Make our utmpx more like System V.

When booting the system, truncate the utx.active file, but do write the
BOOT_TIME record into it afterwards. This allows one to obtain the boot
time of the system as follows:

	struct utmpx u1 = { .ut_type = BOOT_TIME }, *u2;

	setutxent();
	u2 = getutxid(&u1);

Now, the boot time is stored in u2->ut_tv, just like on Linux and other
systems.

We don't open the utx.active file with O_EXLOCK. It's rather unlikely
that other applications use this database at the same time and I want to
prevent the possibility of deadlocks in init(8).

Discussed with:	pluknet

lib/libc/gen/getutxent.3

@@ -301,7 +301,6 @@ The value of
 determines which databases are modified.
 .Pp
 Entries of type
-.Dv BOOT_TIME ,
 .Dv SHUTDOWN_TIME ,
 .Dv OLD_TIME
 and
@@ -335,7 +334,7 @@ In addition, entries of type
 .Dv BOOT_TIME
 and
 .Dv SHUTDOWN_TIME
-will cause all entries in
+will cause all existing entries in
 .Pa /var/run/utx.active
 to be discarded.
 .Pp

lib/libc/gen/pututxline.c

@@ -86,6 +86,9 @@ utx_active_add(const struct futx *fu)
 		return (-1);
 	while (fread(&fe, sizeof(fe), 1, fp) == 1) {
 		switch (fe.fu_type) {
+		case BOOT_TIME:
+			/* Leave these intact. */
+			break;
 		case USER_PROCESS:
 		case INIT_PROCESS:
 		case LOGIN_PROCESS:
@@ -170,6 +173,19 @@ utx_active_remove(struct futx *fu)
 	return (ret);
 }
 
+static void
+utx_active_init(const struct futx *fu)
+{
+	int fd;
+
+	/* Initialize utx.active with a single BOOT_TIME record. */
+	fd = _open(_PATH_UTX_ACTIVE, O_CREAT|O_RDWR|O_TRUNC, 0644);
+	if (fd < 0)
+		return;
+	_write(fd, fu, sizeof(*fu));
+	_close(fd);
+}
+
 static void
 utx_active_purge(void)
 {
@@ -277,9 +293,11 @@ pututxline(const struct utmpx *utmpx)
 
 	switch (fu.fu_type) {
 	case BOOT_TIME:
+		utx_active_init(&fu);
+		utx_lastlogin_upgrade();
+		break;
 	case SHUTDOWN_TIME:
 		utx_active_purge();
-		utx_lastlogin_upgrade();
 		break;
 	case OLD_TIME:
 	case NEW_TIME: