Fail more gracefully from ActiveStorage missing file exceptions

[cbothner]

This PR translates service-specific missing object exceptions into one generic ActiveStorage::FileNotFoundError so that the application can fail more gracefully when a missing file is accessed. Specifically, this PR lets DiskController rescue this error and respond with 404 instead of 500. This improves the development experience especially when working, in development, with a database of Blob records that point to files in the production service.

Outstanding question

RepresentationsController does not presently handle ActiveStorage::FileNotFoundError, since @georgeclaghorn argued it truly is exceptional for the service not to have a file that corresponds to a specific blob in the database. I don’t disagree with this, but hope we can nevertheless find a way to reduce the flood of stack traces that fill our development logs when loading an index view that tries to display many thumbnails of images stored in our production service.

Related issue

Resolves #33647

[georgeclaghorn]

Let’s use bare raise instead of raise e to avoid munging the backtrace:

if e.type == "BlobNotFound"
  raise ActiveStorage::FileNotFoundError
else
  raise
end

[georgeclaghorn]

Can we extract a private handle_errors method to avoid duplicating the error handling logic in download and download_chunk?

def download_chunk(key, range)
  instrument :download_chunk, key: key, range: range do
    handle_errors do
      _, io = blobs.get_blob(container, key, start_range: range.begin, end_range: range.exclude_end? ? range.end - 1 : range.end)
      io.force_encoding(Encoding::BINARY)
    end
  end
end

[cbothner]

I was half tempted to put such a private method on the parent class (taking a service specific exception as an argument) but worried it was gonna be too much magic. Is this just needed because of the extra type checking needed for Azure errors? (Related bug fix PR #33667 that should probably do it the same way…)

[georgeclaghorn]

I wouldn’t put it on the parent class. It’s a service-specific method.

[georgeclaghorn]

You don’t need the begin/end block here:

def handle_errors
  yield
rescue Azure::Core::Http::HTTPError => e
  # ...
end

[georgeclaghorn]

Ditto: you don’t need the begin/end block here.

[georgeclaghorn]

Last thing: can you please squash your commits? I think two are appropriate: one for rescuing Errno::ENOENT in DiskController and one for translating service-specific missing object exceptions.

I also think these changes merit separate changelog entries, since they're independent:

• ActiveStorage::Blob#download and ActiveStorage::Blob#open raise ActiveStorage::FileNotFoundError when the corresponding file is missing from the storage service. Services translate service-specific missing object exceptions (e.g. Google::Cloud::NotFoundError for the GCS service and Errno::ENOENT for the disk service) into ActiveStorage::FileNotFoundError.

• ActiveStorage::DiskController#show generates a 404 Not Found response when the requested file is missing from the disk service. It previously raised Errno::ENOENT.

[cbothner]

Done 👍

Is there anything to do about the errors raised in RepresentationsController for the sake of developer experience, or is that inappropriate to handle at the framework level?

[georgeclaghorn]

The DebugExceptions middleware logs the full backtrace if none of the lines in the trace come from the application, which is the case for ActiveStorage::FileNotFoundErrors triggered by requests to RepresentationsController:

rails/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb

Lines 179 to 188 in cdee520

	trace = wrapper.application_trace
	trace = wrapper.framework_trace if trace.empty?
	ActiveSupport::Deprecation.silence do
	logger.fatal " "
	logger.fatal "#{exception.class} (#{exception.message}):"
	log_array logger, exception.annoted_source_code if exception.respond_to?(:annoted_source_code)
	logger.fatal " "
	log_array logger, trace
	end

I’m not immediately sure what we can do about that. I’m going to think about it and merge this in the meantime.