journal: refresh cached credentials of stdout streams

journald assumes that getsockopt(SO_PEERCRED) correctly identifies the
process on the remote end of the socket. However, this is incorrect
according to man 7 socket:

    The returned  credentials  are  those that were in effect at the
    time of the call to connect(2) or socketpair(2).

This becomes a problem when a new process inherits the stdout stream
from a parent. First, log messages from the child process will
be attributed to the parent. Second, the struct ucred used by journald
becomes invalid as soon as the parent exits. Further sendmsg calls then
fail with ENOENT. Logs for the child process then vanish from the journal.

Fix this by using recvmsg on the stdout stream, and refreshing the cached
struct ucred if SCM_CREDENTIALS indicate a new process.

Fixes #13708

test/TEST-04-JOURNAL/test-journal.sh

@@ -74,6 +74,22 @@ cmp /expected /output
 { journalctl -ball -b  -m 2>&1 || :; } | head -1 > /output
 cmp /expected /output
 
+# https://github.com/systemd/systemd/issues/13708
+ID=$(systemd-id128 new)
+systemd-cat -t "$ID" bash -c 'echo parent; (echo child) & wait' &
+PID=$!
+wait %%
+journalctl --sync
+# We can drop this grep when https://github.com/systemd/systemd/issues/13937
+# has a fix.
+journalctl -b -o export -t "$ID" --output-fields=_PID | grep '^_PID=' >/output
+[[ `grep -c . /output` -eq 2 ]]
+grep -q "^_PID=$PID" /output
+grep -vq "^_PID=$PID" /output
+
+# Add new tests before here, the journald restarts below
+# may make tests flappy.
+
 # Don't lose streams on restart
 systemctl start forever-print-hola
 sleep 3