ARROW-17306: [C++] filesystem: LocalFileSystem: provide optimized ver…

…sion of `GetFileInfoGenerator`

Introduce a helper class `AsyncStatSelector`, which contains
an optimized specialization for `GetFileInfoGenerator` in the
`LocalFileSystem` class.

There are two variants of async discovery functions suported:
1. `DiscoverDirectoryFiles`, which parallelizes traversal of
   individual directories so that each directory results are
   yielded as a separate `FileInfoGenerator` via an underlying
   `DiscoveryImplIterator`, which delivers items in chunks
   (default size is `kDefaultBatchSize == 1K` items).
2. `DiscoverDirectoriesFlattened`, which forwards execution to
   the `DiscoverDirectoryFiles`, with the difference that the
   results from individual sub-directory iterators are merged
   into the single FileInfoGenerator stream.

The implementation makes use of additional attributes in
`LocalFileSystemOptions`, such as `directory_readahead`,
which can be used to tune algorithm behavior and adjust
how many directories can be processed in parallel.

This option is disabled by default, so that individual
directories are processed in serial manner via
`MakeConcatenatedGenerator` under the hood.

Also, internal batching can also be configured by
`LocalFileSystemOptions::batch_size`, which specifies
how many `FileInfo`:s should be batched into a single
`FileInfoVector` to yield in a single `DirectoryImplIterator::Next()`
invocation.

Tests: unit(release)

Signed-off-by: Pavel Solodovnikov <[email protected]>
Co-Authored-by: Igor Seliverstov <[email protected]>

cpp/src/arrow/filesystem/localfs.cc

@@ -29,10 +29,14 @@
 #include <sys/stat.h>
 #endif
 
+#include "arrow/filesystem/filesystem.h"
 #include "arrow/filesystem/localfs.h"
 #include "arrow/filesystem/path_util.h"
+#include "arrow/filesystem/type_fwd.h"
 #include "arrow/filesystem/util_internal.h"
 #include "arrow/io/file.h"
+#include "arrow/io/type_fwd.h"
+#include "arrow/util/async_generator.h"
 #include "arrow/util/io_util.h"
 #include "arrow/util/logging.h"
 #include "arrow/util/uri.h"
@@ -310,6 +314,208 @@ Result<std::vector<FileInfo>> LocalFileSystem::GetFileInfo(const FileSelector& s
   return results;
 }
 
