mds: add warning about encoding new fields

It's never a good idea to put new fields in the middle of other encoded values
unless you're willing to break all backwards-compatibility.

Signed-off-by: Patrick Donnelly <[email protected]>

Author: batrick

src/mds/CDentry.cc

@@ -568,6 +568,7 @@ void CDentry::encode_remote(inodeno_t& ino, unsigned char d_type,
 
   // marker, name, ino
   ENCODE_START(2, 1, bl);
+  // WARNING: always put new fields at the end of bl
   encode(ino, bl);
   encode(d_type, bl);
   encode(alternate_name, bl);

src/mds/CDir.cc

@@ -2506,6 +2506,7 @@ void CDir::_omap_commit_ops(int r, int op_prio, int64_t metapool, version_t vers
       bl.append('i');         // inode
 
       ENCODE_START(2, 1, bl);
+      // WARNING: always put new fields at the end of bl
       encode(item.alternate_name, bl);
       _encode_primary_inode_base(item, dfts, bl);
       ENCODE_FINISH(bl);