serial: remove watch on reset

Otherwise, this can cause serial_xmit to be entered with LSR.TEMT=0,
which is invalid and causes an assertion failure.

Reported-by: Bret Ketchum <bcketchum@gmail.com>
Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -228,6 +228,7 @@ static gboolean serial_watch_cb(GIOChannel *chan, GIOCondition cond,
                                 void *opaque)
 {
     SerialState *s = opaque;
+    s->watch_tag = 0;
     serial_xmit(s);
     return FALSE;
 }
@@ -258,10 +259,12 @@ static void serial_xmit(SerialState *s)
         if (s->mcr & UART_MCR_LOOP) {
             /* in loopback mode, say that we just received a char */
             serial_receive1(s, &s->tsr, 1);
-        } else if (qemu_chr_fe_write(s->chr, &s->tsr, 1) != 1) {
-            if (s->tsr_retry < MAX_XMIT_RETRY &&
-                qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,
-                                      serial_watch_cb, s) > 0) {
+        } else if (qemu_chr_fe_write(s->chr, &s->tsr, 1) != 1 &&
+                   s->tsr_retry < MAX_XMIT_RETRY) {
+            assert(s->watch_tag == 0);
+            s->watch_tag = qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,
+                                                 serial_watch_cb, s);
+            if (s->watch_tag > 0) {
                 s->tsr_retry++;
                 return;
             }
@@ -821,6 +824,11 @@ static void serial_reset(void *opaque)
 {
     SerialState *s = opaque;
 
+    if (s->watch_tag > 0) {
+        g_source_remove(s->watch_tag);
+        s->watch_tag = 0;
+    }
+
     s->rbr = 0;
     s->ier = 0;
     s->iir = UART_IIR_NO_INT;

include/hw/char/serial.h

@@ -56,6 +56,7 @@ struct SerialState {
     int it_shift;
     int baudbase;
     uint32_t tsr_retry;
+    guint watch_tag;
     uint32_t wakeup;
 
     /* Time when the last byte was successfully sent out of the tsr */