Rename kRemoveWithSingleDelete to kPurge (#9951)

Summary:
PR 9929 adds a new CompactionFilter::Decision, i.e.
kRemoveWithSingleDelete so that CompactionFilter can indicate to
CompactionIterator that a PUT can only be removed with SD. However, how
CompactionIterator handles such a key is implementation detail which
should not be implied in the public API. In fact,
such a PUT can just be dropped. This is an optimization which we will apply in the near future.

Discussion thread: https://github.com/facebook/rocksdb/pull/9929#discussion_r863198964

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9951

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D36156590

Pulled By: riversand963

fbshipit-source-id: 7b7d01f47bba4cad7d9cca6ca52984f27f88b372

HISTORY.md

@@ -14,6 +14,7 @@
 * Add rollback_deletion_type_callback to TransactionDBOptions so that write-prepared transactions know whether to issue a Delete or SingleDelete to cancel a previous key written during prior prepare phase. The PR aims to prevent mixing SingleDeletes and Deletes for the same key that can lead to undefined behaviors for write-prepared transactions.
 * EXPERIMENTAL: Add new API AbortIO in file_system to abort the read requests submitted asynchronously.
 * CompactionFilter::Decision has a new value: kRemoveWithSingleDelete. If CompactionFilter returns this decision, then CompactionIterator will use `SingleDelete` to mark a key as removed.
+* Renamed CompactionFilter::Decision::kRemoveWithSingleDelete to kPurge since the latter sounds more general and hides the implementation details of how compaction iterator handles keys.
 
 ### Bug Fixes
 * RocksDB calls FileSystem::Poll API during FilePrefetchBuffer destruction which impacts performance as it waits for read requets completion which is not needed anymore. Calling FileSystem::AbortIO to abort those requests instead fixes that performance issue.

db/compaction/compaction_iterator.cc

@@ -307,7 +307,7 @@ bool CompactionIterator::InvokeFilterIfNeeded(bool* need_skip,
     // no value associated with delete
     value_.clear();
     iter_stats_.num_record_drop_user++;
-  } else if (filter == CompactionFilter::Decision::kRemoveWithSingleDelete) {
+  } else if (filter == CompactionFilter::Decision::kPurge) {
     // convert the current key to a single delete; key_ is pointing into
     // current_key_ at this point, so updating current_key_ updates key()
     ikey_.type = kTypeSingleDeletion;

db/db_compaction_filter_test.cc

@@ -998,7 +998,7 @@ TEST_F(DBTestCompactionFilter, DropKeyWithSingleDelete) {
                       std::string* /*new_value*/,
                       std::string* /*skip_until*/) const override {
       if (key.starts_with("b")) {
-        return Decision::kRemoveWithSingleDelete;
+        return Decision::kPurge;
       }
       return Decision::kRemove;
     }

db_stress_tool/db_stress_compaction_filter.h

@@ -51,8 +51,7 @@ class DbStressCompactionFilter : public CompactionFilter {
     key_mutex->Unlock();
 
     if (!key_exists) {
-      return allow_overwrite ? Decision::kRemove
+      return allow_overwrite ? Decision::kRemove : Decision::kPurge;
-                             : Decision::kRemoveWithSingleDelete;
     }
     return Decision::kKeep;
   }

include/rocksdb/compaction_filter.h

@@ -39,11 +39,11 @@ class CompactionFilter : public Customizable {
   enum class Decision {
     kKeep,
     kRemove,
-    kRemoveWithSingleDelete,
     kChangeValue,
     kRemoveAndSkipUntil,
     kChangeBlobIndex,  // used internally by BlobDB.
     kIOError,          // used internally by BlobDB.
+    kPurge,            // used for keys that can only be SingleDelete'ed
     kUndetermined,
   };