Avoid divide by 0 in ComputeCompactionScore for FIFO compaction (facebook#13767)

Summary:
When max_table_files_size was accidentally configured with 0 value, engine could crash on divide by 0 operation. Although RocksDB do configuration validation during bootstrap, it typically does not do this for runtime dynamic parameter validation. Therefore, there is a chance where max_table_files_size could be set to 0. This PR only focuses on fixing a code path where max_table_files_size ack as divisor.

Pull Request resolved: facebook#13767

Test Plan: Unit test.

Reviewed By: cbi42

Differential Revision: D78420516

Pulled By: xingbowang

fbshipit-source-id: 6fdcc85b28a2c6319066665262b981e513719703

Author: Xingbo Wang

db/version_set.cc

@@ -3509,8 +3509,13 @@ void VersionStorageInfo::ComputeCompactionScore(
       }
 
       if (compaction_style_ == kCompactionStyleFIFO) {
-        score = static_cast<double>(total_size) /
-                mutable_cf_options.compaction_options_fifo.max_table_files_size;
+        auto max_table_files_size =
+            mutable_cf_options.compaction_options_fifo.max_table_files_size;
+        if (max_table_files_size == 0) {
+          // avoid divide 0
+          max_table_files_size = 1;
+        }
+        score = static_cast<double>(total_size) / max_table_files_size;
         if (score < 1 &&
             mutable_cf_options.compaction_options_fifo.allow_compaction) {
           score = std::max(