GH-46971: [C++][Parquet] Use temporary buffers when decrypting Parquet data pages (#46972)

### Rationale for this change

Reduce memory usage required when reading wide, encrypted Parquet files.

### What changes are included in this PR?

Change `SerializedPageReader` so that it doesn't hold a decryption buffer but only allocates one as needed, so it can be freed after pages are decompressed.

### Are these changes tested?

This is only a performance improvement and doesn't change any behaviour so should be covered by existing tests.
The memory improvement has been verified manually (see #46971).

### Are there any user-facing changes?

No

* GitHub Issue: #46971

Authored-by: Adam Reeve <[email protected]>
Signed-off-by: Adam Reeve <[email protected]>

Author: adamreeve

cpp/src/parquet/column_reader.cc

@@ -226,8 +226,7 @@ class SerializedPageReader : public PageReader {
         decompression_buffer_(AllocateBuffer(properties_.memory_pool(), 0)),
         page_ordinal_(0),
         seen_num_values_(0),
-        total_num_values_(total_num_values),
-        decryption_buffer_(AllocateBuffer(properties_.memory_pool(), 0)) {
+        total_num_values_(total_num_values) {
     if (crypto_ctx != nullptr) {
       crypto_ctx_ = *crypto_ctx;
       InitDecryption();
@@ -241,7 +240,7 @@ class SerializedPageReader : public PageReader {
   //
   // The returned Page contains references that aren't guaranteed to live
   // beyond the next call to NextPage(). SerializedPageReader reuses the
-  // decryption and decompression buffers internally, so if NextPage() is
+  // decompression buffer internally, so if NextPage() is
   // called then the content of previous page might be invalidated.
   std::shared_ptr<Page> NextPage() override;
 
@@ -304,8 +303,6 @@ class SerializedPageReader : public PageReader {
   // updated by only the page ordinal.
   std::string data_page_aad_;
   std::string data_page_header_aad_;
-  // Encryption
-  std::shared_ptr<ResizableBuffer> decryption_buffer_;
 };
 
 void SerializedPageReader::InitDecryption() {
@@ -477,14 +474,12 @@ std::shared_ptr<Page> SerializedPageReader::NextPage() {
 
     // Decrypt it if we need to
     if (data_decryptor_ != nullptr) {
-      PARQUET_THROW_NOT_OK(
-          decryption_buffer_->Resize(data_decryptor_->PlaintextLength(compressed_len),
-                                     /*shrink_to_fit=*/false));
-      compressed_len =
-          data_decryptor_->Decrypt(page_buffer->span_as<uint8_t>(),
-                                   decryption_buffer_->mutable_span_as<uint8_t>());
+      auto decryption_buffer = AllocateBuffer(
+          properties_.memory_pool(), data_decryptor_->PlaintextLength(compressed_len));
+      compressed_len = data_decryptor_->Decrypt(
+          page_buffer->span_as<uint8_t>(), decryption_buffer->mutable_span_as<uint8_t>());
 
-      page_buffer = decryption_buffer_;
+      page_buffer = decryption_buffer;
     }
 
     if (page_type == PageType::DICTIONARY_PAGE) {