fix(proxy): add max_header_list_size config for HTTP/2 client

[wahajahmed010]

Summary

After the Hyper 0.14→1.x migration, the HTTP/2 client defaults to DEFAULT_MAX_HEADER_LIST_SIZE = 16 KiB, which is too restrictive for gRPC services with large trailers. The server side already has env var support for max_header_list_size, but the client side did not.

This PR adds client-side configurability via environment variables, following the same pattern already used for server-side h2 params.

Changes

linkerd/http/h2/src/lib.rs

• Added max_header_list_size: Option<u32> to ClientParams struct
• Included max_header_list_size in the override_from method

linkerd/app/src/env/http2.rs

• Added parse_client() function for client-side h2 params parsing (mirrors parse_server())
• Supports env vars for flow control, keep-alive, max frame size, max header list size, and max send buf size
• Includes test coverage

linkerd/app/src/env.rs

• Replaced inline h2::ClientParams construction in outbound/inbound connect configs with calls to http2::parse_client()
• No behavioral change — same defaults when no env vars are set

linkerd/proxy/http/src/h2.rs

• Destructure max_header_list_size from ClientParams
• Apply via builder.max_header_list_size(sz) in the h2 connection setup

Environment Variables

Client params use the prefix LINKERD2_PROXY_OUTBOUND_CONNECT_HTTP2_ and LINKERD2_PROXY_INBOUND_CONNECT_HTTP2_:

• *_MAX_HEADER_LIST_SIZE (e.g., 65536)
• *_MAX_FRAME_SIZE
• *_MAX_SEND_BUF_SIZE
• *_KEEP_ALIVE_TIMEOUT / *_KEEP_ALIVE_INTERVAL / *_KEEP_ALIVE_WHILE_IDLE
• *_ADAPTIVE_FLOW_CONTROL / *_INITIAL_STREAM_WINDOW_SIZE / *_INITIAL_CONNECTION_WINDOW_SIZE

Fixes linkerd/linkerd2#15199

[kostyay]

Thanks for this — the approach looks right, and I was able to validate it locally against the max-header repro (HTTP 22 KiB response headers + gRPC 22 KiB trailers) once the custom proxy was deployed with the new env vars set.

While building the branch, I hit a couple of issues that block a clean cargo build --release:

1. Missing field in api-resolve

Adding max_header_list_size to ClientParams breaks compilation in linkerd/proxy/api-resolve/src/pb.rs:

error[E0063]: missing field `max_header_list_size` in initializer of `ClientParams`
  --> linkerd/proxy/api-resolve/src/pb.rs:166:5

to_http2_client_params() needs to populate the new field (likely None until the proto/API surface exposes it).

2. Dead code / unused imports in env.rs

Switching inbound/outbound connect config to http2::parse_client() leaves behind unused code in linkerd/app/src/env.rs:

• unused import: h2
• unused variables: initial_stream_window_size, initial_connection_window_size
• unused constants: ENV_INITIAL_STREAM_WINDOW_SIZE, ENV_INITIAL_CONNECTION_WINDOW_SIZE

These fail the build with -D warnings.

3. (Optional) Behavior change to call out

Previously, connect flow-control window sizes were driven by the global env vars:

LINKERD2_PROXY_HTTP2_INITIAL_STREAM_WINDOW_SIZE
LINKERD2_PROXY_HTTP2_INITIAL_CONNECTION_WINDOW_SIZE

After this change, connect-side settings only come from the prefixed vars:

LINKERD2_PROXY_OUTBOUND_CONNECT_HTTP2_*
LINKERD2_PROXY_INBOUND_CONNECT_HTTP2_*

If anyone was relying on the global vars for connect tuning, that would silently stop working. Worth documenting in the PR description or preserving backward compatibility.

[kostyay]

You can find a repo to test the issue here, with instructions on how to test it with this custom fixed branch:
https://github.com/kostyay/linkerd-issue-max-header-reproduction

[kostyay]

hi @cratelyn
will you have time to review/merge this?
would love for this to be merged as this bug had been around for a while now

@wahajahmed010 looks like the build is failing for the DCO action.

There is one commit incorrectly signed off. This means that the author of this commit failed to include a Signed-off-by line in the commit message.

Can you resolve that please?

[kostyay]

@cratelyn
bumping this again, would love to get your feedback on it