make tun(4) kq kevent data more like everything else

[David Gwynne]

kqueue stuff lets you provide some data when an event fires. generally
things that you read and write (ie, file descriptors) provide how much
data you can read or write. tun(4) provides how many packets you can
read, and how many bytes you could write.

im arguing that the number of packets is inconsistent with everything
else and should change. im also arguing we should calculate the maximum
write userland can do better too.

this is a very low impact change. i would bet that people use libevent,
not kq directly, and libevent doesn't really provide a way to get
to the kevent data. i just want to stop stumbling over the current code
when im reading tun(4).

ok?

Index: if_tun.c
===================================================================
RCS file: /cvs/src/sys/net/if_tun.c,v
retrieving revision 1.189
diff -u -p -r1.189 if_tun.c
--- if_tun.c    12 Sep 2019 01:28:29 -0000      1.189
+++ if_tun.c    12 Sep 2019 01:37:36 -0000
@@ -1008,9 +1008,6 @@ tun_dev_poll(struct tun_softc *tp, int e
  *
  * The tun driver uses an array of tun_softc's based on the minor number
  * of the device.  kn->kn_hook gets set to the specific tun_softc.
- *
- * filt_tunread() sets kn->kn_data to the iface qsize
- * filt_tunwrite() sets kn->kn_data to the MTU size
  */
 int
 tunkqfilter(dev_t dev, struct knote *kn)
@@ -1082,7 +1079,6 @@ filt_tunread(struct knote *kn, long hint
 {
        struct tun_softc        *tp;
        struct ifnet            *ifp;
-       unsigned int             len;
 
        if (kn->kn_status & KN_DETACHED) {
                kn->kn_data = 0;
@@ -1092,16 +1088,9 @@ filt_tunread(struct knote *kn, long hint
        tp = (struct tun_softc *)kn->kn_hook;
        ifp = &tp->tun_if;
 
-       len = IFQ_LEN(&ifp->if_snd);
-       if (len > 0) {
-               kn->kn_data = len;
+       kn->kn_data = ifq_hdatalen(&ifp->if_snd);
 
-               TUNDEBUG(("%s: tunkqread q=%d\n", ifp->if_xname,
-                   IFQ_LEN(&ifp->if_snd)));
-               return (1);
-       }
-       TUNDEBUG(("%s: tunkqread waiting\n", ifp->if_xname));
-       return (0);
+       return (kn->kn_data > 0);
 }
 
 void
@@ -1131,7 +1120,7 @@ filt_tunwrite(struct knote *kn, long hin
        tp = (struct tun_softc *)kn->kn_hook;
        ifp = &tp->tun_if;
 
-       kn->kn_data = ifp->if_mtu;
+       kn->kn_data = ifp->if_hdrlen + ifp->if_hardmtu;
 
        return (1);
 }