[GitHub] [arrow] lidavidm commented on a change in pull request #9613: PARQUET-1993: [C++] expose way to wait for I/O to complete

[GitBox <gi...@apache.org>]

lidavidm commented on a change in pull request #9613:
URL: https://github.com/apache/arrow/pull/9613#discussion_r587451127



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File path: cpp/src/arrow/io/caching.cc
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@@ -193,6 +195,36 @@ Result<std::shared_ptr<Buffer>> ReadRangeCache::Read(ReadRange range) {
   return Status::Invalid("ReadRangeCache did not find matching cache entry");
 }
 
+Future<> ReadRangeCache::Wait() {
+  struct State {
+    explicit State(std::vector<Future<std::shared_ptr<Buffer>>> f)
+        : futures(std::move(f)), remaining(futures.size()) {}
+    std::vector<Future<std::shared_ptr<Buffer>>> futures;
+    std::atomic<size_t> remaining;
+  };
+
+  std::vector<Future<std::shared_ptr<Buffer>>> futures;
+  for (const auto& entry : impl_->entries) {
+    futures.push_back(entry.future);
+  }
+  auto out = Future<>::Make();
+  auto state = std::make_shared<State>(std::move(futures));
+  for (const auto& future : state->futures) {
+    future.AddCallback([state, out](const Result<std::shared_ptr<Buffer>>&) mutable {
+      if (state->remaining.fetch_sub(1) != 1) return;

Review comment:
       Agreed. I can move this to future.h/future.cc since there's already a very similar helper there. 




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