perf(ext/http): Use flat list of headers for multiple set/get methods (#19336)

This PR attempts to resolve the first item on the list from
https://github.com/denoland/deno/issues/19330 which is about using a
flat list of interleaved key/value pairs, instead of a nested array of
tuples.

I can tackle some more if you can provide a quick example of using raw
v8 arrays, cc @mmastrac

ext/http/00_serve.js

@@ -334,7 +334,12 @@ class InnerRequest {
     if (this.#slabId === undefined) {
       throw new TypeError("request closed");
     }
-    return op_http_get_request_headers(this.#slabId);
+    const headers = [];
+    const reqHeaders = op_http_get_request_headers(this.#slabId);
+    for (let i = 0; i < reqHeaders.length; i += 2) {
+      headers.push([reqHeaders[i], reqHeaders[i + 1]]);
+    }
+    return headers;
   }
 
   get slabId() {
@@ -570,7 +575,7 @@ function mapToCallback(context, callback, onError) {
       if (headers.length == 1) {
         op_http_set_response_header(req, headers[0][0], headers[0][1]);
       } else {
-        op_http_set_response_headers(req, headers);
+        op_http_set_response_headers(req, headers.flat());
       }
     }
 

ext/http/http_next.rs

@@ -256,12 +256,11 @@ pub fn op_http_get_request_header(
 }
 
 #[op]
-pub fn op_http_get_request_headers(
+pub fn op_http_get_request_headers(slab_id: SlabId) -> Vec<ByteString> {
-  slab_id: SlabId,
-) -> Vec<(ByteString, ByteString)> {
   let http = slab_get(slab_id);
   let headers = &http.request_parts().headers;
-  let mut vec = Vec::with_capacity(headers.len());
+  // Two slots for each header key/value pair
+  let mut vec = Vec::with_capacity(headers.len() * 2);
   let mut cookies: Option<Vec<&[u8]>> = None;
   for (name, value) in headers {
     if name == COOKIE {
@@ -272,7 +271,8 @@ pub fn op_http_get_request_headers(
       }
     } else {
       let name: &[u8] = name.as_ref();
-      vec.push((name.into(), value.as_bytes().into()))
+      vec.push(name.into());
+      vec.push(value.as_bytes().into());
     }
   }
 
@@ -283,11 +283,10 @@ pub fn op_http_get_request_headers(
   // TODO(mmastrac): This should probably happen on the JS side on-demand
   if let Some(cookies) = cookies {
     let cookie_sep = "; ".as_bytes();
-    vec.push((
+    vec.push(ByteString::from(COOKIE.as_str()));
-      ByteString::from(COOKIE.as_str()),
+    vec.push(ByteString::from(cookies.join(cookie_sep)));
-      ByteString::from(cookies.join(cookie_sep)),
-    ));
   }
+
   vec
 }
 
@@ -313,18 +312,15 @@ pub fn op_http_set_response_header(slab_id: SlabId, name: &str, value: &str) {
 }
 
 #[op]
-pub fn op_http_set_response_headers(
+pub fn op_http_set_response_headers(slab_id: SlabId, headers: Vec<ByteString>) {
-  slab_id: SlabId,
-  headers: Vec<(ByteString, ByteString)>,
-) {
   let mut http = slab_get(slab_id);
   // TODO(mmastrac): Invalid headers should be handled?
   let resp_headers = http.response().headers_mut();
   resp_headers.reserve(headers.len());
-  for (name, value) in headers {
+  for header in headers.chunks_exact(2) {
     // These are valid latin-1 strings
-    let name = HeaderName::from_bytes(&name).unwrap();
+    let name = HeaderName::from_bytes(&header[0]).unwrap();
-    let value = HeaderValue::from_bytes(&value).unwrap();
+    let value = HeaderValue::from_bytes(&header[1]).unwrap();
     resp_headers.append(name, value);
   }
 }