WriteBatch::Put() overload that gathers key and value from arrays of slices

Summary: In our project, when writing to the database, we want to form the value as the concatenation of a small header and a larger payload.  It's a shame to have to copy the payload just so we can give RocksDB API a linear view of the value.  Since RocksDB makes a copy internally, it's easy to support gather writes.

Test Plan: write_batch_test, new test case

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13947

include/rocksdb/slice.h

@@ -98,6 +98,16 @@ class Slice {
   // Intentionally copyable
 };
 
+// A set of Slices that are virtually concatenated together.  'parts' points
+// to an array of Slices.  The number of elements in the array is 'num_parts'.
+struct SliceParts {
+  SliceParts(const Slice* parts, int num_parts) :
+      parts(parts), num_parts(num_parts) { }
+
+  const Slice* parts;
+  int num_parts;
+};
+
 inline bool operator==(const Slice& x, const Slice& y) {
   return ((x.size() == y.size()) &&
           (memcmp(x.data(), y.data(), x.size()) == 0));

include/rocksdb/write_batch.h

@@ -27,6 +27,7 @@
 namespace rocksdb {
 
 class Slice;
+class SliceParts;
 
 class WriteBatch {
  public:
@@ -36,6 +37,11 @@ class WriteBatch {
   // Store the mapping "key->value" in the database.
   void Put(const Slice& key, const Slice& value);
 
+  // Variant of Put() that gathers output like writev(2).  The key and value
+  // that will be written to the database are concatentations of arrays of
+  // slices.
+  void Put(const SliceParts& key, const SliceParts& value);
+
   // Merge "value" with the existing value of "key" in the database.
   // "key->merge(existing, value)"
   void Merge(const Slice& key, const Slice& value);