Bucket.delete_blobs doesn't call on_error in a batch context

[jd185367]

Trying to delete a file that doesn't exist with Bucket.delete_blobs should call the function passed to the on_error arg (if provided) instead of raising an exception. That's the behavior I observe when calling this method normally, but trying to do this inside a batch deletion seems to ignore the passed function and throw the exception anyway (instead of calling the function like I'd expect).

This appears to be closely related to issue #31, but seemed a distinct enough use case to be worth mentioning.

Environment details

• OS: Windows 10 Enterprise, Version 1909
• Python version: 3.9.6
• pip version: 21.2.4 (using the poetry package manager, v1.1.11)
• google-cloud-storage version: 1.43.0

Steps to reproduce

1. Create a bucket in a Google Cloud project and confirm you can connect to it from a Python script on your machine (GOOGLE_APPLICATION_CREDENTIALS set, etc).
2. Run the example code below on the machine with at least 1 name in filenames that doesn't exist in the bucket.
3. Confirm that print_blob_name is executed if the with gcs_client.batch(): line is removed, but an exception is thrown instead when it's present.

Code example

import google.cloud.storage

gcs_client = google.cloud.storage.Client()
bucket = gcs_client.lookup_bucket("bucket-name")
filenames = ["non_existent.png"]

def print_blob_name(blob: google.cloud.storage.blob.Blob):
    print(f"ERROR - tried to delete missing blob '{str(blob)}'")

with gcs_client.batch():
    bucket.delete_blobs(filenames, on_error=print_blob_name)

Stack trace

Traceback (most recent call last):
  File "C:\Users\jd185367\.pyenv\pyenv-win\versions\3.9.6\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\jd185367\.pyenv\pyenv-win\versions\3.9.6\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\jd185367\<redacted>\gcs_deletion_test.py", line 11, in <module>
    bucket.delete_blobs(filenames, on_error=print_blob_name)
  File "C:\Users\jd185367\AppData\Local\pypoetry\Cache\virtualenvs\<redacted>-I8hA3ptd-py3.9\lib\site-packages\google\cloud\storage\batch.py", line 299, in __exit__
    self.finish()
  File "C:\Users\jd185367\AppData\Local\pypoetry\Cache\virtualenvs\<redacted>-I8hA3ptd-py3.9\lib\site-packages\google\cloud\storage\batch.py", line 285, in finish
    self._finish_futures(responses)
  File "C:\Users\jd185367\AppData\Local\pypoetry\Cache\virtualenvs\<redacted>-I8hA3ptd-py3.9\lib\site-packages\google\cloud\storage\batch.py", line 261, in _finish_futures
    raise exceptions.from_http_response(exception_args)
google.api_core.exceptions.NotFound: 404 BATCH contentid://None: No such object: bucket-name/non_existent.png

[andrewsg]

Thank you for this detailed bug report.

[cojenco]

The current batch design does not support library methods whose return values depend on the response payload. In this case, the error handling via the on_error callable in bucket.delete_blobs() is not supported in a Batch context.

We've updated the docstrings with a warning and added clarifications on the limited Batch support in the python client, see details in #1045