qemu-char: Keep pty slave file descriptor open until the master is closed

If a process opens the slave pts device, writes data to it, then
immediately closes it, the data doesn't reliably get delivered to the
emulated serial port. This seems to be because a read of the master
pty device returns EIO on Linux if no process has the pts device open,
even when data is waiting "in the pipe".

A fix seems to be for QEMU to keep the pts file descriptor open until
the pty is closed, as per the below patch.
Signed-off-by: NAshley Jonathan <jonathan.ashley@altran.com>
Message-Id: <AC19797808C8D548ABDE0CA4A97AA30A30DEB409@XMB-DCFR-37.europe.corp.altran.com>
Reviewed-by: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -1171,6 +1171,7 @@ typedef struct {
     int connected;
     guint timer_tag;
     guint open_tag;
+    int slave_fd;
 } PtyCharDriver;
 
 static void pty_chr_update_read_handler_locked(CharDriverState *chr);
@@ -1347,6 +1348,7 @@ static void pty_chr_close(struct CharDriverState *chr)
 
     qemu_mutex_lock(&chr->chr_write_lock);
     pty_chr_state(chr, 0);
+    close(s->slave_fd);
     object_unref(OBJECT(s->ioc));
     if (s->timer_tag) {
         g_source_remove(s->timer_tag);
@@ -1374,7 +1376,6 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
         return NULL;
     }
 
-    close(slave_fd);
     qemu_set_nonblock(master_fd);
 
     chr = qemu_chr_alloc(common, errp);
@@ -1399,6 +1400,7 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
     chr->explicit_be_open = true;
 
     s->ioc = QIO_CHANNEL(qio_channel_file_new_fd(master_fd));
+    s->slave_fd = slave_fd;
     s->timer_tag = 0;
 
     return chr;