Switch to using task group instead of manually tracking tasks in flig…

…ht. This makes the walker thread safe so we can get rid of the mutex

cpp/src/arrow/filesystem/s3fs.cc

@@ -79,11 +79,13 @@
 #include "arrow/util/future.h"
 #include "arrow/util/logging.h"
 #include "arrow/util/optional.h"
+#include "arrow/util/task_group.h"
 #include "arrow/util/thread_pool.h"
 #include "arrow/util/windows_fixup.h"
 
 namespace arrow {
 
+using internal::TaskGroup;
 using internal::Uri;
 
 namespace fs {
@@ -1142,83 +1144,60 @@ struct TreeWalker : public std::enable_shared_from_this<TreeWalker> {
         recursion_handler_(std::move(recursion_handler)) {}
 
  private:
-  std::mutex mutex_;
-  Future<> future_;
-  std::atomic<int32_t> num_in_flight_;
+  std::shared_ptr<TaskGroup> task_group_;
 
   Status DoWalk() {
-    future_ = decltype(future_)::Make();
-    num_in_flight_ = 0;
+    task_group_ =
+        TaskGroup::MakeThreaded(io_context_.executor(), io_context_.stop_token());
     WalkChild(base_dir_, /*nesting_depth=*/0);
     // When this returns, ListObjectsV2 tasks either have finished or will exit early
-    return future_.status();
+    return task_group_->Finish();
   }
 
-  bool is_finished() const { return future_.is_finished(); }
-
-  void ListObjectsFinished(Status st) {
-    const auto in_flight = --num_in_flight_;
-    if (!st.ok() || !in_flight) {
-      future_.MarkFinished(std::move(st));
-    }
-  }
+  bool ok() const { return task_group_->ok(); }
 
   struct ListObjectsV2Handler {
     std::shared_ptr<TreeWalker> walker;
     std::string prefix;
     int32_t nesting_depth;
     S3Model::ListObjectsV2Request req;
 
-    void operator()(const Result<S3Model::ListObjectsV2Outcome>& result) {
+    Status operator()(const Result<S3Model::ListObjectsV2Outcome>& result) {
       // Serialize calls to operation-specific handlers
-      std::unique_lock<std::mutex> guard(walker->mutex_);
-      if (walker->is_finished()) {
+      if (!walker->ok()) {
         // Early exit: avoid executing handlers if DoWalk() returned
-        return;
+        return Status::OK();
       }
       if (!result.ok()) {
-        HandleError(result.status());
-        return;
+        return result.status();
       }
       const auto& outcome = *result;
       if (!outcome.IsSuccess()) {
-        Status st = walker->error_handler_(outcome.GetError());
-        HandleError(std::move(st));
-        return;
+        return walker->error_handler_(outcome.GetError());
       }
-      HandleResult(outcome.GetResult());
+      return HandleResult(outcome.GetResult());
     }
 
-    void SpawnListObjectsV2() {
+    Status SpawnListObjectsV2() {
       auto walker = this->walker;
       auto req = this->req;
-      auto maybe_fut = walker->io_context_.executor()->Submit(
-          walker->io_context_.stop_token(),
-          [walker, req]() { return walker->client_->ListObjectsV2(req); });
-      if (!maybe_fut.ok()) {
-        HandleError(maybe_fut.status());
-        return;
-      }
-      maybe_fut->AddCallback(*this);
+      auto cb = *this;
+      walker->task_group_->Append([walker, req, cb]() mutable {
+        Result<S3Model::ListObjectsV2Outcome> result =
+            walker->client_->ListObjectsV2(req);
+        return cb(result);
+      });
+      return Status::OK();
     }
 
-    void HandleError(Status status) { walker->ListObjectsFinished(std::move(status)); }
-
-    void HandleResult(const S3Model::ListObjectsV2Result& result) {
+    Status HandleResult(const S3Model::ListObjectsV2Result& result) {
       bool recurse = result.GetCommonPrefixes().size() > 0;
       if (recurse) {
-        auto maybe_recurse = walker->recursion_handler_(nesting_depth + 1);
-        if (!maybe_recurse.ok()) {
-          walker->ListObjectsFinished(maybe_recurse.status());
-          return;
-        }
-        recurse &= *maybe_recurse;
-      }
-      Status st = walker->result_handler_(prefix, result);
-      if (!st.ok()) {
-        walker->ListObjectsFinished(std::move(st));
-        return;
+        ARROW_ASSIGN_OR_RAISE(auto maybe_recurse,
+                              walker->recursion_handler_(nesting_depth + 1));
+        recurse &= maybe_recurse;
       }
+      RETURN_NOT_OK(walker->result_handler_(prefix, result));
       if (recurse) {
         walker->WalkChildren(result, nesting_depth + 1);
       }
@@ -1227,10 +1206,9 @@ struct TreeWalker : public std::enable_shared_from_this<TreeWalker> {
       if (result.GetIsTruncated()) {
         DCHECK(!result.GetNextContinuationToken().empty());
         req.SetContinuationToken(result.GetNextContinuationToken());
-        SpawnListObjectsV2();
-      } else {
-        walker->ListObjectsFinished(Status::OK());
+        RETURN_NOT_OK(SpawnListObjectsV2());
       }
+      return Status::OK();
     }
 
     void Start() {
@@ -1246,7 +1224,6 @@ struct TreeWalker : public std::enable_shared_from_this<TreeWalker> {
 
   void WalkChild(std::string key, int32_t nesting_depth) {
     ListObjectsV2Handler handler{shared_from_this(), std::move(key), nesting_depth, {}};
-    ++num_in_flight_;
     handler.Start();
   }