Fix a SingleDelete related optimization for blob indexes (#7904)

Summary:
There is a small `SingleDelete` related optimization in the
`CompactionIterator` code: when a `SingleDelete`-`Put` pair is preserved
solely for the purposes of transaction conflict checking, the value
itself gets cleared. (This is referred to as "optimization 3" in the
`CompactionIterator` code.) Though the rest of the code got updated to
support `SingleDelete`'ing blob indexes, this chunk was apparently
missed, resulting in an assertion failure (or `ROCKS_LOG_FATAL` in release
builds) when triggered. Note: in addition to clearing the value, we also
need to update the type of the KV to regular value when dealing with
blob indexes here.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7904

Test Plan: `make check`

Reviewed By: ajkr

Differential Revision: D26118009

Pulled By: ltamasi

fbshipit-source-id: 6bf78043d20265e2b15c2e1ab8865025040c42ae

db/compaction/compaction_iterator.cc

@@ -423,8 +423,8 @@ void CompactionIterator::NextFromInput() {
       // In the previous iteration we encountered a single delete that we could
       // not compact out.  We will keep this Put, but can drop it's data.
       // (See Optimization 3, below.)
-      assert(ikey_.type == kTypeValue);
-      if (ikey_.type != kTypeValue) {
+      assert(ikey_.type == kTypeValue || ikey_.type == kTypeBlobIndex);
+      if (ikey_.type != kTypeValue && ikey_.type != kTypeBlobIndex) {
         ROCKS_LOG_FATAL(info_log_,
                         "Unexpected key type %d for compaction output",
                         ikey_.type);
@@ -437,6 +437,11 @@ void CompactionIterator::NextFromInput() {
                         current_user_key_snapshot_, last_snapshot);
       }
 
+      if (ikey_.type == kTypeBlobIndex) {
+        ikey_.type = kTypeValue;
+        current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
+      }
+
       value_.clear();
       valid_ = true;
       clear_and_output_next_key_ = false;

db/compaction/compaction_iterator_test.cc

@@ -970,6 +970,21 @@ TEST_F(CompactionIteratorWithSnapshotCheckerTest,
           2 /*earliest_write_conflict_snapshot*/);
 }
 
+// Same as above but with a blob index. In addition to the value getting
+// trimmed, the type of the KV is changed to kTypeValue.
+TEST_F(CompactionIteratorWithSnapshotCheckerTest,
+       KeepSingleDeletionForWriteConflictChecking_BlobIndex) {
+  AddSnapshot(2, 0);
+  RunTest({test::KeyStr("a", 2, kTypeSingleDeletion),
+           test::KeyStr("a", 1, kTypeBlobIndex)},
+          {"", "fake_blob_index"},
+          {test::KeyStr("a", 2, kTypeSingleDeletion),
+           test::KeyStr("a", 1, kTypeValue)},
+          {"", ""}, 2 /*last_committed_seq*/, nullptr /*merge_operator*/,
+          nullptr /*compaction_filter*/, false /*bottommost_level*/,
+          2 /*earliest_write_conflict_snapshot*/);
+}
+
 // Compaction filter should keep uncommitted key as-is, and
 //   * Convert the latest velue to deletion, and/or
 //   * if latest value is a merge, apply filter to all suequent merges.