cls/rbd: Silence gcc7 maybe-uninitialized warning

[badone]

gcc7 warns that GroupSnapshotNamespace::group_pool may be use
uninitialized in SnapshotNamespaceOnDisk::decode. Adding a non-default
copy constructor resolves this.

Signed-off-by: Brad Hubbard bhubbard@redhat.com

[joscollin]

Good finding. But would it be a workaround for the existing gcc bug, still ?

[badone]

@joscollin if you are referring to https://bugzilla.redhat.com/show_bug.cgi?id=1439647 it was fixed in gcc-7.1.1-3.fc26. I'm using the current version, gcc-7.2.1-2.fc26.x86_64.

[dillaman]

We shouldn't be forced to create copy constructors when C++ should automatically provide a default. If we cannot trust that a std::string is properly copied, I think there are bigger issues w/ the compiler. That snapshot_id, however, could probably use a default to CEPH_NOSNAP (or similar).

[badone]

It's actually the int64_t that appears to be the problem. It appears to have something to do with the copies boost::variant is making and the copy constructor was just a guess that happened to resolve the warning. Not sure how much time we want to throw at this but I guess I can look deeper into it.

[badone]

@dillaman I've created a bug for this issue at https://bugzilla.redhat.com/show_bug.cgi?id=1507358 but it's not at all clear how long that will take to resolve. I've pushed an alternative patch which changes the order of the member variables and, I hope, should be lower impact given I don't think this is a public API? I know we shouldn't have to jump through these sorts of hoops to work around compiler bugs but, if this is a low impact change, maybe we can slip this in with the snapshot_id initialisation change?

[dillaman]

@badone Why not limit the scope of the changes to the single file?

diff --git a/src/cls/rbd/cls_rbd_types.h b/src/cls/rbd/cls_rbd_types.h
index b82bf452e5..98c5904a57 100644
--- a/src/cls/rbd/cls_rbd_types.h
+++ b/src/cls/rbd/cls_rbd_types.h
@@ -252,13 +252,13 @@ struct GroupSnapshotNamespace {
 
   GroupSnapshotNamespace(int64_t _group_pool,
                         const string &_group_id,
-                        const snapid_t &_snapshot_id) :group_pool(_group_pool),
-                                                       group_id(_group_id),
-                                                       snapshot_id(_snapshot_id) {}
+                        snapid_t _snapshot_id)
+    : group_id(_group_id), group_pool(_group_pool), snapshot_id(_snapshot_id) {
+  }
 
+  std::string group_id;
   int64_t group_pool = 0;
-  string group_id;
-  snapid_t snapshot_id;
+  snapid_t snapshot_id = CEPH_NOSNAP;
 
   void encode(bufferlist& bl) const;
   void decode(bufferlist::iterator& it);

[badone]

@dillaman Sure, that's fine, done.

[ceph-jenkins]

submodules for project are unmodified

[ceph-jenkins]

all commits in this PR are signed

[ceph-jenkins]

OK - docs built

[ceph-jenkins]

make check succeeded

[ceph-jenkins]

make check succeeded

[dillaman]

lgtm