MB-29522: CouchKVStore::rollback: check return result of readVBState

As identified by UBSan, CouchKVStore::rollback() doesn't check the
return value of resdVBState; which can result in it attempting to
write a null local document to the rolled-back vBucket:

    [ RUN      ] CouchKVStoreErrorInjectionTest.readVBState_open_local_document
    runtime error: null pointer passed as argument 2, which is declared to never be null

    #0 0x7ffff7b5f30a in encode_root couchstore/src/node_types.cc:75
    #1 0x7ffff7b36033 in db_write_header couchstore/src/couch_db.cc:175
    #2 0x7ffff7b3f487 in couchstore_commit couchstore/src/couch_db.cc:255
    #3 0x12c0e6d in CouchKVStore::rollback(unsigned short, unsigned long, std::shared_ptr<RollbackCB>) kv_engine/engines/ep/src/couch-kvstore/couch-kvstore.cc:2674
    #4 0xd15cc6 in CouchKVStoreErrorInjectionTest_readVBState_open_local_document_Test::TestBody() kv_engine/engines/ep/tests/module_tests/kvstore_test.cc:1030

The local document is essential to interpreting the vBucket file; so
if we can't read it we need to fail the rollback.

Change-Id: I83871de2d4a96197bce17cbc9f1147792795a783
Reviewed-on: http://review.couchbase.org/93761
Tested-by: Build Bot <[email protected]>
Reviewed-by: Trond Norbye <[email protected]>

Author: daverigby

engines/ep/src/couch-kvstore/couch-kvstore.cc

@@ -2666,7 +2666,9 @@ RollbackResult CouchKVStore::rollback(uint16_t vbid, uint64_t rollbackSeqno,
         return RollbackResult(false, 0, 0, 0);
     }
 
-    readVBState(newdb, vbid);
+    if (readVBState(newdb, vbid) != ENGINE_SUCCESS) {
+        return RollbackResult(false, 0, 0, 0);
+    }
     cachedDeleteCount[vbid] = info.deleted_count;
     cachedDocCount[vbid] = info.doc_count;