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ARROW-12040: [C++] Fix potential deadlock in recursive S3 walks
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From a deadlocked run...

```
#0  0x00007f8a5d48dccd in __lll_lock_wait () from /lib64/libpthread.so.0
#1  0x00007f8a5d486f05 in pthread_mutex_lock () from /lib64/libpthread.so.0
#2  0x00007f8a566e7e89 in arrow::internal::FnOnce<void ()>::FnImpl<arrow::Future<Aws::Utils::Outcome<Aws::S3::Model::ListObjectsV2Result, Aws::S3::S3Error> >::Callback<arrow::fs::(anonymous namespace)::TreeWalker::ListObjectsV2Handler> >::invoke() () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#3  0x00007f8a5650efa0 in arrow::FutureImpl::AddCallback(arrow::internal::FnOnce<void ()>) () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#4  0x00007f8a566e67a9 in arrow::fs::(anonymous namespace)::TreeWalker::ListObjectsV2Handler::SpawnListObjectsV2() () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#5  0x00007f8a566e723f in arrow::fs::(anonymous namespace)::TreeWalker::WalkChild(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int) () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#6  0x00007f8a566e827d in arrow::internal::FnOnce<void ()>::FnImpl<arrow::Future<Aws::Utils::Outcome<Aws::S3::Model::ListObjectsV2Result, Aws::S3::S3Error> >::Callback<arrow::fs::(anonymous namespace)::TreeWalker::ListObjectsV2Handler> >::invoke() () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#7  0x00007f8a5650efa0 in arrow::FutureImpl::AddCallback(arrow::internal::FnOnce<void ()>) () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#8  0x00007f8a566e67a9 in arrow::fs::(anonymous namespace)::TreeWalker::ListObjectsV2Handler::SpawnListObjectsV2() () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#9  0x00007f8a566e723f in arrow::fs::(anonymous namespace)::TreeWalker::WalkChild(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int) () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
#10 0x00007f8a566e74b1 in arrow::fs::(anonymous namespace)::TreeWalker::DoWalk() () from /arrow/r/check/arrow.Rcheck/arrow/libs/arrow.so
```

The callback `ListObjectsV2Handler` is being called recursively and the mutex is non-reentrant thus deadlock.

To fix it I got rid of the mutex on `TreeWalker` by using `arrow::util::internal::TaskGroup` instead of manually tracking the #/status of in-flight requests.

Closes #9842 from westonpace/bugfix/arrow-12040

Lead-authored-by: Weston Pace <[email protected]>
Co-authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>
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@westonpace @pitrou
westonpace and pitrou committed Apr 7, 2021
1 parent d95c72f commit be8892e
Showing 1 changed file with 34 additions and 51 deletions.
85 changes: 34 additions & 51 deletions cpp/src/arrow/filesystem/s3fs.cc
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -1142,82 +1144,65 @@ struct TreeWalker : public std::enable_shared_from_this<TreeWalker> {
recursion_handler_(std::move(recursion_handler)) {}

private:
std::shared_ptr<TaskGroup> task_group_;
std::mutex mutex_;
Future<> future_;
std::atomic<int32_t> num_in_flight_;

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;
{
std::lock_guard<std::mutex> guard(walker->mutex_);
return walker->error_handler_(outcome.GetError());
}
}
HandleResult(outcome.GetResult());
return HandleResult(outcome.GetResult());
}

void 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([cb]() mutable {
Result<S3Model::ListObjectsV2Outcome> result =
cb.walker->client_->ListObjectsV2(cb.req);
return cb(result);
});
}

void HandleError(Status status) { walker->ListObjectsFinished(std::move(status)); }

void 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;
Status HandleResult(const S3Model::ListObjectsV2Result& result) {
bool recurse;
{
// Only one thread should be running result_handler_/recursion_handler_ at a time
std::lock_guard<std::mutex> guard(walker->mutex_);
recurse = result.GetCommonPrefixes().size() > 0;
if (recurse) {
ARROW_ASSIGN_OR_RAISE(auto maybe_recurse,
walker->recursion_handler_(nesting_depth + 1));
recurse &= maybe_recurse;
}
recurse &= *maybe_recurse;
}
Status st = walker->result_handler_(prefix, result);
if (!st.ok()) {
walker->ListObjectsFinished(std::move(st));
return;
RETURN_NOT_OK(walker->result_handler_(prefix, result));
}
if (recurse) {
walker->WalkChildren(result, nesting_depth + 1);
Expand All @@ -1228,9 +1213,8 @@ struct TreeWalker : public std::enable_shared_from_this<TreeWalker> {
DCHECK(!result.GetNextContinuationToken().empty());
req.SetContinuationToken(result.GetNextContinuationToken());
SpawnListObjectsV2();
} else {
walker->ListObjectsFinished(Status::OK());
}
return Status::OK();
}

void Start() {
Expand All @@ -1246,7 +1230,6 @@ 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();
}

Expand Down

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