Add comment to DisableManualCompaction() (#8186)

Summary:
Add comment to DisableManualCompaction() which was missing.
Also explictly return from DBImpl::CompactRange() to avoid memtable flush when manual compaction is disabled.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8186

Test Plan: Run existing unit tests.

Reviewed By: jay-zhuang

Differential Revision: D27744517

fbshipit-source-id: 449548a48905903b888dc9612bd17480f6596a71

db/db_impl/db_impl_compaction_flush.cc

@@ -803,6 +803,10 @@ Status DBImpl::CompactRange(const CompactRangeOptions& options,
                             ColumnFamilyHandle* column_family,
                             const Slice* begin_without_ts,
                             const Slice* end_without_ts) {
+  if (manual_compaction_paused_.load(std::memory_order_acquire) > 0) {
+    return Status::Incomplete(Status::SubCode::kManualCompactionPaused);
+  }
+
   const Comparator* const ucmp = column_family->GetComparator();
   assert(ucmp);
   size_t ts_sz = ucmp->timestamp_size();

include/rocksdb/db.h

@@ -1180,7 +1180,13 @@ class DB {
   virtual Status EnableAutoCompaction(
       const std::vector<ColumnFamilyHandle*>& column_family_handles) = 0;
 
+  // After this function call, CompactRange() or CompactFiles() will not
+  // run compactions and fail. The function will wait for all outstanding
+  // manual compactions to finish before returning
   virtual void DisableManualCompaction() = 0;
+  // Re-enable CompactRange() and ComapctFiles() that are disabled by
+  // DisableManualCompaction(). In debug mode, it might hit assertion if
+  // no DisableManualCompaction() was previously called.
   virtual void EnableManualCompaction() = 0;
 
   // Number of levels used for this DB.