Parallel make bug in busybox

[Richard Purdie]

We (as in the Yocto Project/OpenEmbedded) do a lot of builds of busybox
and one of the failures we've seen recently was:

https://autobuilder.yoctoproject.org/main/builders/poky-tiny/builds/908
/steps/BuildImages/logs/stdio

It basically shows multiple executions of:

"make -f scripts/Makefile.build obj=applets"

in parallel which leads to multiple parallel executions of:

"applets/applet_tables include/applet_tables.h include/NUM_APPLETS.h"

which leads to temporarily truncated files and an eventual:

> libbb/appletlib.c:142:5: warning: "NUM_APPLETS" is not defined [
> -Wundef]
>  #if NUM_APPLETS > 8
>      ^~~~~~~~~~~
> libbb/appletlib.c: In function 'find_applet_by_name':
> libbb/appletlib.c:151:5: warning: "NUM_APPLETS" is not defined [
> -Wundef]
>  #if NUM_APPLETS > 8
>      ^~~~~~~~~~~
> libbb/appletlib.c:164:13: error: 'NUM_APPLETS' undeclared (first use
> in this function)
>   while (i < NUM_APPLETS) {
>              ^~~~~~~~~~~

since perhaps applet_tables.h was empty when this ran.

I haven't been able to reproduce this at will, it seems related to way
the makefile 
calls make within make, probably leading to a breakdown of locking. I'd
prefer not to
have to mark the recipe as parallel make unsafe but its looking risky
at the moment. 

One way I did find of breaking the build was:

make; rm libbb/appletlib.o include/NUM_APPLETS.h; make

which results in:

CC      shell/ash.o
shell/ash.c:59:25: fatal error: NUM_APPLETS.h: No such file or
directory

since NUM_APPLETS.h doesn't get regenerated by "make prepare" after its
deleted.

I'm hoping someone with a bit more experience of busybox's
makefiles/kbuild
might be able to spot the issues and fix them?

One nastier workaround would be to generate copies of the header files,
then 
move them into position so an invalid copy couldn't be read if the race
really 
can't be fixed.

Cheers,

Richard