[Qemu-devel] [PATCH 3/3] raw-posix: Re-open host CD-ROM after media change

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

Piggy-back on the guest CD-ROM polling to poll on the host.  Open and
close the host CD-ROM file descriptor to ensure we read the new size and
not a stale size.

Two things are going on here:

1. If hald/udisks is not already polling CD-ROMs on the host then
   re-opening the CD-ROM causes the host to read the new medium's size.

2. There is a bug in Linux which means the CD-ROM file descriptor must
   be re-opened in order for lseek(2) to see the new size.  The
   inode size gets out of sync with the underlying device (which you can
   confirm by checking that /sys/block/sr0/size and lseek(2) do not
   match after media change).  I have raised this with the
   maintainers but we need a workaround for the foreseeable future.

Note that these changes are all in a #ifdef __linux__ section.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block/raw-posix.c |   25 +++++++++++++++++++++----
 1 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index a95c8d4..ac95467 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -1240,10 +1240,27 @@ static int cdrom_is_inserted(BlockDriverState *bs)
     BDRVRawState *s = bs->opaque;
     int ret;
 
-    ret = ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT);
-    if (ret == CDS_DISC_OK)
-        return 1;
-    return 0;
+    /*
+     * Opening and closing on each drive status check ensures the medium size
+     * is refreshed.  If the file descriptor is kept open the size can become
+     * stale.  This is essentially replicating CD-ROM polling but is driven by
+     * the guest.  As the guest polls, we poll the host.
+     */
+
+    if (s->fd == -1) {
+        s->fd = qemu_open(bs->filename, s->open_flags, 0644);
+        if (s->fd < 0) {
+            return 0;
+        }
+    }
+
+    ret = (ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT) == CDS_DISC_OK);
+
+    if (!ret) {
+        close(s->fd);
+        s->fd = -1;
+    }
+    return ret;
 }
 
 static int cdrom_eject(BlockDriverState *bs, int eject_flag)
-- 
1.7.4.1