Fix load_metadata_buffer for empty metadata

PiperOrigin-RevId: 502870428

mediapipe/tasks/python/metadata/metadata.py

@@ -860,6 +860,8 @@ def get_metadata_buffer(model_buf):
     if meta.Name().decode("utf-8") == MetadataPopulator.METADATA_FIELD_NAME:
       buffer_index = meta.Buffer()
       metadata = tflite_model.Buffers(buffer_index)
+      if metadata.DataLength() == 0:
+        continue
       return metadata.DataAsNumpy().tobytes()
 
   return None

mediapipe/tasks/python/test/metadata/metadata_test.py

@@ -550,7 +550,7 @@ class MetadataPopulatorTest(MetadataTest):
         ("The number of output tensors (1) should match the number of "
          "output tensor metadata (0)"), str(error.exception))
 
-  def testLoadMetadataAndAssociatedFilesShouldSucceeds(self):
+  def testLoadMetadataAndAssociatedFilesShouldSucceed(self):
     # Create a src model with metadata and two associated files.
     src_model_buf = self._create_model_buf()
     populator_src = _metadata.MetadataPopulator.with_model_buffer(src_model_buf)
@@ -566,7 +566,7 @@ class MetadataPopulatorTest(MetadataTest):
         populator_src.get_model_buffer())
     populator_dst.populate()
 
-    # Tests if the metadata and associated files are populated correctly.
+    # Test if the metadata and associated files are populated correctly.
     dst_model_file = self.create_tempfile().full_path
     with open(dst_model_file, "wb") as f:
       f.write(populator_dst.get_model_buffer())
@@ -575,6 +575,28 @@ class MetadataPopulatorTest(MetadataTest):
     recorded_files = populator_dst.get_recorded_associated_file_list()
     self.assertEqual(set(recorded_files), set(self.expected_recorded_files))
 
+  def testLoadMetadataAndAssociatedFilesShouldSucceedOnEmptyMetadata(self):
+    # When the user hasn't specified the metadata, but only the associated
+    # files, an empty metadata buffer is created. Previously, it caused an
+    # exception when reading.
+
+    # Create a source model with two associated files but no metadata.
+    src_model_buf = self._create_model_buf()
+    populator_src = _metadata.MetadataPopulator.with_model_buffer(src_model_buf)
+    populator_src.load_associated_files([self._file1, self._file2])
+    populator_src.populate()
+
+    # Create a model to be populated with the files from `src_model_buf`.
+    dst_model_buf = self._create_model_buf()
+    populator_dst = _metadata.MetadataPopulator.with_model_buffer(dst_model_buf)
+    populator_dst.load_metadata_and_associated_files(
+        populator_src.get_model_buffer())
+    populator_dst.populate()
+
+    # Test if the metadata and associated files are populated correctly.
+    packed_files = populator_dst.get_packed_associated_file_list()
+    self.assertEqual(set(packed_files), set(self.expected_recorded_files))
+
   @parameterized.named_parameters(
       {
           "testcase_name": "InputTensorWithBert",