rbd: allow copy of zero-length images. Includes simple test.

Fixes: #3765
Signed-off-by: Dan Mick <dan.mick@inktank.com>

qa/workunits/rbd/copy.sh

@@ -57,6 +57,12 @@ test_others() {
     rbd export testimg1 /tmp/img1.snap1
     cmp /tmp/img2 /tmp/img1.snap1
 
+    # test create, copy of zero-length images
+    rbd rm testimg2
+    rbd rm testimg3
+    rbd create testimg2 -s 0
+    rbd cp testimg2 testimg3
+
     # remove snapshots
     rbd snap rm --snap=snap1 testimg1
     rbd info --snap=snap1 testimg1 2>&1 | grep 'error setting snapshot context: (2) No such file or directory'

src/librbd/internal.cc

@@ -2439,6 +2439,10 @@ namespace librbd {
     if (!snap_exists)
       return -ENOENT;
 
+    // special-case "len == 0" requests: always valid
+    if (*len == 0)
+      return 0;
+
     // can't start past end
     if (off >= image_size)
       return -EINVAL;