block: Deprecate bdrv_set_read_only() and users

bdrv_set_read_only() is used by some block drivers to override the
read-only option given by the user. This is not how read-only images
generally work in QEMU: Instead of second guessing what the user really
meant (which currently includes making an image read-only even if the
user didn't only use the default, but explicitly said read-only=off), we
should error out if we can't provide what the user requested.

This adds deprecation warnings to all callers of bdrv_set_read_only() so
that the behaviour can be corrected after the usual deprecation period.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/block/bochs.c b/block/bochs.c
index a759b6eff0..50c630047b 100644
--- a/block/bochs.c
+++ b/block/bochs.c
@@ -28,6 +28,7 @@
 #include "block/block_int.h"
 #include "qemu/module.h"
 #include "qemu/bswap.h"
+#include "qemu/error-report.h"
 
 /**************************************************************/
 
@@ -110,9 +111,15 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
-    ret = bdrv_set_read_only(bs, true, errp); /* no write support yet */
-    if (ret < 0) {
-        return ret;
+    if (!bdrv_is_read_only(bs)) {
+        error_report("Opening bochs images without an explicit read-only=on "
+                     "option is deprecated. Future versions will refuse to "
+                     "open the image instead of automatically marking the "
+                     "image read-only.");
+        ret = bdrv_set_read_only(bs, true, errp); /* no write support yet */
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     ret = bdrv_pread(bs->file, 0, &bochs, sizeof(bochs));