use bottommost compression when base level is bottommost

Summary:
The previous compression type selection caused unexpected behavior when the base level was also the bottommost level. The following sequence of events could happen:

- full compaction generates files with `bottommost_compression` type
- now base level is bottommost level since all files are in the same level
- any compaction causes files to be rewritten `compression_per_level` type since bottommost compression didn't apply to base level

I changed the code to make bottommost compression apply to base level.
Closes https://github.com/facebook/rocksdb/pull/3141

Differential Revision: D6264614

Pulled By: ajkr

fbshipit-source-id: d7aaa8675126896684154a1f2c9034d6214fde82

db/compaction_picker.cc

@@ -89,7 +89,7 @@ CompressionType GetCompressionType(const ImmutableCFOptions& ioptions,
   // If bottommost_compression is set and we are compacting to the
   // bottommost level then we should use it.
   if (ioptions.bottommost_compression != kDisableCompressionOption &&
-      level > base_level && level >= (vstorage->num_non_empty_levels() - 1)) {
+      level >= (vstorage->num_non_empty_levels() - 1)) {
     return ioptions.bottommost_compression;
   }
   // If the user has specified a different compression level for each level,

db/db_test2.cc

@@ -1152,7 +1152,7 @@ class CompactionCompressionListener : public EventListener {
     }
 
     if (db_options_->bottommost_compression != kDisableCompressionOption &&
-        ci.output_level == bottommost_level && ci.output_level >= 2) {
+        ci.output_level == bottommost_level) {
       ASSERT_EQ(ci.compression, db_options_->bottommost_compression);
     } else if (db_options_->compression_per_level.size() != 0) {
       ASSERT_EQ(ci.compression,

include/rocksdb/options.h

@@ -205,8 +205,7 @@ struct ColumnFamilyOptions : public AdvancedColumnFamilyOptions {
   CompressionType compression;
 
   // Compression algorithm that will be used for the bottommost level that
-  // contain files. If level-compaction is used, this option will only affect
+  // contain files.
-  // levels after base level.
   //
   // Default: kDisableCompressionOption (Disabled)
   CompressionType bottommost_compression = kDisableCompressionOption;