[Libguestfs] [PATCH] Don't use cache=off if device is on tmpfs

[For a bug noted earlier by both Matt Booth and Jim Meyering] If you use the guestfs_add_drive function, then currently it generates a qemu command line element like: -drive ...,cache=off,... This causes qemu to try to open the device with O_DIRECT. Unfortunately some filesystems don't support this flag, notably tmpfs, which means you can't use libguestfs in conjunction with tmpfs. On some systems /tmp is a tmpfs filesystem. This patch fixes this so that if the filesystem doesn't support O_DIRECT, then we omit the cache=off parameter. This seems reasonable from a reliability point of view, because if you're using tmpfs then you probably didn't expect reliability in the case where your system suddenly powers off. Tested using a tmpfs. Without: $ guestfish alloc /mnt/tmp/test.img 10M : run qemu: could not open disk image /mnt/tmp/test.img libguestfs: error: connect: Connection refused libguestfs: error: connect: Connection refused libguestfs: error: connect: Connection refused libguestfs: error: connect: Connection refused With: $ ./fish/guestfish Welcome to guestfish, the libguestfs filesystem interactive shell for editing virtual machine filesystems. Type: 'help' for help with commands 'quit' to quit the shell Rich. -- Richard Jones, Emerging Technologies, Red Hat http://et.redhat.com/~rjones libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://et.redhat.com/~rjones/libguestfs/ See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html