qemu-img: Check for backing image if specified during create

Or, rather, force the open of a backing image if one was specified
for creation. Using a similar -unsafe option as rebase, allow qemu-img
to ignore the backing file validation if possible.

It may not always be possible, as in the existing case when a filesize
for the new image was not specified.

This is accomplished by shifting around the conditionals in
bdrv_img_create, such that a backing file is always opened unless we
provide BDRV_O_NO_BACKING. qemu-img is adjusted to pass this new flag
when -u is provided to create.

Sorry for the heinous looking diffstat, but it's mostly whitespace.

Inspired by: https://bugzilla.redhat.com/show_bug.cgi?id=1213786

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

1 files changed, 52 insertions, 42 deletions

diff --git a/block.c b/block.c
index 98a9209371..2dd9262cd0 100644
--- a/block.c
+++ b/block.c
@@ -4396,55 +4396,65 @@ void bdrv_img_create(const char *filename, const char *fmt,
 
     backing_fmt = qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT);
 
-    // The size for the image must always be specified, with one exception:
-    // If we are using a backing file, we can obtain the size from there
+    /* The size for the image must always be specified, unless we have a backing
+     * file and we have not been forbidden from opening it. */
     size = qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 0);
-    if (size == -1) {
-        if (backing_file) {
-            BlockDriverState *bs;
-            char *full_backing = g_new0(char, PATH_MAX);
-            int64_t size;
-            int back_flags;
-            QDict *backing_options = NULL;
-
-            bdrv_get_full_backing_filename_from_filename(filename, backing_file,
-                                                         full_backing, PATH_MAX,
-                                                         &local_err);
-            if (local_err) {
-                g_free(full_backing);
-                goto out;
-            }
+    if (backing_file && !(flags & BDRV_O_NO_BACKING)) {
+        BlockDriverState *bs;
+        char *full_backing = g_new0(char, PATH_MAX);
+        int back_flags;
+        QDict *backing_options = NULL;
+
+        bdrv_get_full_backing_filename_from_filename(filename, backing_file,
+                                                     full_backing, PATH_MAX,
+                                                     &local_err);
+        if (local_err) {
+            g_free(full_backing);
+            goto out;
+        }
 
-            /* backing files always opened read-only */
-            back_flags = flags;
-            back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
+        /* backing files always opened read-only */
+        back_flags = flags;
+        back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
-            if (backing_fmt) {
-                backing_options = qdict_new();
-                qdict_put_str(backing_options, "driver", backing_fmt);
-            }
+        if (backing_fmt) {
+            backing_options = qdict_new();
+            qdict_put_str(backing_options, "driver", backing_fmt);
+        }
 
-            bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
-                           &local_err);
-            g_free(full_backing);
-            if (!bs) {
-                goto out;
-            }
-            size = bdrv_getlength(bs);
-            if (size < 0) {
-                error_setg_errno(errp, -size, "Could not get size of '%s'",
-                                 backing_file);
-                bdrv_unref(bs);
-                goto out;
+        bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
+                       &local_err);
+        g_free(full_backing);
+        if (!bs && size != -1) {
+            /* Couldn't open BS, but we have a size, so it's nonfatal */
+            warn_reportf_err(local_err,
+                            "Could not verify backing image. "
+                            "This may become an error in future versions.\n");
+            local_err = NULL;
+        } else if (!bs) {
+            /* Couldn't open bs, do not have size */
+            error_append_hint(&local_err,
+                              "Could not open backing image to determine size.\n");
+            goto out;
+        } else {
+            if (size == -1) {
+                /* Opened BS, have no size */
+                size = bdrv_getlength(bs);
+                if (size < 0) {
+                    error_setg_errno(errp, -size, "Could not get size of '%s'",
+                                     backing_file);
+                    bdrv_unref(bs);
+                    goto out;
+                }
+                qemu_opt_set_number(opts, BLOCK_OPT_SIZE, size, &error_abort);
             }
-
-            qemu_opt_set_number(opts, BLOCK_OPT_SIZE, size, &error_abort);
-
             bdrv_unref(bs);
-        } else {
-            error_setg(errp, "Image creation needs a size parameter");
-            goto out;
         }
+    } /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+
+    if (size == -1) {
+        error_setg(errp, "Image creation needs a size parameter");
+        goto out;
     }
 
     if (!quiet) {