Fix cache metrics update when secondary cache is used (#10440)

Summary:
If a secondary cache is configured, its possible that a cache lookup will get a hit in the secondary cache. In that case, the ```LRUCacheShard::Lookup``` doesn't immediately update the ```total_charge``` for the item handle if the ```wait``` parameter is false (i.e caller will call later to check the completeness). However, ```BlockBasedTable::GetEntryFromCache``` assumes the handle is complete and calls ```UpdateCacheHitMetrics```, which checks the usage of the cache item and fails the assert in https://github.com/facebook/rocksdb/blob/main/cache/lru_cache.h#L237 (```assert(total_charge >= meta_charge)```).

To fix this, we call ```UpdateCacheHitMetrics``` later in ```MultiGet```, after waiting for all cache lookup completions.

Test plan -
Run crash test with changes from #10160

Pull Request resolved: #10440

Reviewed By: gitbw95

Differential Revision: D38283968

Pulled By: anand1976

fbshipit-source-id: 31c54ef43517726c6e5fdda81899b364241dd7e1

cache/lru_cache.cc

@@ -458,6 +458,7 @@ Cache::Handle* LRUCacheShard::Lookup(
       memcpy(e->key_data, key.data(), key.size());
       e->value = nullptr;
       e->sec_handle = secondary_handle.release();
+      e->total_charge = 0;
       e->Ref();
 
       if (wait) {

table/block_based/block_based_table_reader.cc

@@ -392,11 +392,16 @@ Cache::Handle* BlockBasedTable::GetEntryFromCache(
     cache_handle = block_cache->Lookup(key, rep_->ioptions.statistics.get());
   }
 
-  if (cache_handle != nullptr) {
-    UpdateCacheHitMetrics(block_type, get_context,
-                          block_cache->GetUsage(cache_handle));
-  } else {
-    UpdateCacheMissMetrics(block_type, get_context);
+  // Avoid updating metrics here if the handle is not complete yet. This
+  // happens with MultiGet and secondary cache. So update the metrics only
+  // if its a miss, or a hit and value is ready
+  if (!cache_handle || block_cache->Value(cache_handle)) {
+    if (cache_handle != nullptr) {
+      UpdateCacheHitMetrics(block_type, get_context,
+                            block_cache->GetUsage(cache_handle));
+    } else {
+      UpdateCacheMissMetrics(block_type, get_context);
+    }
   }
 
   return cache_handle;

table/block_based/block_based_table_reader_sync_and_async.h

@@ -484,14 +484,21 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::MultiGet)
           }
           results[i].UpdateCachedValue();
           void* val = results[i].GetValue();
+          Cache::Handle* handle = results[i].GetCacheHandle();
+          // GetContext for any key will do, as the stats will be aggregated
+          // anyway
+          GetContext* get_context = sst_file_range.begin()->get_context;
           if (!val) {
             // The async cache lookup failed - could be due to an error
             // or a false positive. We need to read the data block from
             // the SST file
             results[i].Reset();
             total_len += BlockSizeWithTrailer(block_handles[i]);
+            UpdateCacheMissMetrics(BlockType::kData, get_context);
           } else {
             block_handles[i] = BlockHandle::NullBlockHandle();
+            UpdateCacheHitMetrics(BlockType::kData, get_context,
+                                  block_cache->GetUsage(handle));
           }
         }
       }