Motivation

Resolves #3832.

Currently, HTTP/1 header size limits can only be indirectly bounded using max_buf_size. However, setting max_buf_size also restricts the maximum single read buffer for streaming request or response bodies. Furthermore, there was an existing TODO in proto/h1/conn.rs noting that h1_max_header_size should be wired up to the chunked decoder for trailers.

This PR adds explicit max_header_size(bytes) configuration options to both server::conn::http1::Builder and client::conn::http1::Builder.

Solution & Architecture

• Server Builder (server::conn::http1::Builder):
  • Exposes pub fn max_header_size(&mut self, val: usize) -> &mut Self.
  • Configures the maximum total size of HTTP/1 request headers (request line + headers) in bytes.
  • When exceeded, the server responds with 431 Request Header Fields Too Large (RFC 6585 §5) and closes the connection.
• Client Builder (client::conn::http1::Builder):
  • Exposes pub fn max_header_size(&mut self, val: usize) -> &mut Self.
  • Configures the maximum total size of HTTP/1 response headers (status line + headers) in bytes.
  • When exceeded, the client returns an error where error.is_parse_too_large() == true.
• Early Enforcement:
  • Enforced in io::Buffered::parse_headers as bytes are read, preventing buffering unbounded amounts of header data.
  • Enforced in Server::parse / Client::parse against parsed_len to protect scenarios where a single read buffer already contains subsequent data.
• Chunked Trailers:
  • Replaced the pre-existing // TODO: remove this when we land h1_max_header_size support placeholder in proto::h1::conn.rs (decode::Decoder::new) with the configured h1_max_header_size.

Verification

• Unit test in src/proto/h1/role.rs:
  • test_h1_max_header_size: Tests exact boundary conditions on both server and client parsers.
• Integration tests in tests/server.rs:
  • max_header_size_exceeded: Verifies server returns 431 Request Header Fields Too Large when request header size exceeds limit.
  • max_header_size_accepted: Verifies server accepts requests when within limit.
  • max_header_size_with_large_max_buf_size: Verifies max_header_size enforcement is decoupled from and strictly enforced even when max_buf_size is large.
• Integration tests in tests/client.rs:
  • client_max_header_size_exceeded: Verifies client errors with is_parse_too_large() == true.
  • client_max_header_size_accepted: Verifies client successfully receives responses within limit.
• All test suites passing (cargo test --lib --features full, cargo test --test server --features full, cargo test --test client --features full).