GH-40068: [C++] Possible data race when reading metadata of a parquet file (#40111)

### Rationale for this change

The `ParquetFileFragment` will cache the parquet metadata when loading it.  The `metadata()` method accesses this metadata (a shared_ptr) but does not grab the lock used to set that shared_ptr.  It's possible then that we are reading a shared_ptr at the same time some other thread is setting the shared_ptr which is technically (I think) undefined behavior.

### What changes are included in this PR?

Guard access to the metadata by grabbing the mutex first

### Are these changes tested?

Existing tests should regress this change

### Are there any user-facing changes?

No
* Closes: #40068

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: westonpace

cpp/src/arrow/dataset/file_parquet.cc

@@ -779,6 +779,11 @@ ParquetFileFragment::ParquetFileFragment(FileSource source,
       parquet_format_(checked_cast<ParquetFileFormat&>(*format_)),
       row_groups_(std::move(row_groups)) {}
 
+std::shared_ptr<parquet::FileMetaData> ParquetFileFragment::metadata() {
+  auto lock = physical_schema_mutex_.Lock();
+  return metadata_;
+}
+
 Status ParquetFileFragment::EnsureCompleteMetadata(parquet::arrow::FileReader* reader) {
   auto lock = physical_schema_mutex_.Lock();
   if (metadata_ != nullptr) {

cpp/src/arrow/dataset/file_parquet.h

@@ -165,7 +165,7 @@ class ARROW_DS_EXPORT ParquetFileFragment : public FileFragment {
   }
 
   /// \brief Return the FileMetaData associated with this fragment.
-  const std::shared_ptr<parquet::FileMetaData>& metadata() const { return metadata_; }
+  std::shared_ptr<parquet::FileMetaData> metadata();
 
   /// \brief Ensure this fragment's FileMetaData is in memory.
   Status EnsureCompleteMetadata(parquet::arrow::FileReader* reader = NULLPTR);