reproducer script for GNU tar generating invalid tar files in multi-volume mode

tar-multivolume-bug-repro.sh

#!/bin/sh

set -x
TAR="${1:-tar}" # allow pointing at a different tar version for convenience

cat > info.sh <<EOF
#!/bin/sh
printf '%s.%d\n' "\$TAR_ARCHIVE" "\$TAR_VOLUME" >&\$TAR_FD
EOF
chmod +x info.sh

echo brokenfile > brokenfile
echo nextfile > nextfile

printf "\nif the previous file ends on a volume boundary, everything is fine:\n\n"
dd if=/dev/zero bs=$(( 4096 - 512 - 512 )) count=1 of=prevfile status=none
$TAR -F "$(pwd)/info.sh" --record-size=4k -L 4k -cf normal.tar prevfile brokenfile nextfile
# normal.tar contains: <type 0 for prevfile, size 3072> <prevfile> <type 0 for brokenfile, size 11> [EOF]
# normal.tar.2 contains: <type M for brokenfile, size 11> <brokenfile> <type 0 for nextfile, size 10> <nextfile> [EOF]

# tar works fine with the volume itself (without -M)
$TAR -tvf normal.tar.2
# M--------- 0/0              11 1970-01-01 01:00 brokenfile--Continued at byte 0--
# -rw-rw-r-- equinox/equinox   9 2026-01-21 12:36 nextfile

printf "\nif the previous file ends one block before a volume boundary, things break:\n\n"
dd if=/dev/zero bs=$(( 4096 - 512 )) count=1 of=prevfile status=none
$TAR -F "$(pwd)/info.sh" --record-size=4k -L 4k -cf broken.tar prevfile brokenfile nextfile
# broken.tar contains: <type 0 for prevfile, size 3584> <prevfile> [EOF]
# broken.tar.2 contains: <type M for brokenfile, size 11> <type 0 for brokenfile, size 11> <brokenfile> <type 0 for nextfile, size 10> <nextfile> [EOF]

# tar chokes because it's "off by one header" now
$TAR -tvf broken.tar.2
# M--------- 0/0              11 1970-01-01 01:00 brokenfile--Continued at byte 0--
# tar: Skipping to next header
# -rw-rw-r-- equinox/equinox   9 2026-01-21 12:36 nextfile
# tar: Exiting with failure status due to previous errors