fix intra-L0 FIFO for uncompressed use case

Summary:
- inflate the argument passed as `max_compact_bytes_per_del_file` by a bit (10%). The intent of this argument is prevent L0 files from being intra-L0 compacted multiple times. Without compression, some intra-L0 compactions exceed this limit (and thus aren't executed), even though none of their files have gone through intra-L0 before.
- fix `FindIntraL0Compaction` as it was rejecting some valid intra-L0 compactions. In particular, `compact_bytes_per_del_file` is the work-per-deleted-file for the span [0, span_len), whereas `new_compact_bytes_per_del_file` is the work-per-deleted-file for the span [0, span_len+1). The former is more correct for checking whether we've found an eligible span.
Closes facebook#3684

Differential Revision: D7530396

Pulled By: ajkr

fbshipit-source-id: cad4f50902bdc428ac9ff6fffb13eb288648d85e

db/compaction_picker.cc

@@ -59,7 +59,7 @@ bool FindIntraL0Compaction(const std::vector<FileMetaData*>& level_files,
   }
 
   if (span_len >= min_files_to_compact &&
-      new_compact_bytes_per_del_file < max_compact_bytes_per_del_file) {
+      compact_bytes_per_del_file < max_compact_bytes_per_del_file) {
     assert(comp_inputs != nullptr);
     comp_inputs->level = 0;
     for (size_t i = 0; i < span_len; ++i) {
@@ -1580,11 +1580,21 @@ Compaction* FIFOCompactionPicker::PickSizeCompaction(
     if (mutable_cf_options.compaction_options_fifo.allow_compaction &&
         level_files.size() > 0) {
       CompactionInputFiles comp_inputs;
+      // try to prevent same files from being compacted multiple times, which
+      // could produce large files that may never TTL-expire. Achieve this by
+      // disallowing compactions with files larger than memtable (inflate its
+      // size by 10% to account for uncompressed L0 files that may have size
+      // slightly greater than memtable size limit).
+      size_t max_compact_bytes_per_del_file =
+          static_cast<size_t>(MultiplyCheckOverflow(
+              static_cast<uint64_t>(mutable_cf_options.write_buffer_size),
+              1.1));
       if (FindIntraL0Compaction(
               level_files,
               mutable_cf_options
-                  .level0_file_num_compaction_trigger /* min_files_to_compact */,
-              mutable_cf_options.write_buffer_size, &comp_inputs)) {
+                  .level0_file_num_compaction_trigger /* min_files_to_compact */
+              ,
+              max_compact_bytes_per_del_file, &comp_inputs)) {
         Compaction* c = new Compaction(
             vstorage, ioptions_, mutable_cf_options, {comp_inputs}, 0,
             16 * 1024 * 1024 /* output file size limit */,