CUURENT: make buildworld fails: install: libegacy.a: No such file or directory

This is strange:

Accidentally I started from two different terminals on the same box
"rm -rf /usr/obj/usr && make -j2 buildworld"

While the first proceess had a headstart, the second killed
the /usr/obj entries and so both jobs died immediately.

But after that, I tried simply again

"make buildworld"

and I get the message/error shown below. How is it possible that the
sources or the /usr/obj get that way corrupted, that I'm unable to
restart a "make buildworld"?

This smells like a "logical bug".

I'm stuck now. I remember that I had this situation once, but I do not
know the solution. Can this be fixed in a more general way? Why can
concurrent make buildworlds corrupt the base system/base sources that
way, that an subsequent make buildworld will fail as shown?

Please do not let me die stupid!

Regards,

Oliver


--------------------------------------------------------------
>>> stage 1.1: legacy release compatibility shims
--------------------------------------------------------------
cd /usr/src; MAKEOBJDIRPREFIX=/usr/obj/usr/src/tmp
INSTALL="sh /usr/src/tools/install.sh"
PATH=/usr/obj/usr/src/tmp/legacy/usr/sbin:/usr/obj/usr/src/tmp/legacy/usr/bin:/usr/obj/usr/src/tmp/legacy/usr/games:/usr/obj/usr/src/tmp/legacy/bin:/sbin:/bin:/usr/sbin:/usr/bin
WORLDTMP=/usr/obj/usr/src/tmp  VERSION="FreeBSD 10.0-CURRENT amd64
1000040"  MAKEFLAGS="-m /usr/src/tools/build/mk  -m /usr/src/share/mk"
COMPILER_TYPE=clang make  -f Makefile.inc1  DESTDIR=
BOOTSTRAPPING=1000040  SSP_CFLAGS=  -DWITHOUT_HTML -DWITHOUT_INFO
-DNO_LINT -DWITHOUT_MAN  -DNO_PIC -DNO_PROFILE -DNO_SHARED
-DNO_CPU_CFLAGS -DNO_WARNS -DNO_CTF -DEARLY_BUILD legacy ===>
tools/build
(obj,includes,depend,all,install) /usr/obj/usr/src/tmp/usr/src/tools/build
created for /usr/src/tools/build set -e; cd /usr/src/tools/build; make
buildincludes; make installincludes rm -f .depend mkdep -f .depend
-a    -I/usr/src/tools/build/../../contrib/libc-pwcache
-I/usr/src/tools/build/../../lib/libc/include
-I/usr/obj/usr/src/tmp/legacy/usr/include
-std=gnu99   /usr/src/tools/build/../../contrib/libc-pwcache/pwcache.c
sh /usr/src/tools/install.sh -C -o root -g wheel -m 444
libegacy.a /usr/obj/usr/src/tmp/legacy/usr/lib install: libegacy.a: No
such file or directory *** Error code 71

Stop.
make: stopped in /usr/src/tools/build
*** Error code 1