Re: Regression caused by Commit: 5bd7ad2b225b ("Preserve the real value of -jN in MAKEFLAGS using jobserver.")

[Eric Melski]

On 10/30/2017 02:22 PM, Paul Smith wrote:
> On Tue, 2017-10-31 at 07:39 +1100, NeilBrown wrote:
> > On Mon, Oct 30 2017, Tim Murphy wrote:
> >
> > > For my money -j is always a top-level setting.   To have it per makefile
> > > and then let makefiles depend on that behaviour would open the door to lots
> > > of scary bugs.
>
> I don't see what scary bugs we might have; I've seen issues where
> makefiles not intended to be used with -j will not work properly with
> -j, but I'm not sure I've ever seen an issue with a makefile intended
> to work with -j that fails when run without it, or with a different
> value.

Here's a trivial example that shows how a build could fail if -j is not 
specified, but work if -j is specified (with any value, in this case), 
assuming "output" does not exist to start:


        all: reader writer
        
        reader:
                sleep 2
                cat output
        
        writer:
                echo PASS > output

Of course "work" is used somewhat loosely here, as the build might 
appear to work but in fact pick up stale data if "output" happens to 
exist prior to the start of the run (eg, in an incremental build).

It's not a common scenario, to be sure, but I have encountered this "in 
the wild".  Just last week I worked with a large commercial build which 
exhibited this behavior, due to a missing dependency between the reader 
and writer jobs.


Regards,

Eric Melski
Chief Architect
Electric Cloud, Inc.