Automatically compress responses from Accept-Encoding #111

Description

Provide automatic response compression negotiated from the client Accept-Encoding, without requiring applications to write or manually assemble compression middleware.

Main has gzip/Brotli decoding helpers, but no response encoder/negotiation middleware in the public core surface. PR #452 proposes explicit Fastly delivery-compression intent; applications still own reader negotiation/fallback, so it covers only part of this story.

Steps to reproduce

1. Inspect the compression module and middleware module for an outgoing compression policy.
2. Compare the scope of Add explicit adapter delivery compression support #452 with automatic, application-facing negotiation.

Expected behavior

An application can enable a supported automatic policy and receive an encoding acceptable to the client with consistent response metadata.

Impact

Applications currently implement negotiation and compression policy themselves, limiting portability and increasing integration work.

Proposed fix

Design a small configuration/adapter integration over supported encoders, including safe identity fallback, encoded-body preservation and streaming behavior. Reuse #452 where applicable without equating explicit intent with automatic negotiation.

Done when

• Automatic selection honors supported Accept-Encoding values and quality/identity behavior.
• Headers such as Content-Encoding, length and Vary match the emitted bytes, including already encoded responses.
• Bodyless/otherwise ineligible responses are handled correctly and streaming behavior is documented.
• An example enables the behavior without custom compression middleware code.

Related

• Parent: Complete streaming and compression across adapters #107.
• Encoded pass-through: Preserve pre-compressed upstream responses #113.
• Explicit adapter-delivery mechanism: Add explicit Fastly delivery-compression support #362.
• Existing gzip and Brotli decoders
• Core public module surface
• Related PR Add explicit adapter delivery compression support #452; this PR alone does not establish completion of this issue.