c: support seek_for_prev

Summary:
support seek_for_prev in c abi.
Closes https://github.com/facebook/rocksdb/pull/1457

Differential Revision: D4135360

Pulled By: lightmark

fbshipit-source-id: 61256b0

db/c.cc

@@ -1031,6 +1031,11 @@ void rocksdb_iter_seek(rocksdb_iterator_t* iter, const char* k, size_t klen) {
   iter->rep->Seek(Slice(k, klen));
 }
 
+void rocksdb_iter_seek_for_prev(rocksdb_iterator_t* iter, const char* k,
+                                size_t klen) {
+  iter->rep->SeekForPrev(Slice(k, klen));
+}
+
 void rocksdb_iter_next(rocksdb_iterator_t* iter) {
   iter->rep->Next();
 }

db/c_test.c

@@ -495,6 +495,10 @@ int main(int argc, char** argv) {
     CheckIter(iter, "foo", "hello");
     rocksdb_iter_seek(iter, "b", 1);
     CheckIter(iter, "box", "c");
+    rocksdb_iter_seek_for_prev(iter, "g", 1);
+    CheckIter(iter, "foo", "hello");
+    rocksdb_iter_seek_for_prev(iter, "box", 3);
+    CheckIter(iter, "box", "c");
     rocksdb_iter_get_error(iter, &err);
     CheckNoError(err);
     rocksdb_iter_destroy(iter);

include/rocksdb/c.h

@@ -347,6 +347,9 @@ extern ROCKSDB_LIBRARY_API void rocksdb_iter_seek_to_first(rocksdb_iterator_t*);
 extern ROCKSDB_LIBRARY_API void rocksdb_iter_seek_to_last(rocksdb_iterator_t*);
 extern ROCKSDB_LIBRARY_API void rocksdb_iter_seek(rocksdb_iterator_t*,
                                                   const char* k, size_t klen);
+extern ROCKSDB_LIBRARY_API void rocksdb_iter_seek_for_prev(rocksdb_iterator_t*,
+                                                           const char* k,
+                                                           size_t klen);
 extern ROCKSDB_LIBRARY_API void rocksdb_iter_next(rocksdb_iterator_t*);
 extern ROCKSDB_LIBRARY_API void rocksdb_iter_prev(rocksdb_iterator_t*);
 extern ROCKSDB_LIBRARY_API const char* rocksdb_iter_key(