fix: Implement singleflight for segcore ChunkCache (#34250)

See also #34249

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/core/src/storage/ChunkCache.cpp

@@ -15,32 +15,56 @@
 // limitations under the License.
 
 #include "ChunkCache.h"
+#include <future>
+#include <memory>
 #include "common/Types.h"
-#include "mmap/Utils.h"
 
 namespace milvus::storage {
 std::shared_ptr<ColumnBase>
 ChunkCache::Read(const std::string& filepath,
                  const MmapChunkDescriptorPtr& descriptor) {
+    // use rlock to get future
     {
         std::shared_lock lck(mutex_);
         auto it = columns_.find(filepath);
         if (it != columns_.end()) {
-            AssertInfo(it->second, "unexpected null column, file={}", filepath);
-            return it->second;
+            lck.unlock();
+            auto result = it->second.second.get();
+            AssertInfo(result, "unexpected null column, file={}", filepath);
+            return result;
         }
     }
 
-    auto field_data = DownloadAndDecodeRemoteFile(cm_.get(), filepath);
-
+    // lock for mutation
     std::unique_lock lck(mutex_);
+    // double check no-futurn
     auto it = columns_.find(filepath);
     if (it != columns_.end()) {
-        return it->second;
+        lck.unlock();
+        auto result = it->second.second.get();
+        AssertInfo(result, "unexpected null column, file={}", filepath);
+        return result;
     }
+
+    std::promise<std::shared_ptr<ColumnBase>> p;
+    std::shared_future<std::shared_ptr<ColumnBase>> f = p.get_future();
+    columns_.emplace(filepath, std::make_pair(std::move(p), f));
+    lck.unlock();
+
+    // release lock and perform download and decode
+    // other thread request same path shall get the future.
+    auto field_data = DownloadAndDecodeRemoteFile(cm_.get(), filepath);
     auto column = Mmap(field_data->GetFieldData(), descriptor);
+
+    // set promise value to notify the future
+    lck.lock();
+    it = columns_.find(filepath);
+    if (it != columns_.end()) {
+        // check pair exists then set value
+        it->second.first.set_value(column);
+    }
+    lck.unlock();
     AssertInfo(column, "unexpected null column, file={}", filepath);
-    columns_.emplace(filepath, column);
     return column;
 }
 
@@ -58,7 +82,7 @@ ChunkCache::Prefetch(const std::string& filepath) {
         return;
     }
 
-    auto column = it->second;
+    auto column = it->second.second.get();
     auto ok = madvise(
         reinterpret_cast<void*>(const_cast<char*>(column->MmappedData())),
         column->ByteSize(),

internal/core/src/storage/ChunkCache.h

@@ -15,6 +15,8 @@
 // limitations under the License.
 
 #pragma once
+#include <future>
+#include <unordered_map>
 #include "storage/MmapChunkManager.h"
 #include "mmap/Column.h"
 
@@ -60,8 +62,10 @@ class ChunkCache {
     CachePath(const std::string& filepath);
 
  private:
-    using ColumnTable =
-        std::unordered_map<std::string, std::shared_ptr<ColumnBase>>;
+    using ColumnTable = std::unordered_map<
+        std::string,
+        std::pair<std::promise<std::shared_ptr<ColumnBase>>,
+                  std::shared_future<std::shared_ptr<ColumnBase>>>>;
 
  private:
     mutable std::shared_mutex mutex_;