Revert "add_drive_ro adds readonly=on option if available." (RHBZ#617200).

Adding the readonly=on option is not so clever.  This causes
qemu to present the disk as read-only to the guest.  (The
expected behaviour of snapshots=on,readonly=on was that it
would open the disk O_RDONLY but present a writable disk to
the guest).

Since the guest sees a read-only disk, we are unable to do any
recovery if a filesystem on the disk is inconsistent.  This basically
prevents most accesses to live disk images.

What we really want is a qemu option which presents a writable
disk to the guest, but only opens the disk on the host side with
O_RDONLY, to alleviate the udev bug RHBZ#571714.

This reverts commit 676462684e05dd8341dd695762dd99a87d8ec022.

2 files changed, 5 insertions, 21 deletions

diff --git a/src/generator.ml b/src/generator.ml
index 69f561c1..372d01e6 100755
--- a/src/generator.ml
+++ b/src/generator.ml
@@ -559,15 +559,13 @@ handle is closed.  We don't currently have any method to enable
 changes to be committed, although qemu can support this.
 
 This is equivalent to the qemu parameter
-C<-drive file=filename,snapshot=on,readonly=on,if=...>.
+C<-drive file=filename,snapshot=on,if=...>.
 
 C<if=...> is set at compile time by the configuration option
 C<./configure --with-drive-if=...>.  In the rare case where you
 might need to change this at run time, use C<guestfs_add_drive_with_if>
 or C<guestfs_add_drive_ro_with_if>.
 
-C<readonly=on> is only added where qemu supports this option.
-
 Note that this call checks for the existence of C<filename>.  This
 stops you from specifying other types of drive which are supported
 by qemu such as C<nbd:> and C<http:> URLs.  To specify those, use
diff --git a/src/guestfs.c b/src/guestfs.c
index 85a042a0..d6c8d603 100644
--- a/src/guestfs.c
+++ b/src/guestfs.c
@@ -836,6 +836,9 @@ int
 guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
                                const char *drive_if)
 {
+  size_t len = strlen (filename) + 64;
+  char buf[len];
+
   if (strchr (filename, ',') != NULL) {
     error (g, _("filename cannot contain ',' (comma) character"));
     return -1;
@@ -846,24 +849,7 @@ guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
     return -1;
   }
 
-  if (qemu_supports (g, NULL) == -1)
-    return -1;
-
-  /* Only SCSI and virtio drivers support readonly mode.
-   * This is only supported as a QEMU feature since 2010/01.
-   */
-  int supports_ro = 0;
-  if ((STREQ (drive_if, "scsi") || STREQ (drive_if, "virtio")) &&
-      qemu_supports (g, "readonly=on"))
-    supports_ro = 1;
-
-  size_t len = strlen (filename) + 100;
-  char buf[len];
-
-  snprintf (buf, len, "file=%s,snapshot=on,%sif=%s",
-            filename,
-            supports_ro ? "readonly=on," : "",
-            drive_if);
+  snprintf (buf, len, "file=%s,snapshot=on,if=%s", filename, drive_if);
 
   return guestfs__config (g, "-drive", buf);
 }