Fix the bug that compressed cache is disabled in read-only DBs (#6990)

Summary:
Compressed block cache is disabled in https://github.com/facebook/rocksdb/pull/4650 for no good reason. Re-enable it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6990

Test Plan: Add a unit test to make sure a general function works with read-only DB + compressed block cache.

Reviewed By: ltamasi

Differential Revision: D22072755

fbshipit-source-id: 2a55df6363de23a78979cf6c747526359e5dc7a1

HISTORY.md

@@ -14,6 +14,7 @@
 * Fix incorrect results from batched MultiGet for duplicate keys, when the duplicate key matches the largest key of an SST file and the value type for the key in the file is a merge value.
 * Best-efforts recovery ignores CURRENT file completely. If CURRENT file is missing during recovery, best-efforts recovery still proceeds with MANIFEST file(s).
 * In best-efforts recovery, an error that is not Corruption or IOError::kNotFound or IOError::kPathNotFound will be overwritten silently. Fix this by checking all non-ok cases and return early.
+* Compressed block cache was automatically disabled with read-only DBs by mistake. Now it is fixed: compressed block cache will be in effective with read-only DB too.
 
 ### Public API Change
 * Flush(..., column_family) may return Status::ColumnFamilyDropped() instead of Status::InvalidArgument() if column_family is dropped while processing the flush request.

db/db_test2.cc

@@ -86,6 +86,64 @@ TEST_F(DBTest2, OpenForReadOnlyWithColumnFamilies) {
   // With create_if_missing false, there should not be a dir in the file system
   ASSERT_NOK(env_->FileExists(dbname));
 }
+
+class TestReadOnlyWithCompressedCache
+    : public DBTestBase,
+      public testing::WithParamInterface<std::tuple<int, bool>> {
+ public:
+  TestReadOnlyWithCompressedCache()
+      : DBTestBase("/test_readonly_with_compressed_cache") {
+    max_open_files_ = std::get<0>(GetParam());
+    use_mmap_ = std::get<1>(GetParam());
+  }
+  int max_open_files_;
+  bool use_mmap_;
+};
+
+TEST_P(TestReadOnlyWithCompressedCache, ReadOnlyWithCompressedCache) {
+  if (use_mmap_ && !IsMemoryMappedAccessSupported()) {
+    return;
+  }
+  ASSERT_OK(Put("foo", "bar"));
+  ASSERT_OK(Put("foo2", "barbarbarbarbarbarbarbar"));
+  ASSERT_OK(Flush());
+
+  DB* db_ptr = nullptr;
+  Options options = CurrentOptions();
+  options.allow_mmap_reads = use_mmap_;
+  options.max_open_files = max_open_files_;
+  options.compression = kSnappyCompression;
+  BlockBasedTableOptions table_options;
+  table_options.block_cache_compressed = NewLRUCache(8 * 1024 * 1024);
+  table_options.no_block_cache = true;
+  options.table_factory.reset(NewBlockBasedTableFactory(table_options));
+  options.statistics = CreateDBStatistics();
+
+  ASSERT_OK(DB::OpenForReadOnly(options, dbname_, &db_ptr));
+
+  std::string v;
+  ASSERT_OK(db_ptr->Get(ReadOptions(), "foo", &v));
+  ASSERT_EQ("bar", v);
+  ASSERT_EQ(0, options.statistics->getTickerCount(BLOCK_CACHE_COMPRESSED_HIT));
+  ASSERT_OK(db_ptr->Get(ReadOptions(), "foo", &v));
+  ASSERT_EQ("bar", v);
+  if (Snappy_Supported()) {
+    if (use_mmap_) {
+      ASSERT_EQ(0,
+                options.statistics->getTickerCount(BLOCK_CACHE_COMPRESSED_HIT));
+    } else {
+      ASSERT_EQ(1,
+                options.statistics->getTickerCount(BLOCK_CACHE_COMPRESSED_HIT));
+    }
+  }
+
+  delete db_ptr;
+}
+
+INSTANTIATE_TEST_CASE_P(TestReadOnlyWithCompressedCache,
+                        TestReadOnlyWithCompressedCache,
+                        ::testing::Combine(::testing::Values(-1, 100),
+                                           ::testing::Bool()));
 #endif  // ROCKSDB_LITE
 
 class PrefixFullBloomWithReverseComparator

table/block_based/block_based_table_reader.cc

@@ -1426,10 +1426,8 @@ Status BlockBasedTable::MaybeReadBlockAndLoadToCache(
   assert(block_entry != nullptr);
   const bool no_io = (ro.read_tier == kBlockCacheTier);
   Cache* block_cache = rep_->table_options.block_cache.get();
-  // No point to cache compressed blocks if it never goes away
   Cache* block_cache_compressed =
-      rep_->immortal_table ? nullptr
-                           : rep_->table_options.block_cache_compressed.get();
+      rep_->table_options.block_cache_compressed.get();
 
   // First, try to get the block from the cache
   //