[SalvatoreT]

Summary

Adds STORAGE_EMULATOR_HOST support to google-cloud-storage so the client can talk to a local Cloud Storage emulator. The Python client supports the same variable (googleapis/google-cloud-python#9219), and the Ruby pubsub, datastore, firestore, and bigtable gems already read their own *_EMULATOR_HOST variables.

Fixes #34022

Behavior

The emulator host can be set three ways, highest precedence first:

Google::Cloud::Storage.new emulator_host: "localhost:9000"
Google::Cloud::Storage.configure { |c| c.emulator_host = "localhost:9000" }
ENV["STORAGE_EMULATOR_HOST"] = "localhost:9000"

With an emulator host set:

• Requests go to the emulator. A host without a scheme gets http://, and a trailing slash is added because the API client appends storage/v1/ to the root URL.
• The client connects anonymously and skips the Application Default Credentials lookup. Credentials passed to Storage.new are still used.
• A project ID is optional.
• The emulator host overrides endpoint, like the other Ruby emulator-aware gems.
• Storage.anonymous uses the emulator host from config or the environment.
• A blank STORAGE_EMULATOR_HOST is treated as unset.

Storage is REST-based, so emulator mode uses nil credentials (as Storage.anonymous already does) instead of the gRPC :this_channel_is_insecure sentinel.

A new EMULATOR.md guide, linked from OVERVIEW.md, documents this.

Testing

• storage_test.rb: the emulator tests build a real Storage::Service and assert on root_url. They cover the env var, argument, and config; bare and full hosts; a blank env var; no project; explicit credentials; precedence over endpoint; Storage.anonymous; and emulator_host passing through Google::Cloud#storage and Google::Cloud.storage.
• bundle exec rake test: 985 runs, 0 failures.
• Against fake-gcs-server 1.50.2 with STORAGE_EMULATOR_HOST=localhost:4443: created a bucket, uploaded (simple and resumable), listed, and downloaded.
• No new rubocop offenses in the changed library files.

🤖 Generated with Claude Code

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[SalvatoreT]

Hey @cpriti-os @aandreassa @bajajneha27 if y'all are open to externally-authored PRs, would one of you be able to review this?

[SalvatoreT]

Ready for a second pass, @kalragauri!