Parallel make bug in busybox

[Richard Purdie]

On Sun, 2016-08-21 at 22:03 +0200, Denys Vlasenko wrote:
> On Fri, Aug 19, 2016 at 3:47 PM, Richard Purdie <rpurdie at rpsys.net>
> wrote:
> > Index: busybox-1.24.1/applets/applet_tables.c
> > ===================================================================
> > --- busybox-1.24.1.orig/applets/applet_tables.c
> > +++ busybox-1.24.1/applets/applet_tables.c
> > @@ -151,23 +151,15 @@ int main(int argc, char **argv)
> >  //     printf("#define MAX_APPLET_NAME_LEN %u\n",
> > MAX_APPLET_NAME_LEN);
> > 
> >         if (argv[2]) {
> > -               char line_old[80];
> >                 char line_new[80];
> >                 FILE *fp;
> > 
> > -               line_old[0] = 0;
> > -               fp = fopen(argv[2], "r");
> > -               if (fp) {
> > -                       fgets(line_old, sizeof(line_old), fp);
> > -                       fclose(fp);
> > -               }
> >                 sprintf(line_new, "#define NUM_APPLETS %u\n",
> > NUM_APPLETS);
> > -               if (strcmp(line_old, line_new) != 0) {
> > -                       fp = fopen(argv[2], "w");
> > -                       if (!fp)
> > -                               return 1;
> > -                       fputs(line_new, fp);
> > -               }
> > +               fp = fopen(argv[2], "w");
> > +               if (!fp)
> > +                       return 1;
> > +               fputs(line_new, fp);
> > +               fclose(fp);
> 
> This is done on purpose: if number of applets did not change, no need
> to rebuild files which depend on NUM_APPLETS.h

I appreciate the idea, however it doesn't work as the makefiles rely on
timestamps of the dependent objects changing and the above code means
the timestamps may or may not change. If they don't change, the rules
can keep being re-triggered as far as I can tell.

For example, if it rebuilds applet_tables for some reason, but when it
runs applet tables, NUM_APPLETS.h doesn't change, it will keep running
it over and over in submakes since NUM_APPLETS.h depends on
applet_tables and its older however it won't get its timestamp updated.

That is bad enough, however it will continually rewrite
applet_tables.h, and that gives rise to a build race since the file may
be getting rewritten when the compiler looks at it.

Having said all this, I applied my patch to our builds and we're still
seeing failures so there is some further problem :(

Cheers,

Richard