Add blob support to DBIter (#7731)

Summary:
The patch adds iterator support to the integrated BlobDB implementation.
Whenever a blob reference is encountered during iteration, the corresponding
blob is retrieved by calling `Version::GetBlob`, assuming the `expose_blob_index`
(formerly `allow_blob`) flag is *not* set. (Note: the flag is set by the old stacked
BlobDB implementation, which has its own blob file handling/blob retrieval logic.)

In addition, `DBIter` now uniformly returns `Status::NotSupported` with the error
message `"BlobDB does not support merge operator."` when encountering a
blob reference while performing a merge (instead of potentially returning a
message that implies the database should be opened using the stacked BlobDB's
`Open`.)

TODO: We can implement support for lazily retrieving the blob value (or in other
words, bypassing the retrieval of blob values based on key) by extending the `Iterator`
API with a new `PrepareValue` method (similarly to `InternalIterator`, which already
supports lazy values).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7731

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D25256293

Pulled By: ltamasi

fbshipit-source-id: c39cd782011495a526cdff99c16f5fca400c4811

1 files changed, 4 insertions, 3 deletions

diff --git a/table/sst_file_reader.cc b/table/sst_file_reader.cc
index f7f22b061..9bb8bdd71 100644
--- a/table/sst_file_reader.cc
+++ b/table/sst_file_reader.cc
@@ -69,11 +69,12 @@ Iterator* SstFileReader::NewIterator(const ReadOptions& roptions) {
                       ? roptions.snapshot->GetSequenceNumber()
                       : kMaxSequenceNumber;
   ArenaWrappedDBIter* res = new ArenaWrappedDBIter();
-  res->Init(r->options.env, roptions, r->ioptions, r->moptions, sequence,
+  res->Init(r->options.env, roptions, r->ioptions, r->moptions,
+            nullptr /* version */, sequence,
             r->moptions.max_sequential_skip_in_iterations,
             0 /* version_number */, nullptr /* read_callback */,
-            nullptr /* db_impl */, nullptr /* cfd */, false /* allow_blob */,
-            false /* allow_refresh */);
+            nullptr /* db_impl */, nullptr /* cfd */,
+            true /* expose_blob_index */, false /* allow_refresh */);
   auto internal_iter = r->table_reader->NewIterator(
       res->GetReadOptions(), r->moptions.prefix_extractor.get(),
       res->GetArena(), false /* skip_filters */,