perf(ext/http): optimize Deno.serve() hot path

[bartlomieju]

Summary

Optimizes the Deno.serve() hot path by reducing per-request object allocations, eliminating unnecessary function calls, and adding fast paths for the most common response patterns.

Benchmark results (wrk, 2 threads, 10 connections, 10s, macOS ARM64)

Benchmark	Before	After	Improvement
new Response("Hello, World!")	~213,000 RPS	~219,000 RPS	+3%
new Response(JSON.stringify(...), {headers})	~170,000 RPS	~179,000 RPS	+5-6%

Changes

ext/http/00_serve.ts — Serve handler hot path

• Sync fast path: The mapped callback is no longer async. When the handler returns a Response synchronously (not a Promise), it processes it without creating a Promise or going through the microtask queue. Async handlers fall through to handleAsyncResponse.
• Ultra-fast path for new Response(string): Responses marked with _fastResponse skip all validation (type check, bodyUsed check, prototype check) and go directly to the combined Rust op.
• Inlined response processing: For the non-tracing path, respondWith and toInnerResponse are inlined to eliminate function call overhead and redundant property accesses.
• Direct bodyUsed check: Checks inner.body.streamOrStatic?.consumed directly instead of going through response.bodyUsed getter (avoids assertBranded + getter chain).
• Skip ServeHandlerInfo allocation: Only created when handler has 2+ parameters.
• Skip InnerRequest/Request creation for 0-arg handlers.
• Fix asyncContextSnapshot property name: Was accessing context.asyncContext (undefined) instead of context.asyncContextSnapshot.

ext/fetch/23_response.js — Response constructor

• Fast constructor for new Response(string): Inlines response creation, skipping webidlConvertersBodyInitDomString, webidlConvertersResponseInitFast, and extractBody. Saves ~2 object allocations.
• Fast constructor for new Response(string, {status, headers}): Similar fast path with init.
• Fast Response.json() path: Avoids extractBody intermediates.
• Lazy Headers creation: Headers wrapper only created when accessed, not in constructor.
• Fast plain-object headers: { "content-type": "application/json" } added directly to headerList without Headers wrapper.
• Inner response prototype: url() method on shared prototype.
• Shared urlList: User-created responses share empty urlList.
• Direct headerList access in initializeAResponse.

ext/fetch/23_request.js — Request construction

• Lazy Headers creation: Same pattern as Response.
• Eliminated closure in fromInnerRequest: Stores guard directly instead of closure.

ext/http/http_next.rs — Rust ops

• Combined op_http_set_response_body_text_with_header: Sets header + body + status in one op.
• Small body fast path: Skip compression check for bodies < 64 bytes.
• Skip Vary: Accept-Encoding: Only when compression is considered.
• Skip response headers borrow: When compression is None.

Test plan

• Deno.serve() with sync handlers
• Deno.serve() with async handlers
• Response.json() correctness
• new Response(body, { headers: {...} }) headers correctness
• new Response(body, { status: 404 }) status correctness
• new Response(null) empty body
• Streaming responses
• WebSocket upgrades
• Existing serve spec tests

🤖 Generated with Claude Code