fdc: Fix inserting read-only media in empty drive

In order to insert a read-only medium (i.e. a read-only block node) to
the BlockBackend of a floppy drive, we must not have taken write
permissions on that BlockBackend, or the operation will fail with the
error message "Block node is read-only".

The device already takes care to remove all permissions when the medium
is ejected, but the state isn't correct if the drive is initially empty:
It uses blk_is_read_only() to check whether write permissions should be
taken, but this function returns false for empty BlockBackends in the
common case.

Fix floppy_drive_realize() to avoid taking write permissions if the
drive is empty.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

hw/block/fdc.c

@@ -514,6 +514,7 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp)
     FloppyDrive *dev = FLOPPY_DRIVE(qdev);
     FloppyBus *bus = FLOPPY_BUS(qdev->parent_bus);
     FDrive *drive;
+    bool read_only;
     int ret;
 
     if (dev->unit == -1) {
@@ -542,6 +543,12 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp)
         dev->conf.blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
         ret = blk_attach_dev(dev->conf.blk, qdev);
         assert(ret == 0);
+
+        /* Don't take write permissions on an empty drive to allow attaching a
+         * read-only node later */
+        read_only = true;
+    } else {
+        read_only = !blk_bs(dev->conf.blk) || blk_is_read_only(dev->conf.blk);
     }
 
     blkconf_blocksizes(&dev->conf);
@@ -559,9 +566,7 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp)
     dev->conf.rerror = BLOCKDEV_ON_ERROR_AUTO;
     dev->conf.werror = BLOCKDEV_ON_ERROR_AUTO;
 
-    if (!blkconf_apply_backend_options(&dev->conf,
-                                       blk_is_read_only(dev->conf.blk),
-                                       false, errp)) {
+    if (!blkconf_apply_backend_options(&dev->conf, read_only, false, errp)) {
         return;
     }