message, and also printed
> > to stdout in another place, so let's just use printf() again
> > here.
> > 
> > Signed-off-by: Julian Andres Klode <julian.klode at canonical.com>
> > ---
> >  kpartx/kpartx.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kpartx/kpartx.c b/kpartx/kpartx.c
> > index c1af1c5e..2e882721 100644
> > --- a/kpartx/kpartx.c
> > +++ b/kpartx/kpartx.c
> > @@ -399,7 +399,7 @@ main(int argc, char **argv){
> >  					       loopdev);
> >  				r = 1;
> >  			} else
> > -				fprintf(stderr, "loop deleted :
> > %s\n", loopdev);
> > +				printf("loop deleted : %s\n",
> > loopdev);
> >  		}
> >  		goto end;
> >  	}
> 
> 
> Hm. This is a log message, and as such, should go to stderr. kpartx, as
> a program primarily intended to run in udev rules, shouldn't print log
> messages to stdout unless we have compelling reasons for it to do so.
> 
> I'm aware that this isn't done consistently in kpartx at the moment,
> but IMO your patch goes in the wrong direction.

It seems very consistent for me: stderr is used for errors and warnings,
and printf() for the rest. There are two exceptions: This one, and

kpartx/devmapper.c:             fprintf(stderr, "found map %s for uuid %s, renaming to %s\n",

But that one really is what I'd consider a log message. Stuff like
"del devmap" and "loop deleted" seem somehow different - you tell
it to delete a mapping, and to be verbose, and then it should tell
you what it deleted, on stdout. Maybe people are parsing this, who
knows? It definitely broke our tests because unexpected stderr output
appeared.

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