virsh: blockresize: Introduce '--extend' flag

Use the new VIR_DOMAIN_BLOCK_RESIZE_EXTEND to prevent accidentally
shrinking a disk and thus destroying data.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/manpages/virsh.rst

@@ -1642,7 +1642,7 @@ blockresize
 
 ::
 
-   blockresize domain path ([size] | [--capacity])
+   blockresize domain path ([size] | [--capacity]) [--extend]
 
 Resize a block device of domain while the domain is running, *path*
 specifies the absolute path of the block device; it corresponds
@@ -1650,6 +1650,11 @@ to a unique target name (<target dev='name'/>) or source file (<source
 file='name'/>) for one of the disk devices attached to *domain* (see
 also ``domblklist`` for listing these names).
 
+The *--extend* flag instructs the hypervisor to ensure that the new size isn't
+smaller than the old size, thus prevents (accidental) shrinking of the block
+device which may lead to data loss. Users are encouraged to always use this
+flag unless communicating with an older hypervisor.
+
 For image formats without metadata (raw) stored inside fixed-size storage (e.g.
 block devices) the --capacity flag can be used to resize the device to the
 full size of the backing device.

tools/virsh-domain.c

@@ -3339,6 +3339,10 @@ static const vshCmdOptDef opts_blockresize[] = {
      .type = VSH_OT_BOOL,
      .help = N_("resize to capacity of source (block device)")
     },
+    {.name = "extend",
+     .type = VSH_OT_BOOL,
+     .help = N_("ensure that the new size is larger than actual capacity (prevent shrink)")
+    },
     {.name = NULL}
 };
 
@@ -3352,6 +3356,9 @@ cmdBlockresize(vshControl *ctl, const vshCmd *cmd)
 
     VSH_ALTERNATIVE_OPTIONS("size", "capacity");
 
+    if (vshCommandOptBool(cmd, "extend"))
+        flags |= VIR_DOMAIN_BLOCK_RESIZE_EXTEND;
+
     if (vshCommandOptString(ctl, cmd, "path", (const char **) &path) < 0)
         return false;