Fix uninitialized data when reading Parquet RLE-encoded data with nulls,

also fix UBSAN issue when converting null Int96 entries to Arrow timestamp.

cpp/src/arrow/util/rle_encoding.h

@@ -126,6 +126,9 @@ class RleDecoder {
   int GetBatchWithDict(const T* dictionary, T* values, int batch_size);
 
   /// Like GetBatchWithDict but add spacing for null entries
+  ///
+  /// Null entries will be zero-initialized in `values` to avoid leaking
+  /// private data.
   template <typename T>
   int GetBatchWithDictSpaced(const T* dictionary, T* values, int batch_size,
                              int null_count, const uint8_t* valid_bits,
@@ -433,6 +436,8 @@ inline int RleDecoder::GetBatchWithDictSpaced(const T* dictionary, T* out, int b
   DCHECK_GE(bit_width_, 0);
   int values_read = 0;
   int remaining_nulls = null_count;
+  T zero;
+  memset(&zero, 0, sizeof(T));
 
   arrow::internal::BitmapReader bit_reader(valid_bits, valid_bits_offset, batch_size);
 
@@ -484,6 +489,7 @@ inline int RleDecoder::GetBatchWithDictSpaced(const T* dictionary, T* out, int b
             *out = dictionary[indices[literals_read]];
             literals_read++;
           } else {
+            *out = zero;
             skipped++;
           }
           ++out;
@@ -494,6 +500,7 @@ inline int RleDecoder::GetBatchWithDictSpaced(const T* dictionary, T* out, int b
         remaining_nulls -= skipped;
       }
     } else {
+      *out = zero;
       ++out;
       values_read++;
       remaining_nulls--;

cpp/src/parquet/arrow/reader_internal.cc

@@ -738,7 +738,13 @@ Status TransferInt96(RecordReader* reader, MemoryPool* pool,
   RETURN_NOT_OK(::arrow::AllocateBuffer(pool, length * sizeof(int64_t), &data));
   auto data_ptr = reinterpret_cast<int64_t*>(data->mutable_data());
   for (int64_t i = 0; i < length; i++) {
-    *data_ptr++ = Int96GetNanoSeconds(values[i]);
+    if (values[i].value[2] == 0) {
+      // Happens for null entries: avoid triggering UBSAN as that Int96 timestamp
+      // isn't representable as a 64-bit Unix timestamp.
+      *data_ptr++ = 0;
+    } else {
+      *data_ptr++ = Int96GetNanoSeconds(values[i]);
+    }
   }
   *out = std::make_shared<TimestampArray>(type, length, data, reader->ReleaseIsValid(),
                                           reader->null_count());