hw/char/serial: retry write if EAGAIN

If the chardev returns -1 with EAGAIN errno on write(), it should try
to send it again (EINTR is handled by the chardev itself).

This fixes commit 019288bf
"hw/char/serial: Only retry if qemu_chr_fe_write returns 0"
Tested-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180716110755.12499-1-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -260,15 +260,20 @@ 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) == 0 &&
-                   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;
+        } else {
+            int rc = qemu_chr_fe_write(&s->chr, &s->tsr, 1);
+
+            if ((rc == 0 ||
+                 (rc == -1 && errno == EAGAIN)) &&
+                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;
+                }
             }
         }
         s->tsr_retry = 0;