Fix the select()-spin bug on disconnected devices.

Addresses Berlios tracker bug #14638: 100% cpu when Bluetooth gps device
vanishes.  Also a couple of reports on the mailing lists.

The underlying problem here was that:

* Disconnecting a USB device causes reads from it to begin returning 0,
  in effect an end-of-file condition.

* select(2) sets an active bit not on "I/O is ready" but on "read would not
  block" -- including the end-of-file condition.

* Therefore, select() will spin any time its fd set includes a disconnected
  device.

The fix is to check for a zero-length read explicitly and always take the
device out of the active set when that happens.

We were actually doing that here, but the test was defective in two ways:

1. The check for a zero return from gpsd_poll(), indicating I/O error
   or zero-length read, needed to be *before* the check for full paccket
   rather than after. This effectively disabled it.

2. There was a conditional arm in the gpsd_poll() code that made it ignore
   zero-length reads for up to a full cycle.  I think this may have been a
   fossil from long ago when I experimented with non-blocking reads on
   devices.

The fix for the gpsmon spin bug is probably quite similar.  That's up next.

1 files changed, 4 insertions, 13 deletions

diff --git a/libgpsd_core.c b/libgpsd_core.c
index 533ea907..72c637ea 100644
--- a/libgpsd_core.c
+++ b/libgpsd_core.c
@@ -621,21 +621,12 @@ gps_mask_t gpsd_poll(struct gps_device_t *session)
     /* update the scoreboard structure from the GPS */
     gpsd_report(LOG_RAW+2, "%s sent %zd new characters\n",
 		session->gpsdata.dev.path, newlen);
-   if (newlen == -1)	{		/* read error */
-	gpsd_report(LOG_INF, "GPS on %s is offline (%lf sec since data)\n",
-		    session->gpsdata.dev.path,
-		    timestamp() - session->gpsdata.online);
+    if (newlen <= 0)	{		/* read error or EOF*/   
+       gpsd_report(LOG_INF, "GPS on %s is offline (%lf sec since data)\n",
+		   session->gpsdata.dev.path,
+		   timestamp() - session->gpsdata.online);
 	session->gpsdata.online = 0;
 	return 0;
-    } else if (newlen == 0) {		/* no new data */
-	if (session->device_type != NULL && timestamp()>session->gpsdata.online+session->gpsdata.dev.cycle+1) {
-	    gpsd_report(LOG_INF, "GPS on %s is offline (%lf sec since data)\n",
-		    session->gpsdata.dev.path,
-		    timestamp() - session->gpsdata.online);
-	    session->gpsdata.online = 0;
-	    return 0;
-	} else
-	    return ONLINE_SET;
     } else if (session->packet.outbuflen == 0) {   /* got new data, but no packet */
 	gpsd_report(LOG_RAW+3, "New data on %s, not yet a packet\n",
 			    session->gpsdata.dev.path);