[PATCH synaptics] Reset num_active_touches on DeviceOff (#52496)

[Peter Hutterer]

When disabling the device, reset num_active_touches to zero. Otherwise,
num_active_touches stays at the value it was on DeviceOff(). Future touches
add to that value until the index may go past priv->open_slots[].
That causes spurious memory corruption on touch ends.

And as of 55fc42e7c9b4948cadd4f98ef7b6a3b12e268e3e we ignore pre-existing
touches anyway.

Test-case:
- place num_touches fingers on the touchpad
- xinput disable <device>
- lift fingers
- xinput enable <device>
- place finger on device, num_active_touches is now (num_touches + 1)

X.Org Bug 52496 <http://bugs.freedesktop.org/show_bug.cgi?id=52496>

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 src/synaptics.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/synaptics.c b/src/synaptics.c
index 27a37d7..0472197 100644
--- a/src/synaptics.c
+++ b/src/synaptics.c
@@ -977,6 +977,7 @@ SynapticsReset(SynapticsPrivate * priv)
     priv->lastButtons = 0;
     priv->prev_z = 0;
     priv->prevFingers = 0;
+    priv->num_active_touches = 0;
     memset(priv->open_slots, 0, priv->num_slots * sizeof(int));
 }
 
@@ -2611,6 +2612,7 @@ UpdateTouchState(InputInfoPtr pInfo, struct SynapticsHwState *hw)
         if (hw->slot_state[i] == SLOTSTATE_OPEN) {
             priv->open_slots[priv->num_active_touches] = i;
             priv->num_active_touches++;
+            BUG_WARN(priv->num_active_touches > priv->num_slots);
         }
         else if (hw->slot_state[i] == SLOTSTATE_CLOSE) {
             Bool found = FALSE;
-- 
1.7.11.2