Fix compression dictionary sampling with dedicated range tombstone SSTs (#8141)

Summary:
Return early in case there are zero data blocks when
`BlockBasedTableBuilder::EnterUnbuffered()` is called. This crash can
only be triggered by applying dictionary compression to SST files that
contain only range tombstones. It cannot be triggered by a low buffer
limit alone since we only consider entering unbuffered mode after
buffering a data block causing the limit to be breached, or `Finish()`ing the file. It also cannot
be triggered by a totally empty file because those go through
`Abandon()` rather than `Finish()` so unbuffered mode is never entered.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8141

Test Plan: added a unit test that repro'd the "Floating point exception"

Reviewed By: riversand963

Differential Revision: D27495640

Pulled By: ajkr

fbshipit-source-id: a463cfba476919dc5c5c380800a75a86c31ffa23

HISTORY.md

@@ -8,6 +8,7 @@
 * Use thread-safe `strerror_r()` to get error messages.
 * Fixed a potential hang in shutdown for a DB whose `Env` has high-pri thread pool disabled (`Env::GetBackgroundThreads(Env::Priority::HIGH) == 0`)
 * Made BackupEngine thread-safe and added documentation comments to clarify what is safe for multiple BackupEngine objects accessing the same backup directory.
+* Fixed crash (divide by zero) when compression dictionary is applied to a file containing only range tombstones.
 
 ### Performance Improvements
 * On ARM platform, use `yield` instead of `wfe` to relax cpu to gain better performance.

db/db_range_del_test.cc

@@ -73,6 +73,15 @@ TEST_F(DBRangeDelTest, FlushOutputHasOnlyRangeTombstones) {
   } while (ChangeOptions(kRangeDelSkipConfigs));
 }
 
+TEST_F(DBRangeDelTest, DictionaryCompressionWithOnlyRangeTombstones) {
+  Options opts = CurrentOptions();
+  opts.compression_opts.max_dict_bytes = 16384;
+  Reopen(opts);
+  ASSERT_OK(db_->DeleteRange(WriteOptions(), db_->DefaultColumnFamily(), "dr1",
+                             "dr2"));
+  ASSERT_OK(db_->Flush(FlushOptions()));
+}
+
 TEST_F(DBRangeDelTest, CompactionOutputHasOnlyRangeTombstone) {
   do {
     Options opts = CurrentOptions();

table/block_based/block_based_table_builder.cc

@@ -1696,6 +1696,11 @@ void BlockBasedTableBuilder::EnterUnbuffered() {
                                   ? r->compression_opts.zstd_max_train_bytes
                                   : r->compression_opts.max_dict_bytes;
   const size_t kNumBlocksBuffered = r->data_block_and_keys_buffers.size();
+  if (kNumBlocksBuffered == 0) {
+    // The below code is neither safe nor necessary for handling zero data
+    // blocks.
+    return;
+  }
 
   // Abstract algebra teaches us that a finite cyclic group (such as the
   // additive group of integers modulo N) can be generated by a number that is