Phase 7b: user-supplied middleware, client interceptors, Beast graceful drain

[aaylward]

Second Phase 7 hardening slice: the user-supplied middleware seams (recorded in the PLAN per review feedback), client interceptors, and server-transport robustness.

Server middleware (//runtime:server)

• smithy::server::Middleware (function<RequestHandler(RequestHandler)>) and Chain(), composing first-outermost around the plain RequestHandler every generated server already exposes — auth checks, logging, metrics wrap outside the generated router and work with any transport; no codegen involved.
• Observe() — the built-in structured-logging/metrics hook: one callback per served request with {method, target, status, duration} and an injectable clock for deterministic tests.

Client interceptors (//runtime:client)

• smithy::Interceptor hooks around every HTTP attempt a generated client makes: ModifyBeforeTransmit(request, attempt) mutates a fresh per-attempt copy (auth headers, tracing ids — edits never accumulate across retries or leak into the caller's request), ReadAfterTransmit(request, outcome, attempt) observes.
• Registered on ClientConfig::interceptors, run in registration order inside SendWithRetries; generated clients pass them through (goldens regenerated).
• A weather e2e test drives a bearer-token interceptor through the generated client against an auth-rejecting middleware chain on the generated server — both new seams exercised together across the wire.

Beast server robustness

• Graceful drain: Stop() closes the acceptor immediately (posted onto its strand), keep-alive reads cease, and requests already read off the wire get drain_timeout_seconds (default 10) to finish writing before the thread pool is torn down. New test stops the server mid-handler and asserts the response still arrives intact.
• Header limits: new max_header_bytes option (default 8 KiB) caps request headers alongside the existing body limit, with a rejection test.

Docs

Production guide gains interceptor and middleware sections with worked examples plus the hardened-transport summary; server guide, runtime overview, and README updated. Also carries the PLAN entry recording user-supplied middleware as a Phase 7 deliverable.

Validation

• bazel test //... — 44/44 green locally, plus ASan+UBSan (Beast targets are CI-verified: boost sources aren't fetchable in the local sandbox)
• out-of-tree consumer module green against the updated generator
• regeneration byte-stable; clang-format/clang-tidy/buildifier clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01WjaNFwBZxoHdqagvq8ycQf

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