Add simple unit test

Author: archang19

db/db_compaction_test.cc

@@ -10733,6 +10733,42 @@ TEST_F(DBCompactionTest, RecordNewestKeyTimeForTtlCompaction) {
   ASSERT_OK(dbfull()->TEST_WaitForCompact());
   ASSERT_EQ(NumTableFilesAtLevel(0), 0);
 }
+
+TEST_F(DBCompactionTest, RecordNumberCompactionInputIterators) {
+  Options options;
+  SetTimeElapseOnlySleepOnReopen(&options);
+  options.env = CurrentOptions().env;
+  options.compaction_style = kCompactionStyleFIFO;
+  options.statistics = ROCKSDB_NAMESPACE::CreateDBStatistics();
+  options.write_buffer_size = 10 << 10;  // 10KB
+  options.arena_block_size = 4096;
+  options.compression = kNoCompression;
+  options.create_if_missing = true;
+  options.compaction_options_fifo.allow_compaction = true;
+  options.num_levels = 1;
+  env_->SetMockSleep();
+  options.env = env_;
+  options.ttl = 1 * 60 * 60;  // 1 hour
+  ASSERT_OK(TryReopen(options));
+
+  // Generate and flush 4 files, each about 10KB
+  Random rnd(301);
+  for (int i = 0; i < 4; i++) {
+    for (int j = 0; j < 10; j++) {
+      ASSERT_OK(Put(std::to_string(i * 20 + j), rnd.RandomString(980)));
+    }
+    ASSERT_OK(Flush());
+    env_->MockSleepForSeconds(5);
+  }
+  ASSERT_OK(dbfull()->TEST_WaitForCompact());
+  ASSERT_EQ(NumTableFilesAtLevel(0), 1);
+  HistogramData num_compaction_input_iterators;
+  options.statistics->histogramData(NUM_COMPACTION_INPUT_ITERATORS,
+                                    &num_compaction_input_iterators);
+  ASSERT_EQ(num_compaction_input_iterators.sum, 4);
+
+  ASSERT_OK(options.statistics->Reset());
+}
 }  // namespace ROCKSDB_NAMESPACE
 
 int main(int argc, char** argv) {

db/db_impl/db_impl_compaction_flush.cc

@@ -64,6 +64,7 @@ bool DBImpl::EnoughRoomForCompaction(
 size_t DBImpl::GetNumberCompactionInputIterators(Compaction* c) {
   assert(c);
   if (c->start_level() == 0) {
+    assert(0 < c->num_input_levels());
     size_t num_l0_files = c->num_input_files(0);
     size_t num_non_l0_levels = c->num_input_levels() - 1;
     return num_l0_files + num_non_l0_levels;