+namespace {
+
+/// Workhorse for streaming async implementation of `GetFileInfo`
+/// (`GetFileInfoGenerator`).
+///
+/// There are two variants of async discovery functions suported:
+/// 1. `DiscoverDirectoryFiles`, which parallelizes traversal of individual directories
+///    so that each directory results are yielded as a separate `FileInfoGenerator` via
+///    an underlying `DiscoveryImplIterator`, which delivers items in chunks (default size
+///    is 1K items).
+/// 2. `DiscoverDirectoriesFlattened`, which forwards execution to the
+///    `DiscoverDirectoryFiles`, with the difference that the results from individual
+///    sub-directory iterators are merged into the single FileInfoGenerator stream.
+///
+/// The implementation makes use of additional attributes in `LocalFileSystemOptions`,
+/// such as `directory_readahead`, which can be used to tune algorithm
+/// behavior and adjust how many directories can be processed in parallel.
+/// This option is disabled by default, so that individual directories are processed
+/// in serial manner via `MakeConcatenatedGenerator` under the hood.
+class AsyncStatSelector {
+ public:
+  using FileInfoGeneratorProducer = PushGenerator<FileInfoGenerator>::Producer;
+
+  /// The main procedure to start async streaming discovery using a given `FileSelector`.
+  ///
+  /// The result is a two-level generator, i.e. "generator of FileInfoGenerator:s",
+  /// where each individual generator represents an FileInfo item stream from coming an
+  /// individual sub-directory under the selector's `base_dir`.
+  static Result<AsyncGenerator<FileInfoGenerator>> DiscoverDirectoryFiles(
+      FileSelector selector, LocalFileSystemOptions fs_opts) {
+    PushGenerator<FileInfoGenerator> file_gen;
+
+    ARROW_ASSIGN_OR_RAISE(
+        auto base_dir, arrow::internal::PlatformFilename::FromString(selector.base_dir));
+    ARROW_RETURN_NOT_OK(DoDiscovery(std::move(base_dir), 0, std::move(selector),
+                                    file_gen.producer(), fs_opts.batch_size));
+
+    return file_gen;
+  }
+
+  /// Version of `DiscoverDirectoryFiles` which flattens the stream of generators
+  /// into a single FileInfoGenerator stream.
+  /// Makes use of `LocalFileSystemOptions::directory_readahead` to determine how much
+  /// readahead should happen.
+  static arrow::Result<FileInfoGenerator> DiscoverDirectoriesFlattened(
+      FileSelector selector, LocalFileSystemOptions fs_opts) {
+    int32_t dir_readahead = fs_opts.directory_readahead;
+    ARROW_ASSIGN_OR_RAISE(
+        auto part_gen, DiscoverDirectoryFiles(std::move(selector), std::move(fs_opts)));
+    return dir_readahead > 1
+               ? MakeSequencedMergedGenerator(std::move(part_gen), dir_readahead)
+               : MakeConcatenatedGenerator(std::move(part_gen));
+  }
+
+ private:
+  /// The class, which implements iterator interface to traverse a given
+  /// directory at the fixed nesting depth, and possibly recurses into
+  /// sub-directories (if specified by the selector), spawning more
+  /// `DiscoveryImplIterators`, which feed their data into a single producer.
+  class DiscoveryImplIterator {
+    const PlatformFilename dir_fn_;
+    const int32_t nesting_depth_;
+    const FileSelector selector_;
+    const uint32_t batch_size_;
+
+    FileInfoGeneratorProducer file_gen_producer_;
+    FileInfoVector current_chunk_;
+    std::vector<PlatformFilename> child_fns_;
+    size_t idx_ = 0;
+    bool initialized_ = false;
+
+   public:
+    DiscoveryImplIterator(PlatformFilename dir_fn, int32_t nesting_depth,
+                          FileSelector selector,
+                          FileInfoGeneratorProducer file_gen_producer,
+                          uint32_t batch_size)
+        : dir_fn_(std::move(dir_fn)),
+          nesting_depth_(nesting_depth),
+          selector_(std::move(selector)),
+          batch_size_(batch_size),
+          file_gen_producer_(std::move(file_gen_producer)) {}
+
+    /// Pre-initialize the iterator by listing directory contents and caching
+    /// in the current instance.
+    Status Initialize() {
+      auto result = arrow::internal::ListDir(dir_fn_);
+      if (!result.ok()) {
+        auto status = result.status();
+        if (selector_.allow_not_found && status.IsIOError()) {
+          auto exists = FileExists(dir_fn_);
+          if (exists.ok() && !*exists) {
+            return Status::OK();
+          } else {
+            return exists.ok() ? arrow::Status::UnknownError(
+                                     "Failed to discover directory: ", dir_fn_.ToString())
+                               : exists.status();
+          }
+        }
+        return status;
