Discovered in Gentoo (https://bugs.gentoo.org/922896):

app-arch/tar-1.35 fails random tests (often test 160 multiv04.at, sometimes others), if run with MAKEOPTS="-j7 -l6". With MAKEOPTS=-j1 tests run fine.

160: split directory members in a MV archive         FAILED (multiv04.at:39)
[...]
## ------------- ##
## Test results. ##
## ------------- ##

ERROR: 227 tests were run,
1 failed unexpectedly.
17 tests were skipped.
## -------------------------- ##
## testsuite.log was created. ##
## -------------------------- ##

Please send `tests/testsuite.log' and all information you think might help:

   To: <bug-tar@gnu.org>
   Subject: [GNU tar 1.35] testsuite: 160 failed

You may investigate any problem if you feel able to do so, in which
case the test suite provides a good starting point.  Its output may
be found below `tests/testsuite.dir'.

make[2]: * [Makefile:2347: check-local] Error 1
make[2]: uscita dalla directory «/var/tmp/portage/app-arch/tar-1.35/work/tar-1.35/tests»
make[1]: * [Makefile:2193: check-am] Error 2
make[1]: uscita dalla directory «/var/tmp/portage/app-arch/tar-1.35/work/tar-1.35/tests»
make: * [Makefile:1882: check-recursive] Error 1

• ERROR: app-arch/tar-1.35::gentoo failed (test phase)

Relevant part of testsuite.log says:
#                             -*- compilation -*-
160. multiv04.at:36: testing split directory members in a MV archive ...
./multiv04.at:39:
mkdir gnu
(cd gnu
TEST_TAR_FORMAT=gnu
export TEST_TAR_FORMAT
TAR_OPTIONS="-H gnu"
export TAR_OPTIONS
rm -rf *


mkdir directory
awk 'BEGIN { for (i = 0; i < 1024; i++) printf("directory/%014x\n", i); }' </dev/null | genfile --files-from -

exec <&-

sleep 2

tar --listed-incremental=list -c -f archive.a directory

sleep 2

tar --listed-incremental=list -c --record-size 1024 -L 16 -f arc.1 -f arc.2 -v directory

echo separator

tar -MRt -f arc.1 -f arc.2)
./multiv04.at:39:
mkdir oldgnu
(cd oldgnu
TEST_TAR_FORMAT=oldgnu
export TEST_TAR_FORMAT
TAR_OPTIONS="-H oldgnu"
export TAR_OPTIONS
rm -rf *


mkdir directory
awk 'BEGIN { for (i = 0; i < 1024; i++) printf("directory/%014x\n", i); }' </dev/null | genfile --files-from -

exec <&-

sleep 2

tar --listed-incremental=list -c -f archive.a directory

sleep 2

tar --listed-incremental=list -c --record-size 1024 -L 16 -f arc.1 -f arc.2 -v directory

echo separator

tar -MRt -f arc.1 -f arc.2)
--- /dev/null   2024-01-22 16:45:05.792907304 +0100
+++ /var/tmp/portage/app-arch/tar-1.35/work/tar-1.35/tests/testsuite.dir/at-groups/160/stderr   2024-01-25 10:32:13.889659493 +0100
@@ -0,0 +1,6 @@
+tar: directory: Directory is new
+^GPrepare volume #3 for 'arc.1' and hit return: tar: EOF where user reply was expected
+tar: WARNING: Archive is incomplete
+tar: Error is not recoverable: exiting now
+^GPrepare volume #3 for 'arc.1' and hit return: tar: EOF where user reply was expected
+tar: Error is not recoverable: exiting now
--- -   2024-01-25 10:32:13.897657806 +0100
+++ /var/tmp/portage/app-arch/tar-1.35/work/tar-1.35/tests/testsuite.dir/at-groups/160/stdout   2024-01-25 10:32:13.889659493 +0100
@@ -1,5 +1,64 @@
 directory/
+directory/00000000000000
+directory/00000000000001
+directory/00000000000002
+directory/00000000000003
+directory/00000000000004
+directory/00000000000005
+directory/00000000000006
+directory/00000000000007
+directory/00000000000008
+directory/00000000000009
+directory/0000000000000a
+directory/0000000000000b
+directory/0000000000000c
+directory/0000000000000d
+directory/0000000000000e
+directory/0000000000000f
+directory/00000000000010
+directory/00000000000011
+directory/00000000000012
+directory/00000000000013
+directory/00000000000014
+directory/00000000000015
+directory/00000000000016
+directory/00000000000017
+directory/00000000000018
+directory/00000000000019
+directory/0000000000001a
+directory/0000000000001b
+directory/0000000000001c
+directory/0000000000001d
+directory/0000000000001e
 separator
 block 0: directory/
-block 35: * Block of NULs *
+block 35: directory/00000000000000
+block 36: directory/00000000000001
+block 37: directory/00000000000002
+block 38: directory/00000000000003
+block 39: directory/00000000000004
+block 40: directory/00000000000005
+block 41: directory/00000000000006
+block 42: directory/00000000000007
+block 43: directory/00000000000008
+block 44: directory/00000000000009
+block 45: directory/0000000000000a
+block 46: directory/0000000000000b
+block 47: directory/0000000000000c
+block 48: directory/0000000000000d
+block 49: directory/0000000000000e
+block 50: directory/0000000000000f
+block 51: directory/00000000000010
+block 52: directory/00000000000011
+block 53: directory/00000000000012
+block 54: directory/00000000000013
+block 55: directory/00000000000014
+block 56: directory/00000000000015
+block 57: directory/00000000000016
+block 58: directory/00000000000017
+block 59: directory/00000000000018
+block 60: directory/00000000000019
+block 61: directory/0000000000001a
+block 62: directory/0000000000001b
+block 63: directory/0000000000001c
 
./multiv04.at:39: exit code was 2, expected 0
160. multiv04.at:36: 160. split directory members in a MV archive (multiv04.at:36): FAILED (multiv04.at:39)