launch: Treat /dev/null specially, for old KVM.

Old KVM can't add /dev/null readonly.  Treat /dev/null as a special
case.

We also fix a few tests where /dev/null was being used with
format=qcow2.  This was always incorrect behaviour, but qemu appears
to tolerate it.

1 files changed, 19 insertions, 0 deletions

diff --git a/src/launch.c b/src/launch.c
index 72eca511..cd785014 100644
--- a/src/launch.c
+++ b/src/launch.c
@@ -347,6 +347,7 @@ guestfs__add_drive_opts (guestfs_h *g, const char *filename,
   char *iface;
   char *name;
   int use_cache_off;
+  int is_null;
 
   if (strchr (filename, ':') != NULL) {
     error (g, _("filename cannot contain ':' (colon) character. "
@@ -374,6 +375,24 @@ guestfs__add_drive_opts (guestfs_h *g, const char *filename,
     goto err_out;
   }
 
+  /* Traditionally you have been able to use /dev/null as a filename,
+   * as many times as you like.  Treat this as a special case, because
+   * old versions of qemu have some problems.
+   */
+  is_null = STREQ (filename, "/dev/null");
+  if (is_null) {
+    if (format && STRNEQ (format, "raw")) {
+      error (g, _("for device '/dev/null', format must be 'raw'"));
+      goto err_out;
+    }
+    /* Ancient KVM (RHEL 5) cannot handle the case where we try to add
+     * a snapshot on top of /dev/null.  Modern qemu can handle it OK,
+     * but the device size is still 0, so it shouldn't matter whether
+     * or not this is readonly.
+     */
+    readonly = 0;
+  }
+
   /* For writable files, see if we can use cache=off.  This also
    * checks for the existence of the file.  For readonly we have
    * to do the check explicitly.