Fix blob db compression bug

Summary:
`CompressBlock()` will return the uncompressed slice (i.e. `Slice(value_unc)`) if compression ratio is not good enough. This is undesired. We need to always assign the compressed slice to `value`.
Closes https://github.com/facebook/rocksdb/pull/2447

Differential Revision: D5244682

Pulled By: yiwu-arbug

fbshipit-source-id: 6989dd8852c9622822ba9acec9beea02007dff09

utilities/blob_db/blob_db_impl.cc

@@ -897,15 +897,8 @@ Status BlobDBImpl::Write(const WriteOptions& opts, WriteBatch* updates) {
       last_file_ = bfile;
       has_put_ = true;
 
-      Slice value = value_unc;
       std::string compression_output;
-      if (impl_->bdb_options_.compression != kNoCompression) {
-        CompressionType ct = impl_->bdb_options_.compression;
-        CompressionOptions compression_opts;
-        value = CompressBlock(value_unc, compression_opts, &ct,
-                              kBlockBasedTableVersionFormat, Slice(),
-                              &compression_output);
-      }
+      Slice value = impl_->GetCompressedSlice(value_unc, &compression_output);
 
       std::string headerbuf;
       Writer::ConstructBlobHeader(&headerbuf, key, value, expiration, -1);
@@ -1018,6 +1011,18 @@ Status BlobDBImpl::PutWithTTL(const WriteOptions& options,
           : -1);
 }
 
+Slice BlobDBImpl::GetCompressedSlice(const Slice& raw,
+                                     std::string* compression_output) const {
+  if (bdb_options_.compression == kNoCompression) {
+    return raw;
+  }
+  CompressionType ct = bdb_options_.compression;
+  CompressionOptions compression_opts;
+  CompressBlock(raw, compression_opts, &ct, kBlockBasedTableVersionFormat,
+                Slice(), compression_output);
+  return *compression_output;
+}
+
 Status BlobDBImpl::PutUntil(const WriteOptions& options,
                             ColumnFamilyHandle* column_family, const Slice& key,
                             const Slice& value_unc, int32_t expiration) {
@@ -1028,15 +1033,8 @@ Status BlobDBImpl::PutUntil(const WriteOptions& options,
 
   if (!bfile) return Status::NotFound("Blob file not found");
 
-  Slice value = value_unc;
   std::string compression_output;
-  if (bdb_options_.compression != kNoCompression) {
-    CompressionType ct = bdb_options_.compression;
-    CompressionOptions compression_opts;
-    value = CompressBlock(value_unc, compression_opts, &ct,
-                          kBlockBasedTableVersionFormat, Slice(),
-                          &compression_output);
-  }
+  Slice value = GetCompressedSlice(value_unc, &compression_output);
 
   std::string headerbuf;
   Writer::ConstructBlobHeader(&headerbuf, key, value, expiration, -1);

utilities/blob_db/blob_db_impl.h

@@ -212,6 +212,9 @@ class BlobDBImpl : public BlobDB {
                    const std::string& index_entry, std::string* value,
                    SequenceNumber* sequence = nullptr);
 
+  Slice GetCompressedSlice(const Slice& raw,
+                           std::string* compression_output) const;
+
   // Just before flush starts acting on memtable files,
   // this handler is called.
   void OnFlushBeginHandler(DB* db, const FlushJobInfo& info);

utilities/blob_db/blob_db_test.cc

@@ -205,7 +205,7 @@ TEST_F(BlobDBTest, Override) {
 }
 
 #ifdef SNAPPY
-TEST_F(BlobDBTest, DISABLED_Compression) {
+TEST_F(BlobDBTest, Compression) {
   Random rnd(301);
   BlobDBOptionsImpl bdb_options;
   bdb_options.disable_background_tasks = true;