perf: lazy header instantiation for HTTP requests

This commit introduces a performance optimization for the native HTTP
server. From my testing it is about 2-6% faster than `main`. Request
headers in the HTTP servers are now lazilly instatated when they are
accessed, rather than being preemptively wrapped in the `Headers` class.

op_crates/fetch/26_fetch.js

@@ -686,6 +686,7 @@
  // fastBody and dontValidateUrl allow users to opt out of certain behaviors
  const fastBody = Symbol("Body#fast");
  const dontValidateUrl = Symbol("dontValidateUrl");
+ const lazyHeaders = Symbol("lazyHeaders");
 
  class Body {
  #contentType = "";
@@ -946,7 +947,7 @@
  #method = "GET";
  /** @type {string} */
  #url = "";
- /** @type {Headers} */
+ /** @type {Headers | string[][]} */
  #headers;
  /** @type {"include" | "omit" | "same-origin" | undefined} */
  #credentials = "omit";
@@ -985,26 +986,42 @@
  }
 
  let headers;
+ let contentType = "";
  // prefer headers from init
  if (init.headers) {
- headers = new Headers(init.headers);
+ if (init[lazyHeaders] && Array.isArray(init.headers)) {
+ // Trust the headers are valid, and only put them into the `Headers`
+ // strucutre when the user accesses the property. We also assume that
+ // all passed headers are lower-case (as is the case when they come
+ // from hyper in Rust), and that headers are of type
+ // `[string, string][]`.
+ headers = init.headers;
+ for (const tuple of headers) {
+ if (tuple[0] === "content-type") {
+ contentType = tuple[1];
+ }
+ }
+ } else {
+ headers = new Headers(init.headers);
+ contentType = headers.get("content-type") || "";
+ }
  } else if (input instanceof Request) {
  headers = input.headers;
+ contentType = headers.get("content-type") || "";
  } else {
  headers = new Headers();
  }
 
- const contentType = headers.get("content-type") || "";
  super(b, { contentType });
  this.#headers = headers;
 
  if (input instanceof Request) {
  if (input.bodyUsed) {
  throw TypeError(BodyUsedError);
  }
+ // headers are already set above. no reason to do it again
  this.#method = input.method;
  this.#url = input.url;
- this.#headers = new Headers(input.headers);
  this.#credentials = input.credentials;
  } else {
  // Constructing a URL just for validation is known to be expensive.
@@ -1071,6 +1088,9 @@
  }
 
  get headers() {
+ if (!(this.#headers instanceof Headers)) {
+ this.#headers = new Headers(this.#headers);
+ }
  return this.#headers;
  }
 
@@ -1501,5 +1521,6 @@
  createHttpClient,
  fastBody,
  dontValidateUrl,
+ lazyHeaders,
  };
 })(this);

runtime/js/40_http.js

@@ -2,7 +2,7 @@
 "use strict";
 
 ((window) => {
- const { Request, dontValidateUrl, fastBody, Response } =
+ const { Request, dontValidateUrl, lazyHeaders, fastBody, Response } =
  window.__bootstrap.fetch;
  const { Headers } = window.__bootstrap.headers;
  const errors = window.__bootstrap.errors.errors;
@@ -61,8 +61,9 @@
  const request = new Request(url, {
  body,
  method,
- headers: new Headers(headersList),
+ headers: headersList,
  [dontValidateUrl]: true,
+ [lazyHeaders]: true,
  });
 
  const respondWith = createRespondWith(responseSenderRid, this.#rid);