perf: lazy header instantiation for HTTP requests (#10150)

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.

1 files changed, 25 insertions, 4 deletions

diff --git a/op_crates/fetch/26_fetch.js b/op_crates/fetch/26_fetch.js
index 0bcd8f85c..e713b48ed 100644
--- a/op_crates/fetch/26_fetch.js
+++ b/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 = "";
@@ -960,7 +961,7 @@
     #method = "GET";
     /** @type {string} */
     #url = "";
-    /** @type {Headers} */
+    /** @type {Headers | string[][]} */
     #headers;
     /** @type {"include" | "omit" | "same-origin" | undefined} */
     #credentials = "omit";
@@ -999,16 +1000,32 @@
       }
 
       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;
 
@@ -1016,9 +1033,9 @@
         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.
@@ -1085,6 +1102,9 @@
     }
 
     get headers() {
+      if (!(this.#headers instanceof Headers)) {
+        this.#headers = new Headers(this.#headers);
+      }
       return this.#headers;
     }
 
@@ -1515,5 +1535,6 @@
     createHttpClient,
     fastBody,
     dontValidateUrl,
+    lazyHeaders,
   };
 })(this);