rbd: return 0 and an empty list when pool is entirely empty

PendingReleaseNotes

@@ -1,3 +1,12 @@
+v0.75
+~~~~~
+
+- 'rbd ls' on a pool which never held rbd images now exits with code
+  0. It outputs nothing in plain format, or an empty list in
+  non-plain format. This is consistent with the behavior for a pool
+  which used to hold images, but contains none. Scripts relying on
+  this behavior should be updated.
+
 v0.74
 ~~~~~
 

src/rbd.cc

@@ -251,6 +251,8 @@ static int do_list(librbd::RBD &rbd, librados::IoCtx& io_ctx, bool lflag,
 {
   std::vector<string> names;
   int r = rbd.list(io_ctx, names);
+  if (r == -ENOENT)
+    r = 0;
   if (r < 0)
     return r;
 
@@ -2777,14 +2779,7 @@ if (!set_conf_param(v, p1, p2, p3)) { \
   case OPT_LIST:
     r = do_list(rbd, io_ctx, lflag, formatter.get());
     if (r < 0) {
-      switch (r) {
-      case -ENOENT:
-        cerr << "rbd: pool " << poolname << " doesn't contain rbd images"
-	     << std::endl;
-        break;
-      default:
-        cerr << "rbd: list: " << cpp_strerror(-r) << std::endl;
-      }
+      cerr << "rbd: list: " << cpp_strerror(-r) << std::endl;
       return -r;
     }
     break;

src/test/cli-integration/rbd/formatted-output.t

@@ -1,3 +1,12 @@
+ls on empty pool never containing images
+========================================
+  $ rados -p rbd rm rbd_directory || true
+  $ rbd ls
+  $ rbd ls --format json
+  [] (no-eol)
+  $ rbd ls --format xml
+  <images></images> (no-eol)
+
 create
 =======
   $ rbd create -s 1024 foo