fix(codec): respect server's enabled encodings when selecting response compression

[kv-cache]

Motivation

CompressionEncoding::from_accept_encoding_header selects a response compression encoding based purely on the client's grpc-accept-encoding header and the compile-time feature set, not the runtime set of encodings the server enabled via Server::send_compressed(...).

When the server has only zstd enabled but the client advertises gzip,zstd,identity, tonic compresses the response with gzip, which is an encoding the server never opted into and would not have advertised back.

The sibling from_encoding_header (used for request decompression) already has the correct if enabled_encodings.is_enabled(...) guards per match arm. This PR brings the accept-side in line.

Observed in production with tonic 0.14.6: server configured send_compressed(Zstd) only, ~38% of server CPU in miniz_oxide::deflate. Worked around downstream with a tower layer that rewrites the incoming grpc-accept-encoding header to "zstd,identity" to force the buggy selection onto the correct path. After the fix, that work-around will no longer be needed.

The bug affects all four RPC kinds — from_accept_encoding_header is called from unary / server_streaming / client_streaming / streaming in tonic/src/server/grpc.rs.

Solution

Add a runtime is_enabled guard to each match arm of from_accept_encoding_header so the function returns the first encoding in the intersection of (server-enabled, client-accepted), preserving the client's preference order. Returns None if the intersection is empty so the server falls back to identity-encoded responses.

Adds three unit tests in mod tests of tonic/src/codec/compression.rs:

Test	Server enabled	Client grpc-accept-encoding	Expected
from_accept_encoding_header_respects_server_enabled_encodings	Zstd	gzip,zstd,identity	Some(Zstd)
from_accept_encoding_header_returns_none_when_no_overlap	Zstd	gzip,identity	None
from_accept_encoding_header_uses_client_preference_order	Zstd, Gzip	gzip,zstd,identity	Some(Gzip)

All three unit tests fail on master without the code fix; all pass with it. Existing tonic unit tests (79) and compression integration tests (63) continue to pass.

Backport note

This fix applies cleanly. Flagging that the v0.14.x branch head appears to be behind the tonic-v0.14.6 tag (latest commit there is chore: prepare v0.14.5 release), so I targeted master like v0.14.6 did. Happy to open a parallel PR against v0.14.x if maintainers want a backport.

[linux-foundation-easycla]

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• ✅ login: kv-cache / name: Vineeth Rao Kanaparthi (155107d)