kv: implement value_as_ptr() and use it in .get()

src/kv/KeyValueDB.h

@@ -163,6 +163,14 @@ class KeyValueDB {
     virtual std::pair<std::string,std::string> raw_key() = 0;
     virtual bool raw_key_is_prefixed(const std::string &prefix) = 0;
     virtual bufferlist value() = 0;
+    virtual bufferptr value_as_ptr() {
+      bufferlist bl = value();
+      if (bl.length()) {
+        return *bl.buffers().begin();
+      } else {
+        return bufferptr();
+      }
+    }
     virtual int status() = 0;
     virtual ~WholeSpaceIteratorImpl() { }
   };
@@ -223,6 +231,9 @@ class KeyValueDB {
     bufferlist value() {
       return generic_iter->value();
     }
+    bufferptr value_as_ptr() {
+      return generic_iter->value_as_ptr();
+    }
     int status() {
       return generic_iter->status();
     }

src/kv/LevelDBStore.cc

@@ -267,12 +267,13 @@ int LevelDBStore::get(const string &prefix,
 		  const string &key,
 		  bufferlist *value)
 {
+  assert(value && (value->length() == 0));
   utime_t start = ceph_clock_now(g_ceph_context);
   int r = 0;
   KeyValueDB::Iterator it = get_iterator(prefix);
   it->lower_bound(key);
   if (it->valid() && it->key() == key) {
-    *value = it->value();
+    value->append(it->value_as_ptr());
   } else {
     r = -ENOENT;
   }

src/kv/LevelDBStore.h

@@ -298,6 +298,12 @@ class LevelDBStore : public KeyValueDB {
     bufferlist value() {
       return to_bufferlist(dbiter->value());
     }
+
+    bufferptr value_as_ptr() {
+      leveldb::Slice data = dbiter->value();
+      return bufferptr(data.data(), data.size());
+    }
+
     int status() {
       return dbiter->status().ok() ? 0 : -1;
     }

src/kv/RocksDBStore.cc

@@ -366,12 +366,13 @@ int RocksDBStore::get(
     const string &key,
     bufferlist *out)
 {
+  assert(out && (out->length() == 0));
   utime_t start = ceph_clock_now(g_ceph_context);
   int r = 0;
   KeyValueDB::Iterator it = get_iterator(prefix);
   it->lower_bound(key);
   if (it->valid() && it->key() == key) {
-    *out = it->value();
+    out->append(it->value_as_ptr());
   } else {
     r = -ENOENT;
   }
@@ -589,6 +590,13 @@ bufferlist RocksDBStore::RocksDBWholeSpaceIteratorImpl::value()
 {
   return to_bufferlist(dbiter->value());
 }
+
+bufferptr RocksDBStore::RocksDBWholeSpaceIteratorImpl::value_as_ptr()
+{
+  rocksdb::Slice val = dbiter->value();
+  return bufferptr(val.data(), val.size());
+}
+
 int RocksDBStore::RocksDBWholeSpaceIteratorImpl::status()
 {
   return dbiter->status().ok() ? 0 : -1;
@@ -601,7 +609,6 @@ string RocksDBStore::past_prefix(const string &prefix)
   return limit;
 }
 
-
 RocksDBStore::WholeSpaceIterator RocksDBStore::_get_iterator()
 {
   return std::shared_ptr<KeyValueDB::WholeSpaceIteratorImpl>(

src/kv/RocksDBStore.h

@@ -192,6 +192,7 @@ class RocksDBStore : public KeyValueDB {
     pair<string,string> raw_key();
     bool raw_key_is_prefixed(const string &prefix);
     bufferlist value();
+    bufferptr value_as_ptr();
     int status();
   };
 

src/mon/MonitorDBStore.h

@@ -507,12 +507,8 @@ class MonitorDBStore
   }
 
   int get(const string& prefix, const string& key, bufferlist& bl) {
-    bufferlist outbl;
-    int r = db->get(prefix, key, &outbl);
-    if (!r) {
-      bl.append(outbl);
-    }
-    return r;
+    assert(bl.length() == 0);
+    return db->get(prefix, key, &bl);
   }
 
   int get(const string& prefix, const version_t ver, bufferlist& bl) {