journal: include kmsg lines from the systemd process which exec()d us (systemd#8078)

Let the journal capture messages emitted by systemd, before it ran
exec("/usr/lib/systemd/systemd-journald").  Usually such messages will only
appear with `systemd.log_level=debug`.  kmsg lines written after the exec()
will be ignored as before.

In other words, we are avoiding reading our own lines, which start
"systemd-journald[100]: " assuming we are PID 100.  But now we will start
allowing ourself to read lines which start "systemd[100]: ", or any other
prefix which is not "systemd-journald[100]: ".

So this can't help you see messages when we fail to exec() journald :). But,
it makes it easier to see what the pre-exec() messages look like in
the successful case.  Comparing messages like this can be useful when
debugging.  Noticing weird omissions of messages, otoh, makes me anxious.

Author: sourcejedi

src/journal/journald-kmsg.c

@@ -98,15 +98,17 @@ void server_forward_kmsg(
                 log_debug_errno(errno, "Failed to write to /dev/kmsg for logging: %m");
 }
 
-static bool is_us(const char *pid) {
-        pid_t t;
+static bool is_us(const char *identifier, const char *pid) {
+        pid_t pid_num;
 
-        assert(pid);
+        if (!identifier || !pid)
+                return false;
 
-        if (parse_pid(pid, &t) < 0)
+        if (parse_pid(pid, &pid_num) < 0)
                 return false;
 
-        return t == getpid_cached();
+        return pid_num == getpid_cached() &&
+               streq(identifier, program_invocation_short_name);
 }
 
 static void dev_kmsg_record(Server *s, const char *p, size_t l) {
@@ -292,7 +294,7 @@ static void dev_kmsg_record(Server *s, const char *p, size_t l) {
 
                 /* Avoid any messages we generated ourselves via
                  * log_info() and friends. */
-                if (pid && is_us(pid))
+                if (is_us(identifier, pid))
                         goto finish;
 
                 if (identifier) {