minor fixes

Author: raulcd

cpp/src/arrow/filesystem/s3fs.cc

@@ -347,6 +347,14 @@ Result<T> GetOption(const std::string& key, const std::any& value) {
   return Status::Invalid("S3 filesystem option '", key, "' has the wrong type");
 }
 
+template <typename T>
+Result<std::shared_ptr<const T>> GetConstSharedPtrOption(const std::string& key,
+                                                         const std::any& value) {
+  if (const auto* v = std::any_cast<std::shared_ptr<const T>>(&value)) return *v;
+  if (const auto* v = std::any_cast<std::shared_ptr<T>>(&value)) return *v;
+  return Status::Invalid("S3 filesystem option '", key, "' has the wrong type");
+}
+
 }  // namespace
 
 Result<S3Options> S3Options::FromUriAndOptions(const ::arrow::util::Uri& uri,
@@ -367,7 +375,7 @@ Result<S3Options> S3Options::FromUriAndOptions(const ::arrow::util::Uri& uri,
                             GetOption<std::shared_ptr<S3RetryStrategy>>(key, value));
     } else if (key == "default_metadata") {
       ARROW_ASSIGN_OR_RAISE(default_metadata,
-                            GetOption<std::shared_ptr<KeyValueMetadata>>(key, value));
+                            GetConstSharedPtrOption<KeyValueMetadata>(key, value));
     } else {
       return Status::Invalid("Unexpected option for S3 filesystem: '", key, "'");
     }

cpp/src/arrow/filesystem/s3fs.h

@@ -299,6 +299,7 @@ struct ARROW_EXPORT S3Options {
   /// - "secret_key" (std::string)
   /// - "session_token" (std::string)
   /// - "retry_strategy" (std::shared_ptr<S3RetryStrategy>)
+  /// - "default_metadata" (std::shared_ptr<const KeyValueMetadata>)
   /// Options take precedence over the URI; unknown keys or invalid values return
   /// Status::Invalid.
   static Result<S3Options> FromUriAndOptions(const ::arrow::util::Uri& uri,