+      }
+      child_fns_ = result.MoveValueUnsafe();
+
+      const size_t dirent_count = child_fns_.size();
+      current_chunk_.reserve(dirent_count >= batch_size_ ? batch_size_ : dirent_count);
+
+      initialized_ = true;
+      return Status::OK();
+    }
+
+    Result<FileInfoVector> Next() {
+      if (!initialized_) {
+        auto init = Initialize();
+        if (!init.ok()) {
+          return Finish(init);
+        }
+      }
+      while (idx_ < child_fns_.size()) {
+        auto full_fn = dir_fn_.Join(child_fns_[idx_++]);
+        auto res = StatFile(full_fn.ToNative());
+        if (!res.ok()) {
+          return Finish(res.status());
+        }
+
+        auto info = res.MoveValueUnsafe();
+
+        // Try to recurse into subdirectories, if needed.
+        if (info.type() == FileType::Directory &&
+            nesting_depth_ < selector_.max_recursion && selector_.recursive) {
+          auto status = DoDiscovery(std::move(full_fn), nesting_depth_ + 1, selector_,
+                                    file_gen_producer_, batch_size_);
+          if (!status.ok()) {
+            return Finish(status);
+          }
+        }
+        // Everything is ok. Add the item to the current chunk of data.
+        current_chunk_.emplace_back(std::move(info));
+        // Keep `current_chunk_` as large, as `batch_size_`.
+        // Otherwise, yield the complete chunk to the caller.
+        if (current_chunk_.size() == batch_size_) {
+          FileInfoVector yield_vec;
+          std::swap(yield_vec, current_chunk_);
+          const size_t items_left = child_fns_.size() - idx_;
+          current_chunk_.reserve(items_left >= batch_size_ ? batch_size_ : items_left);
+          return yield_vec;
+        }
+      }  // while (idx_ < child_fns_.size())
+
+      // Flush out remaining items
+      if (!current_chunk_.empty()) {
+        FileInfoVector yield_vec;
+        std::swap(yield_vec, current_chunk_);
+        return yield_vec;
+      }
+      return Finish();
+    }
+
+   private:
+    /// Close the producer end of stream and return iteration end marker.
+    Result<FileInfoVector> Finish(Status status = Status::OK()) {
+      file_gen_producer_.Close();
+      ARROW_RETURN_NOT_OK(status);
+      return IterationEnd<FileInfoVector>();
+    }
+  };
+
+  /// Create an instance of  `DiscoveryImplIterator` under the hood for the
+  /// specified directory, wrap it in the `BackgroundGenerator + TransferredGenerator`
+  /// bundle and feed the results to the main producer queue.
+  static Status DoDiscovery(const PlatformFilename& dir_fn, int32_t nesting_depth,
+                            FileSelector selector,
+                            FileInfoGeneratorProducer file_gen_producer,
+                            uint32_t batch_size) {
+    ARROW_RETURN_IF(file_gen_producer.is_closed(),
+                    arrow::Status::Cancelled("Discovery cancelled"));
+
+    ARROW_ASSIGN_OR_RAISE(
+        auto gen,
+        MakeBackgroundGenerator(Iterator<FileInfoVector>(DiscoveryImplIterator(
+                                    std::move(dir_fn), nesting_depth, std::move(selector),
+                                    file_gen_producer, batch_size)),
+                                io::default_io_context().executor()));
+    gen = MakeTransferredGenerator(std::move(gen), arrow::internal::GetCpuThreadPool());
+    ARROW_RETURN_IF(!file_gen_producer.Push(std::move(gen)),
+                    arrow::Status::Cancelled("Discovery cancelled"));
+    return arrow::Status::OK();
+  }
+};
+
+}  // anonymous namespace
+
+FileInfoGenerator LocalFileSystem::GetFileInfoGenerator(const FileSelector& select) {
+  auto path_status = ValidatePath(select.base_dir);
+  if (!path_status.ok()) {
+    return MakeFailingGenerator<FileInfoVector>(path_status);
+  }
+  auto fileinfo_gen = AsyncStatSelector::DiscoverDirectoriesFlattened(select, options());
+  if (!fileinfo_gen.ok()) {
+    return MakeFailingGenerator<FileInfoVector>(fileinfo_gen.status());
+  }
+  return fileinfo_gen.MoveValueUnsafe();
+}
+
 Status LocalFileSystem::CreateDir(const std::string& path, bool recursive) {
   RETURN_NOT_OK(ValidatePath(path));
   ARROW_ASSIGN_OR_RAISE(auto fn, PlatformFilename::FromString(path));

cpp/src/arrow/filesystem/localfs.h

@@ -87,6 +87,7 @@ class ARROW_EXPORT LocalFileSystem : public FileSystem {
   /// \endcond
   Result<FileInfo> GetFileInfo(const std::string& path) override;
   Result<std::vector<FileInfo>> GetFileInfo(const FileSelector& select) override;
+  FileInfoGenerator GetFileInfoGenerator(const FileSelector& select) override;
 
   Status CreateDir(const std::string& path, bool recursive = true) override;