flatcar-tmpfiles: Always copy missing entries over to the database

When a user or group entry is missing but the passwd/group file exists
it is not added by flatcar-tmpfiles and systemd-tmpfiles will then fail
because it can't resolve the user or group. We only care about the
fixed set of users like "core" because those are the ones that the user
will normally customize and this is also enough to fix the bug because
tmpfiles.d/baselayout-home.conf is the only place where not "root" is
used but another user which is "core".
Always copy the missing entries (from the fixed list of entries to be
copied) over to the database under /etc/, regardless if it exists or
not.

scripts/flatcar-tmpfiles

@@ -16,18 +16,21 @@ mkdir -p "${ROOT}/etc"
 
 # readable files
 umask 022
-if [[ ! -e "${ROOT}/etc/passwd" ]]; then
-    grep -E -e "^(${COPY_USERS}):" "${BASE}/passwd" > "${ROOT}/etc/passwd"
-fi
-if [[ ! -e "${ROOT}/etc/group" ]]; then
-    grep -E -e "^(${COPY_GROUPS}):" "${BASE}/group" > "${ROOT}/etc/group"
-fi
+# Output those lines in BASE/passwd (that are to be copied) when they are not in /etc/passwd already and append to /etc/passwd.
+# But since we don't want lines/half lines being added also be used as patterns, we copy the file first to memory
+# (if it doesn't exist we are fine with empty output as patterns which means to match empty lines which inverted is all).
+PATTERNS=$(cat "${ROOT}/etc/passwd" || true)
+grep -v -F -x -f <(echo "${PATTERNS}") <(grep -E -e "^(${COPY_USERS}):" "${BASE}/passwd") >> "${ROOT}/etc/passwd"
+PATTERNS=$(cat "${ROOT}/etc/group" || true)
+grep -v -F -x -f <(echo "${PATTERNS}") <(grep -E -e "^(${COPY_GROUPS}):" "${BASE}/group") >> "${ROOT}/etc/group"
 
 # secure files
 umask 027
-if [[ ! -e "${ROOT}/etc/shadow" ]]; then
-    grep -E -e "^(${COPY_USERS}):" "${BASE}/shadow" > "${ROOT}/etc/shadow"
-fi
-if [[ ! -e "${ROOT}/etc/gshadow" ]]; then
-    grep -E -e "^(${COPY_GROUPS}):" "${BASE}/gshadow" > "${ROOT}/etc/gshadow"
-fi
+PATTERNS=$(cat "${ROOT}/etc/shadow" || true)
+grep -v -F -x -f <(echo "${PATTERNS}") <(grep -E -e "^(${COPY_USERS}):" "${BASE}/shadow") >> "${ROOT}/etc/shadow"
+PATTERNS=$(cat "${ROOT}/etc/gshadow" || true)
+grep -v -F -x -f <(echo "${PATTERNS}") <(grep -E -e "^(${COPY_GROUPS}):" "${BASE}/gshadow") >> "${ROOT}/etc/gshadow"
+
+# The script runs without set -euo pipefail and allows grep to return 1, thus the last statement must not be grep
+# because it would propagate the exit code 1 and let the systemd unit fail.
+exit 0